What are vaccines?

Vaccines play an important role in keeping us healthy. They protect us from serious and sometimes deadly diseases. Vaccines are injections (shots), liquids, pills, or nasal sprays that you take to teach your body's immune system to recognize and defend against harmful germs. The germs could be viruses or bacteria.

Some types of vaccines contain germs that cause disease. But the germs have been killed or weakened enough that they won't make you sick. Some vaccines only contain a part of a germ. Other types of vaccines include instructions for your cells to make a protein of the germ.

These different vaccine types all spark an immune response, which helps your body fight off the germs. Your immune system will also remember the germ and attack it if that germ ever invades again. This protection against a certain disease is called immunity.

These diseases can be very serious. Because of this, getting immunity from a vaccine is safer than getting immunity by being sick with the disease. And for a few vaccines, getting vaccinated can actually give you a better immune response than getting the disease would.

Do vaccines cause side effects?

As with medicines, any vaccine can cause side effects. Most of the time the side effects are minor, such as a sore arm, fatigue, or mild fever. They usually go away within a few days. These common side effects are often a sign that your body is starting to build immunity against a disease.

Serious side effects from vaccines can happen, but they are very rare. These side effects could include a severe allergic reaction. Other possible side effects are different for each vaccine. Talk with your health care provider if you're concerned about your health after getting vaccinated.

Some people worry that childhood vaccines could cause autism spectrum disorder (ASD). But many scientific studies have looked at this and have found no link between vaccines and ASD.

How are vaccines tested for safety?

Every vaccine that is approved in the United States goes through extensive safety testing. It starts with testing and evaluation of the vaccine before it's approved by the Food and Drug Administration (FDA). This process can often take several years.

• First, the vaccine is tested in labs. Based on those tests, the FDA decides whether to test the vaccine with people.
• Testing with people is done through clinical trials. In these trials, the vaccines are tested on volunteers. Clinical trials usually start with 20 to 100 volunteers, but eventually include thousands of volunteers.
• The clinical trials have three phases.
• The trials are looking for the answer to important questions such as:
  • Is the vaccine safe?
  • What dose (amount) works best?
  • How does the immune system react to it?
  • How effective is it?
• During the process, the FDA works closely with the company who makes the vaccine to evaluate the vaccine's safety and effectiveness. If the vaccine is found to be safe and effective, it will be approved and licensed by the FDA.
• After a vaccine is licensed, experts may consider adding it to the recommended vaccine, or immunization, schedule. This schedule is from the Centers for Disease Control and Prevention (CDC). It lists which vaccines are recommended for different groups of people. They list which age groups should get which vaccines, how many doses they need, and when they should get them.

Testing and monitoring continue after the vaccine is approved:

• The company making the vaccines tests every batch of vaccines for quality and safety. The FDA reviews the results of these tests. It also inspects the factories where the vaccine is made. These checks help make sure the vaccines meet standards for quality and safety.
• The FDA, CDC, and other federal agencies continue to monitor its safety, to watch for possible side effects. They have systems to track any safety issues with the vaccines.

These high safety standards and testing help to make sure that vaccines in the United States are safe. Vaccines help protect against serious, even deadly, diseases. They not only protect you, but also help to keep these diseases from spreading to others.

What is an adjuvant and why is it added to a vaccine?

An adjuvant is an ingredient used in some vaccines that helps create a stronger immune response in people receiving the vaccine. In other words, adjuvants help vaccines work better. Some vaccines that are made from weakened or killed germs contain naturally occurring adjuvants and help the body produce a strong protective immune response. However, most vaccines developed today include just small components of germs, such as their proteins, rather than the entire virus or bacteria. Adjuvants help the body to produce an immune response strong enough to protect the person from the disease he or she is being vaccinated against. Adjuvanted vaccines can cause more local reactions (such as redness, swelling, and pain at the injection site) and more systemic reactions (such as fever, chills and body aches) than non-adjuvanted vaccines.

Adjuvants have been used safely in vaccines for decades.

Aluminum salts, such as aluminum hydroxide, aluminum phosphate, and aluminum potassium sulfate have been used safely in vaccines for more than 70 years. Aluminum salts were initially used in the 1930s, 1940s, and 1950s with diphtheria and tetanus vaccines after it was found they strengthened the body’s immune response to these vaccines.

Newer adjuvants have been developed to target specific components of the body’s immune response, so that protection against disease is stronger and lasts longer.

In all cases, vaccines containing adjuvants are tested for safety and effectiveness in clinical trials before they are licensed for use in the United States, and they are continuously monitored by CDC and FDA once they are approved.

Several different adjuvants are used in U.S. vaccines.

Aluminum
Aluminum-containing adjuvants are vaccine ingredients that have been used in vaccines since the 1930s. Small amounts of aluminum are added to help the body build stronger immunity against the germ in the vaccine. Aluminum is one of the most common metals found in nature and is present in air, food, and water. Scientific research has shown the amount of aluminum exposure in people who follow the recommended vaccine schedule is low and is not readily absorbed by the body.

AS04
Beginning in 2009, monophosphoryl lipid A (MPL) was used in one U.S. vaccine (Cervarix^®); however, the vaccine is no longer available in the United States due to low market demand. This immune-boosting substance was isolated from the surface of bacteria.

MF59
MF59 is the adjuvant contained in Fluad (an influenza vaccine licensed for adults aged 65 or older). MF59 is an oil-in-water emulsion composed of squalene, which is a naturally occurring oil found in many plant and animal cells, as well as in humans. MF59, used in flu vaccines in Europe since 1997 and in the United States since 2016, has been given to millions of people and has an excellent safety record.

AS01_B
AS01_B is an adjuvant suspension used with the antigen component of Shingrix vaccine. Shingrix is the recombinant zoster vaccine recommended for persons aged 50 years or older. AS01_B is made of up of monophosphoryl lipid A (MPL), an immune-boosting substance isolated from the surface of bacteria, and QS-21, a natural compound extracted from the Chilean soapbark tree (Quillaja saponaria Molina). In pre-licensure clinical trials, AS01_B was associated with local and systemic reactions, but the overall safety profile was reassuring.

AS01_B is also a component of vaccines currently being tested in clinical trials, including malaria and HIV vaccines. To date, these trials have included over 15,000 people.

CpG 1018
CpG 1018 is a recently developed adjuvant used in Heplisav-B vaccine. It is made up of cytosine phosphoguanine (CpG) motifs, which is a synthetic form of DNA that mimics bacterial and viral genetic material. When CpG 1018is included in a vaccine, it increases the body’s immune response.

In pre-licensure clinical trials, adverse events after Heplisav-B were comparable to those observed after another U.S.-licensed, non-adjuvanted hepatitis B vaccine.

Autism spectrum disorder (ASD) is a developmental disability that can cause significant social, communication, and behavioral challenges. Recent estimates from CDC’s Autism and Developmental Disabilities Monitoring Network found that about 1 in 54 children have been identified with ASD in communities across the United States. CDC is committed to providing essential data on ASD, searching for causes of and factors that increase the risk for ASD, and developing resources that help identify children with ASD as early as possible.

Vaccines do not cause autism.

Some people have had concerns that ASD might be linked to the vaccines children receive, but studies have shown that there is no link between receiving vaccines and developing ASD. In 2011, an Institute of Medicine (IOM) report on eight vaccines given to children and adults found that with rare exceptions, these vaccines are very safe.

A 2013 CDC study added to the research showing that vaccines do not cause ASD. The study looked at the number of antigens (substances in vaccines that cause the body’s immune system to produce disease-fighting antibodies) from vaccines during the first two years of life. The results showed that the total amount of antigen from vaccines received was the same between children with ASD and those that did not have ASD.

Vaccine ingredients do not cause autism.

One vaccine ingredient that has been studied specifically is thimerosal, a mercury-based preservative used to prevent contamination of multidose vials of vaccines. Research shows that thimerosal does not cause ASD. In fact, a 2004 scientific review by the IOM concluded that “the evidence favors rejection of a causal relationship between thimerosal–containing vaccines and autism.” Since 2003, there have been nine CDC-funded or conducted studies that have found no link between thimerosal-containing vaccines and ASD, as well as no link between the measles, mumps, and rubella (MMR) vaccine and ASD in children.

Between 1999 and 2001, thimerosal was removed or reduced to trace amounts in all childhood vaccines except for some flu vaccines. This was done as part of a broader national effort to reduce all types of mercury exposure in children before studies were conducted that determined that thimerosal was not harmful. It was done as a precaution. Currently, the only childhood vaccines that contain thimerosal are flu vaccines packaged in multidose vials. Thimerosal-free alternatives are also available for flu vaccine. For more information, see the Timeline for Thimerosal in Vaccines.

Besides thimerosal, some people have had concerns about other vaccine ingredients in relation to ASD as well. However, no links have been found between any vaccine ingredients and ASD.

Fainting, also called syncope, is a temporary loss of consciousness caused by a decreased blood flow to the brain. Although fainting has a variety of possible causes, it is usually triggered by pain or anxiety. Sometimes people faint after vaccination. People who faint might fall and injure themselves if they are not sitting or lying down at the time that they lose consciousness. Sometimes when people faint, their muscles twitch and their bodies make jerking movements; this can sometimes be confused with seizures but are not actual seizures.

Fainting can happen after many types of vaccinations.

Fainting can be triggered by many types of medical procedures. In fact, CDC has received reports of people fainting after nearly all vaccines. Fainting after getting a vaccine is most commonly reported after three vaccines given to adolescents: HPV, MCV4, and Tdap. Because the ingredients of these three vaccines are different, yet fainting is seen with all of them, scientists think that fainting is due to the vaccination process and not to the vaccines themselves. However, there is not yet a definite answer about whether an ingredient of the vaccines is responsible for the fainting or if adolescents are simply more likely than children or adults to experience fainting.

About 3% of men and 3.5% of women report fainting at least once during their lifetimes, but it is not known just how often fainting happens after vaccination. Because fainting usually has no lasting effects, it is hard to study using medical records-based systems. However, the Vaccine Adverse Event Reporting System (VAERS), receives many reports of syncope each year, and many more are likely to go unreported.

Fainting can be common among adolescents after vaccination.

Reports from the Vaccine Adverse Event Reporting System (VAERS) shows that fainting after vaccinations is common in adolescents. One study of VAERS reports found that 62% of syncope reports were among adolescents 11 to 18 years old. However, because syncope may not always be reported, VAERS data cannot be used to determine how often fainting happens after vaccination.

Falls after fainting can cause injuries.

Fainting itself is generally not serious, but harm from related falls or other accidents can cause injury. The main concern is head injury. In a study of syncope-related VAERS reports, 7% of the fainting reports were coded as serious; 12% of these involved head injuries. Although fainting itself might or might not be preventable, it is important to prevent injuries when people do faint.

Fainting and related injuries after immunization can be prevented.

Giving patients a beverage, a snack, or some reassurance about the procedure has been shown to prevent some fainting. Studies are being done to look more into these strategies. However, many falls due to fainting can be prevented by having the patient sit or lie down. For this reason, experts recommend having patients sit in a chair or lay down when they receive a vaccination. In addition, patients should be observed for 15 minutes after vaccination.

If a patient does faint after a vaccination, she or he should be observed by medical personnel until she or he regains consciousness so that further treatment needs can be determined. If fainting happens outside the medical setting and the patient does not recover immediately, contact local emergency medical services. Patients who faint after vaccination generally recover within a few minutes.

What is a febrile seizure?

Sometimes, fevers can cause a child to experience spasms or jerky movements called seizures. Seizures caused by fever are called “febrile seizures.” They are most common with fevers of 102°F (38.9°C) or higher, but they can also happen at lower body temperatures or when a fever is going down. Most febrile seizures last for less than one or two minutes.

Febrile seizures can be frightening, but nearly all children who have a febrile seizure recover quickly. Febrile seizures do not cause any permanent harm and do not have any lasting effects.

Febrile seizures can happen with any condition that causes a fever.

Fevers can be caused by common childhood illnesses like colds, the flu, an ear infection, or roseola. Vaccines can sometimes cause fevers, but febrile seizures are uncommon after vaccination.

Infants and young children are most at risk for febrile seizures.

Up to 5% of young children will have a febrile seizure at some time in their life. Febrile seizures happen in children between the ages of 6 months and 5 years, with most occurring between 14–18 months of age. About 1 out of every 3 children who have a febrile seizure will have at least one more during childhood. There is a small increased risk for febrile seizures after MMR and MMRV vaccines.

Studies have shown a small increased risk for febrile seizures during the 5 to 12 days after a child has received their first vaccination with the measles, mumps, rubella (MMR) vaccine. The risk is slightly higher with the measles, mumps, rubella, varicella (MMRV) combination vaccine, but the risk is still small. Studies have not shown an increased risk for febrile seizures after the separate varicella (chickenpox) vaccine.

There is a small increased risk for febrile seizures when inactivated influenza vaccine (flu shot) is given at the same doctor visit as either the PCV13 (pneumococcal) vaccine or the DTaP vaccine.

A CDC study of children aged 6 months to 2 years has shown a small increased risk for febrile seizures during the 24 hours after a child receives the inactivated influenza vaccine (flu shot) at the same time as the pneumococcal 13-valent conjugate (PCV13) vaccine or the diphtheria, tetanus, acellular pertussis (DTaP) vaccine. The flu shot was not associated with an increased risk of febrile seizures when it was given on a different day from the other two vaccines. Studies have not shown an increased risk for febrile seizures after the DTaP vaccine, except when it is given at the same time as the flu shot. There may be a small increase in the risk of febrile seizure when PCV13 (pneumococcal) vaccine is given by itself.

The risk of febrile seizure with any combination of these vaccines is small (at most 30 febrile seizures in 100,000 children vaccinated) and CDC’s Advisory Committee on Immunization Practices (ACIP) continues to encourage the vaccination of children according to the vaccination schedule, which allows for the flu, pneumococcal and DTaP vaccinations to be given during the same doctor’s visit.

Vaccines can also help prevent febrile seizures.

Vaccinating children at the recommended age may prevent some febrile seizures by protecting children against measles, mumps, rubella, chickenpox, influenza, pneumococcal infections and other diseases that can cause fever and febrile seizures.

Guillain-Barré syndrome (GBS) is a rare disorder where the body’s immune system damages nerve. The damage to the nerves causes muscle weakness and sometimes paralysis. While its cause is not fully understood, the syndrome often follows infection with a virus or bacteria. Each year in the United States, an estimated 3,000 to 6,000 people develop GBS. Most people fully recover from GBS, but some have permanent nerve damage.

GBS is rare.

Anyone can develop GBS, but people older than 50 are at greatest risk. About two-thirds of people with GBS were sick with diarrhea or respiratory illness days or weeks before developing symptoms. Infection with the bacteria Campylobacter jejuni, which causes gastroenteritis (including symptoms of nausea, vomiting and diarrhea), is one of the most common risk factors for GBS. People also can develop GBS after having the flu or other infections such as cytomegalovirus and Epstein-Barr virus. On very rare occasions, people develop GBS in the days or weeks after getting a vaccination.

To study whether a new vaccine might be causing GBS, CDC compares the usual rate of GBS to the observed rate of GBS in vaccinated people. This helps to determine whether a vaccine could be causing more cases.

GBS and the link to flu vaccine

In response to a potential pandemic strain of a flu virus in 1976, there was a national campaign for people to get the swine flu vaccination. Over 45 million people were vaccinated and there were increased reports of GBS following vaccination. Those who received the 1976 swine flu vaccine had a slight increased risk for developing GBS of approximately one additional case of GBS for every 100,000 people who got the swine flu vaccine. Scientists have several theories about the cause, but the exact reason for this link remains unknown.

CDC monitors GBS cases during each flu season. From data collected, the association between seasonal flu vaccine and GBS has been found to vary from season to season. When there has been an increased risk, it has consistently been in the range of 1-2 additional GBS cases per million flu vaccine doses administered.

Additional studies have been conducted on the risk of GBS following flu vaccination. Results of these studies suggest that it is more likely that a person will get GBS after getting the flu than after vaccination. It is important to keep in mind flu infection can potentially cause severe illness and death. The best way to prevent flu infection and its complications is to get the flu vaccine.

Early vaccination is important to prevent diseases

Vaccines are the best defense against infections that may have serious complications such as pneumonia, meningitis, cancer, and even death. CDC recommends vaccinations before the age of two years to protect children against 14 infectious diseases: measles, mumps, rubella (German measles), varicella (chickenpox), hepatitis A, hepatitis B, diphtheria, tetanus, pertussis (whooping cough), Haemophilus influenzae Type b (Hib), polio, influenza (flu), rotavirus, and pneumococcal disease.

Children are given shots (vaccines) at a young age because this is when they are at highest risk of getting sick or dying if they get these diseases. Newborn babies are immune to some diseases because they have antibodies they get from their mothers, usually before they are born. However, this immunity lasts a few months. Most babies do not get protective antibodies against diphtheria, whooping cough, polio, tetanus, hepatitis B, or Hib from their mothers. This is why it’s important to vaccinate a child before she or he is exposed to a disease.

Vaccines contain weakened or killed versions of the germs that cause a disease. These elements of vaccines, and other molecules and micro-organisms that stimulate the immune system, are called “antigens.” Babies are exposed to thousands of germs and other antigens in the environment from the time they are born. When a baby is born, his or her immune system is ready to respond to the many antigens in the environment and the selected antigens in vaccines.

Different childhood vaccines can be given at the same time.

Many vaccines are recommended early in life to protect young children from dangerous infectious diseases. In order to reduce the number of shots a child receives in a doctor’s visit, some vaccines are offered as combination vaccines. A combination vaccine is two or more different vaccines that have been combined into a single shot. Combination vaccines have been in use in the United States since the mid-1940s. Examples of combination vaccines are: DTap (diphtheria-tetanus-pertussis), trivalent IPV (three strains of inactivated polio vaccine), MMR (measles-mumps-rubella), DTap-Hib, and Hib-Hep B.

Often, more than one shot will be given during the same doctor’s visit, usually in separate limbs (e.g. one in each arm). For example, a baby might get DTaP in one arm or leg and IPV in another arm or leg during the same visit.

Giving a child several vaccines during the same visit offers two advantages.

First, children should be given their vaccines as quickly as possible to give them protection during the vulnerable early months of their lives. Second, giving several shots at the same time means fewer office visits. This saves parents time and money, and can be less traumatic for the child.

Getting multiple vaccines at the same time has been shown to be safe.

Scientific data show that getting several vaccines at the same time does not cause any chronic health problems. A number of studies have been done to look at the effects of giving various combinations of vaccines, and when every new vaccine is licensed, it has been tested along with the vaccines already recommended for a particular aged child. The recommended vaccines have been shown to be as effective in combination as they are individually. Sometimes, certain combinations of vaccines given together can cause fever, and occasionally febrile seizures; these are temporary and do not cause any lasting damage. Based on this information, both the Advisory Committee on Immunization Practices and the American Academy of Pediatrics recommend getting all routine childhood vaccines on time.

CDC’s recommended childhood vaccine schedule ensures children get the best protection during the many different stages in growth and development.

From the moment babies are born, they are exposed to numerous bacteria and viruses on a daily basis. Eating food introduces new bacteria into the body; numerous bacteria live in the mouth and nose; and an infant places his or her hands or other objects in his or her mouth hundreds of times every hour, exposing the immune system to still more germs. When a child has a cold, he or she is exposed to up to 10 antigens, and exposure to “strep throat” is about 25 to 50 antigens. Each vaccine in the childhood vaccination schedule has between 1-69 antigens. A child who receives all the recommended vaccines in the 2018 childhood immunization schedule may be exposed to up to 320 antigens through vaccination by the age of 2.

In fact, a 1994 report from the Institute of Medicine, Adverse Events Associated with Childhood Vaccines, states: “In the face of these normal events, it seems unlikely that the number of separate antigens contained in childhood vaccines …would represent an appreciable added burden on the immune system that would be immunosuppressive.”

Vaccines have not been shown to cause sudden infant death syndrome (SIDS).

Babies receive multiple vaccines when they are between 2 to 4 months old. This age range is also the peak age for sudden infant death syndrome (SIDS). The timing of the 2 month and 4 month shots and SIDS has led some people to question whether they might be related. However, studies have found that vaccines do not cause and are not linked to SIDS.

Multiple research studies and safety reviews have looked at possible links between vaccines and SIDS. The evidence accumulated over many years do not show any links between childhood immunization and SIDS.

Placing healthy babies on their backs to sleep reduces the risk of SIDS.

SIDS is the sudden, unexpected death of a baby younger than 1 year of age that doesn’t have a known cause even after a complete investigation. These deaths often happen during sleep or in the baby’s sleep area.

In 1992, the American Academy of Pediatrics recommended that healthy babies be placed on their backs to sleep. That recommendation, along with the National Institute of Child Health and Human Development’s 1994 “Back-to-Sleep” campaign (now known as the Safe to Sleep® campaign), encouraged caregivers to place infants on their backs to sleep, and coincided with a dramatic reduction in the SIDS rate in the United States.

Also, research has found the rate of SIDS declined dramatically following the 1994 “Back-to-Sleep” campaign, and then stabilized in the 2000s at a time when the number of infant immunizations was increasing. The findings provide strong evidence that immunization is not linked to SIDS. See the published article on SIDS rates.

CDC and FDA closely monitor the safety of all vaccines.

CDC and the Food and Drug Administration (FDA) are committed to ensuring that vaccines provided to the public are safe and effective. Once vaccines are licensed in the United States, CDC and FDA continuously monitor them through several safety systems.

Thimerosal is a mercury-based preservative that has been used for decades in the United States in multi-dose vials (vials containing more than one dose) of medicines and vaccines. There is no evidence of harm caused by the low doses of thimerosal in vaccines, except for minor reactions like redness and swelling at the injection site. However, in July 1999, the Public Health Service agencies, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure.

About Thimerosal

Mercury is a naturally occurring element found in the earth’s crust, air, soil, and water. Two types of mercury to which people may be exposed — methylmercury and ethylmercury — are very different.

Methylmercury is the type of mercury found in certain kinds of fish. At high exposure levels methylmercury can be toxic to people. In the United States, federal guidelines keep as much methylmercury as possible out of the environment and food, but over a lifetime, everyone is exposed to some methylmercury.

Thimerosal contains ethylmercury, which is cleared from the human body more quickly than methylmercury, and is therefore less likely to cause any harm.

Thimerosal prevents the growth of bacteria in vaccines.

Thimerosal is added to vials of vaccine that contain more than one dose (multi-dose vials) to prevent growth of germs, like bacteria and fungi. Introduction of bacteria and fungi has the potential to occur when a syringe needle enters a vial as a vaccine is being prepared for administration. Contamination by germs in a vaccine could cause severe local reactions, serious illness or death. In some vaccines, preservatives, including thimerosal, are added during the manufacturing process to prevent germ growth.

The human body eliminates thimerosal easily.

Thimerosal does not stay in the body a long time so it does not build up and reach harmful levels. When thimerosal enters the body, it breaks down to ethylmercury and thiosalicylate, which are readily eliminated.

Thimerosal has been shown to be safe when used in vaccines.

Thimerosal use in medical products has a record of being very safe. Data from many studies show no evidence of harm caused by the low doses of thimerosal in vaccines.

There are some side effects of thimerosal in vaccines.

The most common side-effects are minor reactions like redness and swelling at the injection site. Although rare, some people may be allergic to thimerosal.

Scientific research does not show a connection between thimerosal and autism.

Research does not show any link between thimerosal in vaccines and autism, a neurodevelopmental disorder. Many well conducted studies have concluded that thimerosal in vaccines does not contribute to the development of autism. Even after thimerosal was removed from almost all childhood vaccines, autism rates continued to increase, which is the opposite of what would be expected if thimerosal caused autism.

Thimerosal was taken out of childhood vaccines in the United States in 2001.

Measles, mumps, and rubella (MMR) vaccines do not and never did contain thimerosal. Varicella (chickenpox), inactivated polio (IPV), and pneumococcal conjugate vaccines have also never contained thimerosal.

Influenza (flu) vaccines are currently available in both thimerosal-containing (for multi-dose vaccine vials) and thimerosal-free versions.

Some parents have questions about the safety of ingredients – like thimerosal (“THY-mayr-uh-sal”) – in children’s shots (vaccines).

We want you to know that thimerosal is no longer used in children’s shots, except some types of flu shots. You can ask for a flu shot without thimerosal.

Check out these answers to common questions about thimerosal.

What is thimerosal?

Thimerosal is a vaccine additive, added to some vaccines to prevent germs (like bacteria and fungi) from growing in them. If germs grow in vaccines, they can cause illness—or even death.

Why do some people worry about thimerosal in vaccines?

You may have heard that thimerosal has mercury in it. Not all types of mercury are the same. Some types of mercury, like mercury in some kinds of fish, stay in the human body and can make people sick. Thimerosal is a different kind of mercury. It doesn’t stay in the body, and is unlikely to make us sick.

Is thimerosal safe?

Yes. Thimerosal has been used safely in vaccines for a long time (since the 1930s).

Scientists have been studying the use of thimerosal in vaccines for many years. They haven’t found any evidence that thimerosal causes harm.

Is thimerosal still used in vaccines for children?

No. Thimerosal hasn’t been used in vaccines for children since 2001.

However, thimerosal is still used in some flu vaccines. Yearly flu vaccines are recommended for all children. If you are worried about thimerosal, you can ask for a flu vaccine without it.

Does thimerosal cause autism?

No. Research does not show any link between thimerosal and autism.

Are there side effects from thimerosal in vaccines?

Most people don’t have any side effects from thimerosal, but some people will have mild side effects like redness and swelling at the place where the shot was given, which will only last 1 to 2 days. It’s very unlikely you will have an allergic reaction to thimerosal.

How can I find out if thimerosal is in a vaccine?

Ask your doctor or pharmacist.

Ask to see the vaccine’s list of ingredients. All vaccine packages come with information (called an insert) that lists the ingredients.

Are vaccines safe during pregnancy?

Certain vaccines are safe and recommended for women before, during, and after pregnancy to help keep them and their babies healthy. The antibodies mothers develop in response to these vaccines not only protect them, but also cross the placenta and help protect their babies from serious diseases early in life. Vaccinating during pregnancy also helps protect a mother from getting a serious disease and then giving it to her newborn.

Which vaccines should I get if I am pregnant?

If you are pregnant or planning a pregnancy, the specific vaccines you need are determined by your age, lifestyle, medical conditions, travel, and previous vaccinations.

If you are planning a pregnancy, talk with your healthcare provider about getting up to date on all your vaccines. Some vaccines, such as the measles, mumps, rubella (MMR) vaccine, should be given a month or more before pregnancy. Your healthcare provider can help you determine if you need this or any other catch-up vaccine.

CDC recommends that pregnant women get two vaccines during every pregnancy: the inactivated flu vaccine (the injection, not the live nasal flu vaccine) and the Tdap vaccine.

Flu vaccine

CDC recommends getting the flu vaccine if you are pregnant during flu season. While flu seasons vary in their timing, CDC recommends getting vaccinated by the end of October, if possible. Getting vaccinated later during flu season, though, can still be beneficial. Flu vaccines have been given to millions of pregnant women over the years, and scientific evidence shows that it is safe. Getting the flu vaccine during pregnancy is one of the best ways to protect yourself and your baby for several months after birth from flu-related complications.

Tdap vaccine

Pregnant women are also encouraged to get the Tdap vaccine at any time during pregnancy, but optimally between 27 and 36 weeks of each pregnancy, to protect yourself and your baby from pertussis, also known as whooping cough. This vaccine is recommended during every pregnancy, regardless of how long it has been since you previously received the Tdap vaccine. If you did not get a Tdap vaccine during your pregnancy and have never gotten it, CDC recommends that you get the vaccine immediately after giving birth.

It is safe for women to receive most vaccines right after giving birth, even while breastfeeding. More information about the safety of vaccines during breastfeeding.

Which vaccines should I not get if I am pregnant?

Some vaccines are not recommended during pregnancy, such as:

• Human papillomavirus (HPV) vaccine
• Measles, mumps, and rubella (MMR) vaccine
• Live influenza vaccine (nasal flu vaccine)
• Varicella (chicken pox) vaccine
• Certain travel vaccines: yellow fever, typhoid fever, and Japanese encephalitis
  Note: these travel vaccines should generally not be given during pregnancy, unless your healthcare provider determines that the benefits outweigh the risks.

If you get any of these vaccines and then find out you are pregnant, talk to your doctor. Further doses of the vaccines, if needed, should be given after you have completed the pregnancy.

Can a vaccine harm my developing baby?

Some vaccines, especially live vaccines, should not be given to pregnant women because they may be harmful to the baby. Keep in mind that vaccine recommendations for pregnant women are developed with the highest safety concerns for both mothers and babies.

Are vaccines safe if I am breastfeeding?

Yes. It is safe to receive routine vaccines right after giving birth, even while you are breastfeeding. However, yellow fever vaccine is not recommended for breastfeeding women unless travel to certain countries is unavoidable and a healthcare provider determines that the benefits of vaccination outweigh the risks. Talk with your provider if you are considering yellow fever vaccine.

Any vaccine can cause side effects. For the most part these are minor (for example, a sore arm or low-grade fever) and go away within a few days. Listed below are vaccines licensed in the United States and side effects that have been associated with each of them. This information is copied directly from CDC’s Vaccine Information Statements (VISs), which in turn are derived from the Advisory Committee on Immunization Practices (ACIP) recommendations for each vaccine.

Remember, vaccines are continually monitored for safety, and like any medication, vaccines can cause side effects. However, a decision not to immunize a child also involves risk and could put the child and others who come into contact with him or her at risk of contracting a potentially deadly disease.

Regularly Recommended Vaccines

DTaP vaccine side effects (Diphtheria, Tetanus, and acellular Pertussis)

What are the risks from DTaP vaccine?

• Soreness or swelling where the shot was given, fever, fussiness, feeling tired, loss of appetite, and vomiting sometimes happen after DTaP vaccination.
• More serious reactions, such as seizures, non-stop crying for 3 hours or more, or high fever (over 105°F) after DTaP vaccination happen much less often. Rarely, vaccination is followed by swelling of the entire arm or leg, especially in older children when they receive their fourth or fifth dose.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Hepatitis A vaccine side effects

What are the risks from hepatitis A vaccine?

• Soreness or redness where the shot is given, fever, headache, tiredness, or loss of appetite can happen after hepatitis A vaccination.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Hepatitis B vaccine side effects

What are the risks from hepatitis B vaccine?

• Soreness where the shot is given or fever can happen after hepatitis B vaccination.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Hib vaccine side effects

What are the risks from Hib (Haemophilus influenzae type b) vaccine?

• Redness, warmth, and swelling where the shot is given and fever can happen after Hib vaccination.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

HPV-Gardasil-9 vaccine side effects (Human Papillomavirus Gardasil-9 vaccine)

What are the risks from HPV (Human Papillomavirus) vaccine?

• Soreness, redness, or swelling where the shot is given can happen after HPV vaccination.
• Fever or headache can happen after HPV vaccination.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Influenza (inactivated) vaccine side effects

What are the risks from inactivated influenza vaccine?

• Soreness, redness, and swelling where the shot is given, fever, muscle aches, and headache can happen after influenza vaccination.
• There may be a very small increased risk of Guillain-Barré Syndrome (GBS) after inactivated influenza vaccine (the flu shot).

Young children who get the flu shot along with pneumococcal vaccine (PCV13) and/or DTaP vaccine at the same time might be slightly more likely to have a seizure caused by fever. Tell your health care provider if a child who is getting flu vaccine has ever had a seizure.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Influenza (live) vaccine side effects

What are the risks from LAIV?

• Runny nose or nasal congestion, wheezing, and headache can happen after LAIV vaccination.
• Vomiting, muscle aches, fever, sore throat, and cough are other possible side effects.

If these problems occur, they usually begin soon after vaccination and are mild and short-lived.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

MMR vaccine side effects (Measles, Mumps, and Rubella)

What are the risks from MMR vaccine?

• Sore arm from the injection or redness where the shot is given, fever, and a mild rash can happen after MMR vaccination.
• Swelling of the glands in the cheeks or neck or temporary pain and stiffness in the joints (mostly in teenage or adult women) sometimes occur after MMR vaccination.
• More serious reactions happen rarely. These can include seizures (often associated with fever) or temporary low platelet count that can cause unusual bleeding or bruising.
• In people with serious immune system problems, this vaccine may cause an infection that may be life-threatening. People with serious immune system problems should not get MMR vaccine.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

MMRV vaccine side effects (Measles, Mumps, Rubella, and Varicella)

What are the risks from MMRV vaccine?

• Sore arm from the injection, redness where the shot is given, fever, and a mild rash can happen after MMRV vaccination.
• Swelling of the glands in the cheeks or neck or temporary pain and stiffness in the joints sometimes occur after MMRV vaccination.
• Seizures, often associated with fever, can happen after MMRV vaccine. The risk of seizures is higher after MMRV than after separate MMR and varicella vaccines when given as the first dose of the two-dose series in younger children. Your health care provider can advise you about the appropriate vaccines for your child.
• More serious reactions happen rarely, including temporary low platelet count, which can cause unusual bleeding or bruising.
• In people with serious immune system problems, this vaccine may cause an infection that may be life-threatening. People with serious immune system problems should not get MMRV vaccine.

If a person develops a rash after MMRV vaccination, it could be related to either the measles or the varicella component of the vaccine. The varicella vaccine virus could be spread to an unprotected person. Anyone who gets a rash should stay away from infants and people with a weakened immune system until the rash goes away. Talk with your health care provider to learn more.

Some people who are vaccinated against chickenpox get shingles (herpes zoster) years later. This is much less common after vaccination than after chickenpox disease.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Meningococcal ACWY vaccine side effects

What are the risks from meningococcal vaccines?

• Redness or soreness where the shot is given can happen after meningococcal ACWY vaccination.
• A small percentage of people who receive meningococcal ACWY vaccine experience muscle pain, headache, or tiredness.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Meningococcal B vaccine side effects

What are the risks from meningococcal vaccines?

• Soreness, redness, or swelling where the shot is given, tiredness, fatigue, headache, muscle or joint pain, fever, chills, nausea, or diarrhea can happen after meningococcal B vaccination. Some of these reactions occur in more than half of the people who receive the vaccine.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

PCV vaccine side effects (Pneumococcal Conjugate Vaccine)

What are the risks from PCV?

• Redness, swelling, pain, or tenderness where the shot is given, and fever, loss of appetite, fussiness (irritability), feeling tired, headache, muscle aches, joint pain, and chills can happen after pneumococcal conjugate vaccination.

Young children may be at increased risk for seizures caused by fever after PCV13 if it is administered at the same time as inactivated influenza vaccine. Ask your health care provider for more information.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

PPSV23 vaccine side effects (Pneumococcal Polysaccharide)

What are the risks from PPSV?

• Redness or pain where the shot is given, feeling tired, fever, or muscle aches can happen after PPSV23.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Polio vaccine side effects

• A sore spot with redness, swelling, or pain where the shot is given can happen after polio vaccination.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Rabies vaccine side effects

What are the risks from rabies vaccine?

• Soreness, redness, swelling, or itching at the site of the injection, and headache, nausea, abdominal pain, muscle aches, or dizziness can happen after rabies vaccine.
• Hives, pain in the joints, or fever sometimes happen after booster doses.
• Very rarely, nervous system disorders such as Guillain-Barré syndrome (GBS) have been reported after rabies vaccine.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Rotavirus vaccine side effects

What are the risks from rotavirus vaccine?

• Irritability or mild, temporary diarrhea or vomiting can happen after rotavirus vaccine.

Intussusception is a type of bowel blockage that is treated in a hospital and could require surgery. It happens naturally in some infants every year in the United States, and usually there is no known reason for it. There is also a small risk of intussusception from rotavirus vaccination, usually within a week after the first or second vaccine dose. This additional risk is estimated to range from about 1 in 20,000 U.S. infants to 1 in 100,000 U.S. infants who get rotavirus vaccine. Your health care provider can give you more information.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Td vaccine (Adult Tetanus & Diphtheria)

What are the risks from Td vaccine?

• Pain, redness, or swelling where the shot was given, mild fever, headache, feeling tired, and nausea, vomiting, diarrhea, or stomachache sometimes happen after Td vaccination.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Tdap vaccine (Combined Tetanus, Diphtheria & Pertussis)

What are the risks from Tdap vaccine?

• Pain, redness, or swelling where the shot was given, mild fever, headache, feeling tired, and nausea, vomiting, diarrhea, or stomachache sometimes happen after Tdap vaccination.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Varicella (Chickenpox) vaccine side effects

What are the risks from chickenpox vaccine?

• Sore arm from the injection, redness or rash where the shot is given, or fever can happen after varicella vaccination.
• More serious reactions happen very rarely. These can include pneumonia, infection of the brain and/or spinal cord covering, or seizures that are often associated with fever.
• In people with serious immune system problems, this vaccine may cause an infection that may be life-threatening. People with serious immune system problems should not get varicella vaccine.

It is possible for a vaccinated person to develop a rash. If this happens, the varicella vaccine virus could be spread to an unprotected person. Anyone who gets a rash should stay away from infants and people with a weakened immune system until the rash goes away. Talk with your health care provider to learn more.

Some people who are vaccinated against chickenpox get shingles (herpes zoster) years later. This is much less common after vaccination than after chickenpox disease.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Yellow Fever vaccine side effects

What are the risks from Yellow Fever vaccine?

• Soreness, redness, or swelling where the shot was given are common after yellow fever vaccine.
• Fever sometimes happens.
• Headache and muscle aches can occur.
• More serious reactions happen rarely after yellow fever vaccine. These can include:
  • Nervous system reactions such as inflammation of the brain (encephalitis) and/or spinal cord covering (meningitis), or Guillain-Barré Syndrome (GBS), among others.
  • Life-threatening severe illness with organ dysfunction or failure.

People 60 years and older and people with weakened immune systems might be more likely to experience serious reactions to yellow fever vaccine.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Zoster (Shingles) Vaccine, RZV side effects

What are the risks from shingles vaccine?

• A sore arm with mild or moderate pain is very common after recombinant shingles vaccine. Redness and swelling can also happen at the site of the injection.
• Tiredness, muscle pain, headache, shivering, fever, stomach pain, and nausea are common after recombinant shingles vaccine.

These side effects may temporarily prevent a vaccinated person from doing regular activities. Symptoms usually go away on their own in 2 to 3 days. You should still get the second dose of recombinant shingles vaccine even if you had one of these reactions after the first dose.

Guillain-Barré syndrome (GBS), a serious nervous system disorder, has been reported very rarely after recombinant zoster vaccine.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Travel & Special Circumstance Vaccines

Adenovirus vaccine side effects

What are the risks from Adenovirus vaccine?

• Headache, upper respiratory tract infection, stuffy nose, sore throat, abdominal pain, cough, nausea, diarrhea, fever or joint pain can happen after adenovirus vaccine.
• More serious problems including blood in the urine or stool, pneumonia, or inflammation of the stomach or intestines occur rarely after adenovirus vaccination.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Note: Adenovirus vaccine is approved for use only among military personnel.

Anthrax vaccine side effects

What are the risks from anthrax vaccine?

After getting a shot of anthrax vaccine, you may have:

• Tenderness, redness, itching, or a lump or bruise where the shot is given
• Muscle aches or short-term trouble moving your arm
• Headaches or fatigue

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Cholera vaccine side effects

What are the risks from Cholera vaccine?

• Tiredness, headache, abdominal pain, nausea, vomiting, lack of appetite, and diarrhea can happen after cholera vaccine.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

JE-IXIARO vaccine side effects (Japanese Encephalitis - IXIARO)

What are the risks from Japanese encephalitis vaccine?

• Pain, tenderness, redness, or swelling where the shot was given are common after JE vaccine.
• Fever sometimes happens (more often in children).
• Headache or muscle aches can occur (mainly in adults).

Studies have shown that severe reactions to JE vaccine are very rare.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.

Smallpox (Vaccinia) vaccine side effects

This medication guide replaces the Smallpox VIS. It is to be used before one receives the vaccination. Medical Guide for vaccination with ACAM2000.

Typhoid vaccine side effects

What are the risks from typhoid vaccine?

• Pain from the shot, redness, or swelling at the site of the injection, fever, and headache, and general discomfort can happen after inactivated typhoid vaccine.
• Fever, headache, abdominal pain, diarrhea, nausea, and vomiting can happen after live typhoid vaccine.

People sometimes faint after medical procedures, including vaccination. Tell your provider if you feel dizzy or have vision changes or ringing in the ears.

As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death.