About Influenza (Flu)

Influenza (flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness. Serious complications of flu infection can result in hospitalization or death. Some people, such as older people, young children, and people with certain health conditions, are at higher risk for developing serious flu complications.

There are two main types of influenza (flu) viruses: Types A and B. Influenza A and B viruses that routinely spread in people (human influenza viruses) are responsible for seasonal flu epidemics each year. Learn more about flu.

There are safe vaccines that can help reduce the risk from flu and its potentially serious complications.

Available Vaccines

In the United States, there are currently two ways someone can get a flu vaccine—by injection (a shot) or nasal spray. There is no preferential recommendation for any one flu vaccine over another. Note: In any given flu season, there are multiple FDA-licensed flu vaccines from different manufacturers available for use.

Injectable flu vaccines

Flu shots include inactivated influenza vaccines (IIV) and recombinant influenza vaccines (RIV). Flu shots do not contain live flu viruses. These vaccines come in various types of trivalent, quadrivalent, high-dose, adjuvanted, cell-based, and recombinant formulations.

Nasal spray flu vaccine

Also known as live, attenuated influenza vaccine (LAIV), this nasal spray contains live viruses. However, the viruses are attenuated (weakened), so that they will not cause influenza

Learn more about different types of flu vaccines.

Who Should Get Flu Vaccine

CDC recommends annual influenza vaccination for everyone 6 months and older with any flu vaccine licensed by the Food and Drug Administration (FDA) that is appropriate for the recipient’s age and health status.

Vaccination is particularly important for people who are at high risk of developing serious complications from influenza.

Specific high-risk groups include:

• Adults 65 and older
• People who have diabetes
• Pregnant women
• People who have compromised immune systems, such as that due to HIV/AIDS, other medical conditions, and medications
• Young children
• People who have cancer
• Children with neurologic conditions
• People with asthma
• People with a history of heart disease and stroke
• People with some other chronic medical conditions, such as kidney, liver, blood, and metabolic disorders

Common Side Effects

Vaccines, like any medicine, can have side effects. Many people who get an annual flu vaccine have no side effects at all. Most side effects from flu vaccination are mild and go away on their own in a few days without any treatment.

Severe allergic reactions following vaccination are rare, but can be life threatening.
Symptoms of a severe allergic reaction can include hives, swelling of the face and throat, difficulty breathing, a fast heartbeat, dizziness, and weakness.

If such reactions occur, call 9-1-1 and get the person to the nearest hospital.

Inactivated Influenza Vaccine (IIV) and Recombinant Influenza Vaccine (RIV)

Common Side Effects

• Injection site reactions that include soreness, redness and swollen arm
• Fever
• Muscle Aches
• Headache
• Fatigue

Who Should Not Get IIV or RIV (flu shot)

People should not get a flu shot if they:

• Are younger than 6 months of age.
• Have severe, life-threatening allergies to flu vaccine or any ingredient in the vaccine. This might include gelatin, antibiotics, or other ingredients.

People should talk to their healthcare provider before getting a flu shot if they:

• Have an allergy to eggs or any of the ingredients in the vaccine.
• Have ever had Guillain-Barré Syndrome (GBS). Some people with a history of GBS should not get a flu vaccine.

People who are moderately or severely ill should usually wait until they recover before getting a flu shot.

In some cases, the healthcare provider may decide to postpone influenza vaccination to a future visit.

Live, Attenuated Influenza Vaccine (LAIV, nasal spray)

Side Effects

In children, side effects from the nasal spray may include:

• Runny nose
• Wheezing
• Headache
• Vomiting
• Muscle aches
• Fever (low grade)

In adults, side effects from the nasal spray vaccine may include:

• Runny nose
• Headache
• Sore throat
• Cough

Who Should Not Get LAIV (flu nasal spray vaccine)

People should not get a nasal spray flu vaccine if they:

• Are younger than 2 years of age.
• Are 50 years of age and older.
• Are pregnant.
• Have severe, life-threatening allergies to nasal spray flu vaccine or any ingredient in the vaccine.
• Are between 2-17 years of age and are receiving aspirin, aspirin-containing, or salicylate-containing products.
• Have weakened immune systems (immunosuppression).
• Care for or are close contacts of severely immunocompromised persons who require a protected environment (or otherwise avoid contact with those persons for 7 days after getting the nasal spray vaccine).
• Are 2 years through 4 years and have asthma or have had a history of wheezing in the past 12 months.
• Have recently taken influenza antiviral drugs. People who have taken these medications should talk with their healthcare provider about which influenza antiviral was taken, and when the last dose was taken.
• LAIV is not recommended for people with some kinds of chronic health conditions because the safety and effectiveness of this vaccine in people with those conditions has not been established.

People should talk to their healthcare provider before getting a nasal spray flu vaccine if they:

• Are 5 years and older and have asthma.
• Have other underlying medical conditions that can put them at high risk of developing serious flu complications. These include conditions such as chronic lung diseases, heart disease (except isolated hypertension), kidney disease, liver disorders, neurologic and neuromuscular disorders, blood disorders, or metabolic disorders (such as diabetes).
• Have moderate or severe acute illness with or without fever.
• Have had Guillain-Barré Syndrome after a previous dose of influenza vaccine.

People who are moderately or severely ill should usually wait until they recover before getting a flu vaccine.

In some cases, the healthcare provider may decide to postpone influenza vaccination to a future visit or suggest a different influenza vaccine.

A Closer Look at the Safety Data

Findings from vaccine safety monitoring systems and scientific studies have shown that the flu vaccines have an excellent safety profile. Hundreds of millions of Americans have safely received flu vaccines for more than 50 years and the body of scientific evidence overwhelmingly supports their safety.

The safety of flu vaccines is monitored by CDC and FDA. Certain safety outcomes are commonly evaluated, including Guillain-Barré Syndrome, maternal and infant safety, and febrile seizures.

Guillain-Barré Syndrome (GBS)

The data on an association between seasonal inactivated influenza vaccine and GBS have been variable from season-to-season. When there has been an increased risk, it has been in the range of 1-2 additional GBS cases per million flu vaccine doses administered. The data also indicate that a person is more likely to get GBS after flu disease than after getting a flu vaccine.

Maternal and Infant Safety

Pregnant women are considered at high risk for developing serious complications from flu. Several studies have shown that influenza vaccination can protect pregnant women during and after pregnancy and protect the baby from influenza infection for several months after birth.

Febrile Seizure

Febrile seizures are seizures caused by fevers in young children. They can occur in all children, including those who have no serious underlying health issues. Febrile seizures can occur after common childhood febrile illnesses, like colds and flu. Less commonly, febrile seizures can occur as a result of post-vaccination fever. Although febrile seizures can be frightening for parents, children recover quickly and are not at increased risk for seizure disorders later in life. Children eventually outgrow them.

Are flu vaccines safe?

Flu vaccines have a good safety record. Hundreds of millions of Americans have safely received flu vaccines over the past 50 years, and there has been extensive research supporting the safety of flu vaccines.

A flu vaccine is the first and best way to reduce your chances of getting the flu and spreading it to others. CDC recommends that everyone 6 months of age and older receive a flu vaccine every year.

Can I get the flu from the flu vaccine?

No, the flu vaccine cannot cause flu. The vaccines either contain inactivated virus, meaning the viruses are no longer infectious, or a particle designed to look like a flu virus to your immune system. While the nasal spray flu vaccine does contain a live virus, the viruses are changed so that they cannot give you the flu.

Do flu vaccines cause any side effects?

Like any medical product, vaccines can cause side effects. Side effects of the flu vaccine are generally mild and go away on their own within a few days.

Common side effects from the flu shot include:

• Soreness, redness, and/or swelling from the shot
• Headache
• Fever
• Nausea
• Muscle aches

The flu shot, like other injections, can occasionally cause fainting.

Some studies have found a possible small association of injectable flu vaccine with Guillain-Barré syndrome (GBS). Overall, these studies estimated the risk for GBS after vaccination as fewer than 1 or 2 cases of GBS per one million people vaccinated. Other studies have not found any association. GBS also, rarely, occurs after flu illness. Even though GBS following flu illness is rare, GBS is more common following flu illness than following flu vaccination. GBS has not been associated with the nasal spray vaccine.

Are there signs or symptoms that should cause concern after getting a flu vaccine?

With any vaccine, look for any unusual conditions, such as a high fever, behavior changes, or signs of a severe allergic reaction after vaccination.

Signs of a severe allergic reaction can include:

• Difficulty breathing
• Hoarseness or wheezing
• Swelling around the eyes or lips
• Hives
• Paleness
• Weakness
• A fast heart beat or dizziness

Life threatening allergic reactions to the flu shot are rare. These signs would most likely happen within a few minutes to a few hours after the vaccine is given.

What should I do if I think I am having a severe reaction to a flu vaccine?

If you think it is a severe allergic reaction or other emergency that can’t wait, call 9-1-1 and get to the nearest hospital. Otherwise, call your doctor.

Afterward, the reaction should be reported to the Vaccine Adverse Event Reporting System (VAERS). Your doctor might file this report, or you can do it yourself through the VAERS website, or by calling 1-800-822-7967.

What should I do if I think I have been injured by the flu vaccine?

If you believe you have been injured by a flu vaccine you may be eligible to receive compensation from the federal government for your injuries if certain criteria are met. To learn more visit the National Vaccine Injury Compensation Program website or call 1-800-338-2382.

Are there some people who should not receive a flu vaccine?

CDC recommends everyone 6 months of age and older should receive an annual flu vaccination with rare exceptions. Individuals who can’t get the flu shot include:

• Children younger than 6 months, since they are too young to get a flu shot.
• Individuals with severe, life-threatening allergies to flu vaccine or any ingredient(s) in the vaccine.

Individuals should talk with their doctor before getting the flu shot if they:

• Have had a severe allergy to eggs or any of the ingredients in the vaccine.
  • See Special Considerations Regarding Egg Allergy for more information about egg allergies and flu vaccine.
• Have had Guillain-Barré syndrome (GBS).
• Are not feeling well.

There are multiple flu vaccines available, and not all flu vaccines can be given to people of all ages. Talk to your doctor if you have any questions regarding which flu vaccine options are best for you and your family. See Vaccination: Who Should Do It, Who Should Not and Who Should Take Precautions) for more information.

Should pregnant women receive a flu vaccine?

Yes, pregnant women should get a flu shot to protect themselves and their developing babies. To learn more about flu vaccine safety during pregnancy, visit Flu Vaccine Safety and Pregnancy.

How is the safety of flu vaccines monitored?

CDC and the Food and Drug Administration (FDA) closely monitor the safety of vaccines approved for use in the United States. CDC uses two primary systems to monitor the safety of flu vaccines:

1. Vaccine Adverse Event Reporting System (VAERS): an early warning system that helps CDC and FDA monitor problems following vaccination. Anyone can report possible vaccine side effects to VAERS. Generally, VAERS reports cannot determine if an adverse event was caused by a vaccine, but these reports can help determine if further investigations are needed.
2. Vaccine Safety Datalink (VSD): A collaboration between CDC and nine health care organizations which allows ongoing monitoring and proactive searches of vaccine-related data.

People with egg allergies can receive any licensed, recommended age-appropriate influenza (flu) vaccine (IIV4, RIV4, ccIIV4, or LAIV4) that is otherwise appropriate. People who have a history of severe egg allergy (those who have had any symptom other than hives after exposure to egg) should be vaccinated in a medical setting, supervised by a health care provider who is able to recognize and manage severe allergic reactions. Two completely egg-free flu vaccine options are available: Flublok Quadrivalent recombinant flu vaccine and Flucelvax Quadrivalent cell-based flu shot.

What are febrile seizures?

The word “febrile” refers to having a fever. A seizure is a convulsion or fit of uncontrolled body movements. A “febrile seizure” refers to a seizure/convulsion associated with a fever in a child. Febrile seizures usually last around one or two minutes and can occur with any illness that causes fever, such as colds, influenza, or ear infection. 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. A person experiencing a febrile seizure may lose consciousness.

Who is likely to have febrile seizures?

Most febrile seizures happen in children between the ages of 6 months and 5 years. Up to 5% of young children will have at least one febrile seizure. The most common age range for children to have febrile seizures is 14–18 months.

About 1 in 3 children who have one febrile seizure will have more febrile seizures during childhood. If a member of a child’s immediate family (a brother, sister, or parent) has had febrile seizures, that child is more likely to have a febrile seizure.

How serious are febrile seizures?

Febrile seizures can be frightening, but nearly all children who have a febrile seizure recover quickly, are healthy afterwards, and do not have any permanent neurological damage. Febrile seizures do not make children more likely to develop epilepsy or any other seizure disorder.

Can flu vaccines cause febrile seizures in children?

Vaccines can cause fevers, but febrile seizures are rare after vaccination. Importantly, getting sick with the flu also can cause febrile seizures.

Flu illness can cause high fever and febrile seizures in children. Flu vaccine can protect children against flu illness and its complications. Flu vaccine cannot cause the flu because of the way that it’s made. See Misconceptions about Seasonal Flu and Flu Vaccines for more information.

Several studies of children in the United States have been conducted to see if there is an increased risk for febrile seizures following flu vaccination.

• Flu vaccine was not found to be associated with febrile seizures in one study that looked at 45,000 children aged 6 months through 23 months of age who received a flu vaccine from 1991 through 2003 (Hambidge et al, 2006).
• Seasonal flu vaccine and the 2009 H1N1 flu vaccine were not found to be associated with febrile seizures in children during the 2009-10 flu season (Lee et al, 2011).
• Some studies have detected a small increased risk of febrile seizures in young children following the flu shot in some flu seasons. In these studies, the risk of febrile seizures was increased for children 12 through 23 months of age, particularly when the flu shot was given at the same time as pneumococcal conjugate vaccine (PCV13) (Tse et al, 2012) and diphtheria, tetanus, and pertussis (DTaP)-containing vaccine (Duffy et al, 2016).

The CDC carefully reviewed the data on febrile seizures and considered the benefits of vaccinating children against these illnesses, and decided that no changes in the childhood immunization recommendations should be made.

What is considered an egg allergy? What are the signs and symptoms of an egg allergic reaction?   

Persons who are able to eat lightly cooked egg (e.g., scrambled egg) without a reaction are unlikely to be allergic. Egg-allergic persons might tolerate egg in baked products (e.g., bread or cake). Tolerance to egg-containing foods does not exclude the possibility of egg allergy. Egg allergy can be confirmed by a consistent medical history of adverse reactions to eggs and egg-containing foods, plus skin or blood testing for immunoglobulin E directed against egg proteins.

How common is egg allergy in children and adults?

Egg allergy affects about 1.3 % of all children and 0.2 % of all adults.

What vaccine should I get if I am egg allergic, but I can eat lightly cooked eggs?

If you are able to eat lightly cooked egg (e.g., scrambled egg) without reaction, you are unlikely to have an egg allergy and can get any licensed flu vaccine (i.e., any form of IIV, LAIV, or RIV) that is otherwise appropriate for your age and health status.

What flu vaccine should I get if I get hives after eating egg-containing foods?

If you are someone with a history of egg allergy, who has experienced only hives after exposure to egg, you can get any licensed flu vaccine (i.e., any form of IIV, LAIV, or RIV) that is otherwise appropriate for your age and health.

What kind of flu vaccine should I get if I have more serious reactions to eating eggs or egg-containing foods like cardiovascular changes or a reaction requiring epinephrine?

If you are someone who has more serious reactions to eating eggs or egg-containing foods, like angioedema, respiratory distress, lightheadedness, or recurrent emesis; or who required epinephrine or another emergency medical intervention, you can get any licensed flu vaccine (i.e., any form of IIV, LAIV, or RIV) that is otherwise appropriate for your age and health status, but the vaccine should be given in an inpatient or outpatient medical setting (including but not necessarily limited to hospitals, clinics, health departments, and physician offices), under the supervision of a health care provider who is able to recognize and manage severe allergic conditions.

Are there still people with egg allergies who should not get flu vaccine?

People with egg allergy can receive flu vaccines according to the recommendations above. A person who has previously experienced a severe allergic reaction to flu vaccine, regardless of the component suspected of being responsible for the reaction, should not get a flu vaccine again.

Why do flu vaccines contain egg protein?

Most flu vaccines today are produced using an egg-based manufacturing process and thus contain a small amount of egg protein called ovalbumin.

For the 2019-2020 flu season, there are two vaccines licensed for use that are manufactured without the use of eggs and are considered egg-free:

• Flublok Quadrivalent (licensed for use in adults 18 years and older)
• Flucelvax Quadrivalent (licensed for use in people 4 years and older)

How much egg protein is in flu vaccine?

While not all manufacturers disclose the amount of ovalbumin in their vaccines, those that did from 2011–12 through 2014–15 reported maximum amounts of ≤1 µg/0.5 mL dose for flu shots and 0.24 µg/0.2 mL dose for the nasal spray vaccine. Recombinant vaccine (Flublok Quadrivalent) and cell-based vaccine (Flucelvax Quadrivalent) are the only vaccines currently available that are completely egg free.

Can egg protein in flu vaccine cause allergic reactions in persons with a history of egg allergy?

Yes, allergic reactions can happen, but they occur very rarely with the flu vaccines available in the United States today. Occasional cases of anaphylaxis, a severe life-threatening reaction that involves multiple organ systems and can progress rapidly, in egg-allergic persons have been reported to the Vaccine Adverse Event Reporting System (VAERS) after administration of flu vaccine. Flu vaccines contain various components that may cause allergic reactions, including anaphylaxis. In a Vaccine Safety Datalink study, there were 10 cases of anaphylaxis after more than 7.4 million doses of inactivated flu vaccine, trivalent (IIV3) given without other vaccines, (rate of 1.35 per one million doses). Most of these cases of anaphylaxis were not related to the egg protein present in the vaccine. CDC and the Advisory Committee on Immunization Practices continue to review available data regarding anaphylaxis cases following flu vaccines.

How long after flu vaccination does a reaction occur in persons with a history of egg allergy?

Allergic reactions can begin very soon after vaccination. However, the onset of symptoms is sometimes delayed. In a Vaccine Safety Datalink study of more than 25.1 million doses of vaccines of various types given to children and adults over 3 years, only 33 people had anaphylaxis. Of patients with a documented time to onset of symptoms, eight cases had onset within 30 minutes of vaccination, while in another 21 cases, symptoms were delayed more than 30 minutes following vaccination, including one case with symptom onset on the following day.

What is Guillain-Barré syndrome (GBS)?

Guillain-Barré syndrome (GBS) is a rare disorder in which a person’s own immune system damages their nerve cells, causing muscle weakness and sometimes paralysis. GBS can cause symptoms that usually last for a few weeks. Most people recover fully from GBS, but some people have long-term nerve damage. In very rare cases, people have died of GBS, usually from difficulty breathing. In the United States, an estimated 3,000 to 6,000 people develop GBS each year.

What causes GBS?

The exact cause of GBS is unknown, but about two-thirds of people who develop GBS experience symptoms several days or weeks after they have been sick with diarrhea or a respiratory illness. Infection with the bacterium Campylobacter jejuni 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, they may develop GBS in the days or weeks after getting a vaccination.

Who is at risk for developing GBS?

Anyone can develop GBS; however, it is more common among older adults. The incidence of GBS increases with age, and people older than 50 years are at greatest risk for developing GBS.

How common is GBS? How common is it among people who have been vaccinated against flu?

The background rate for GBS in the Unites States is about 80 to 160 cases of GBS each week, regardless of vaccination. The data on the association between GBS and seasonal flu vaccination are variable and inconsistent across flu seasons. If there is an increased risk of GBS following flu vaccination it is small, on the order of one to two additional GBS cases per million doses of flu vaccine administered.

What is thimerosal?

Thimerosal is an ethyl mercury-based preservative used in vials that contain more than one dose of a vaccine (multi-dose vials) to prevent germs, bacteria and/or fungi from contaminating the vaccine.

Do flu vaccines contain thimerosal?

Flu vaccines in multi-dose vials contain thimerosal to safeguard against contamination of the vial. Most single-dose vials and pre-filled syringes of flu shot and the nasal spray flu vaccine do not contain a preservative because they are intended to be used once.

Why are preservatives sometimes used in vaccines?

Preservatives are used to protect vaccines packaged in multi-dose vials. Each time a vaccine dose is drawn from a multi-dose vial, bacteria or fungi can enter the vial. Receiving a vaccine contaminated with bacteria or fungi can be dangerous. Preservatives are needed to prevent contamination of multi-dose vials each time individual doses are drawn.

Thimerosal use in vaccines and other 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.

Influenza (flu) viruses typically spread in fall and winter, with activity peaking between December and February. Getting vaccinated in the fall can lower your chances of getting the flu.

Flu is a serious disease, caused by influenza viruses, that can lead to hospitalization and even death. Every flu season is different, and the substantial health impacts can vary widely from season to season, with some flu seasons being worse than others. Your best defense is vaccination, which provides protection from flu and its potential complications.

Flu vaccines are approved by the U.S. Food and Drug Administration to prevent influenza, and they provide important benefits. The Centers for Disease Control and Prevention estimate that during the 2021-22 flu season there were:

• 8 million to 13 million flu illnesses.
• 3.7 million to 6.1 million flu-related medical visits.
• 82,000 to 170,000 flu-related hospitalizations.
• 5,000 to 14,000 flu-related deaths.

Flu vaccination prevents millions of flu-related illnesses and visits to a health care professional. For example, according to the CDC, during the 2019-2020 influenza season, flu vaccination prevented an estimated 7.5 million influenza illnesses, 3.7 million influenza-associated medical visits, 105,000 influenza-associated hospitalizations, and 6,300 influenza-associated deaths.

COVID-19 and Flu

It’s likely that flu viruses and the virus that causes COVID-19 will continue to circulate together in fall and winter. The flu vaccine does not prevent COVID-19. The FDA has approved two vaccines for the prevention of COVID-19 and issued emergency use authorizations (EUAs) for other vaccines for the prevention of COVID-19.

Flu vaccines are approved by the FDA for the prevention of influenza disease and to protect against four different virus strains of influenza. Getting vaccinated to prevent this disease can help keep you out of the doctor’s office for a sick visit and preserve health care resources for patients with other diseases and medical conditions. The FDA plays a key role in ensuring that safe and effective flu vaccines are available every flu season.

Flu viruses are spread by droplets when people infected with the flu cough, sneeze, or talk. Flu may also be spread when a person touches a surface or object that has flu viruses on it and then touches their own mouth, nose, or eyes.

The CDC recommends that children older than 6 months and adults get a flu vaccine by the end of October. Even if you wait until after October, go ahead and get vaccinated. Immunization will still be beneficial and provide protection.

If you have already been sick with the flu this season without getting vaccinated, getting a flu vaccine is still important because it helps prevent disease caused by four different strains of flu viruses. Presumably, you were infected with one type of flu virus strain, so the vaccine would offer protection against the strains that you haven’t already had.

A Flu Vaccine Is the Best Prevention

Flu seasons and their severity are unpredictable. Annual vaccination is the best way to prevent influenza in people ages 6 months and older.

An annual vaccination to prevent flu is the best way to reduce the risk of getting the flu and spreading it to others. When more people get vaccinated, it is less likely that the flu viruses will spread through a community.

The vaccine typically changes each year and contains the four flu virus strains that are expected to circulate and cause illness in the U.S. during the upcoming flu season. The effectiveness of influenza vaccines varies depending on several factors, such as the age and health of the recipient, the types of circulating influenza viruses, and the degree of similarity between circulating viruses and those included in the vaccine.

The task of producing a new vaccine for the next flu season starts well before the current season ends.

The flu vaccine will trigger your immune system to produce antibodies to protect against influenza disease — it will not make you sick with the flu. It can take about two weeks after vaccination for antibodies to develop in the body, which is an important reason to get your flu vaccine early, before flu activity starts.

Why We Need New Flu Vaccines Every Year

There are several reasons a new flu vaccine must be made each year.

Flu viruses can change from year to year, so the vaccine is updated to protect against the influenza virus strains that are expected to circulate in the U.S. In addition, the protection provided by the flu vaccine a person received in the previous year will diminish over time and may be too low to prevent influenza disease into next year’s flu season.

Who’s Most at Risk of Getting Sick with Flu

Typically, children and older people are most at risk of getting sick with influenza. The best way to protect babies who are too young to be vaccinated is to make sure the people around them are vaccinated. Occasionally, a flu virus will circulate that disproportionately affects young and middle-age adults.

You also can reduce the spread of the flu and its effects by taking such practical measures as washing your hands, covering coughs and sneezes, and staying home when you’re sick.

The FDA has approved numerous vaccines for the prevention of influenza. But if you do get the flu, there are FDA-approved antiviral drugs, available by prescription, to treat it. There are several FDA-approved antiviral drugs recommended by the CDC for use against circulating influenza viruses. These drugs work best if started soon after the onset of symptoms (within 48 hours).

How Flu Vaccine Virus Strains Are Selected

Every year, in late February or early March, before that year’s flu season ends, the FDA, the World Health Organization (WHO), the CDC, and other public health experts collaborate on collecting and reviewing data from around the world to identify the flu viruses likely to cause the most illnesses during the next flu season.

Following that process, the FDA convenes its vaccines advisory committee, consisting of outside experts, to discuss the WHO recommendations and to consider which flu viruses are expected to circulate in the U.S. The committee reviews data about which flu viruses have caused illnesses in the past year, how the viruses are changing, and disease trends for the U.S. The FDA takes that information into account before it selects the virus strains for FDA-licensed manufacturers to include in their vaccines for use in the U.S.

The closer the match between the virus strains chosen for the vaccine and the circulating strains causing disease during flu season, the better the protection that the flu vaccine provides. Although the vaccine and viruses may not be an exact match in some years, that does not mean the vaccine is not benefiting people. Available data show that the vaccine can reduce the severity of illness in people who get vaccinated but still get sick.

The Flu Vaccine Is Safe

To ensure that the flu vaccine is safe, effective, and of high quality, the FDA prepares and provides reagents (materials to standardize vaccines) to manufacturers that they need to make their vaccine and to verify its identity and potency. The FDA also inspects manufacturing facilities regularly and evaluates each manufacturer’s vaccine annually before it can be approved for use for the upcoming flu season.

The FDA’s oversight doesn’t end there. After manufacturers have distributed their vaccines for use by the public, the FDA and CDC work together to routinely evaluate reports of adverse events following vaccination submitted by vaccine manufacturers, health care professionals and vaccine recipients (or their parents/guardians) to the Vaccine Adverse Event Reporting System (VAERS).

Additional systems are in place to monitor the safety of vaccines. The FDA partners with private organizations that collect health care data (such as health insurance companies) and other federal agencies (such as Centers for Medicare & Medicaid Services, Veterans Health Administration and CDC) to further evaluate the safety and effectiveness of the influenza vaccines and other vaccines that the FDA has approved or authorized for emergency use.

The Biologics Effectiveness and Safety (BEST) Initiative is one of the programs the FDA utilizes to assess vaccine safety and effectiveness in real-world conditions, reflecting patient care and the real-world use of the influenza vaccine and other vaccines in the U.S. In addition, the CDC maintains the Vaccine Safety Datalink (VSD) program, which evaluates the safety of vaccines similar to the BEST Initiative. VSD receives its data from nine integrated health care organizations in the U.S.