In 2000, measles had all but stopped spreading in the U.S. That's because vaccination rates were high enough to keep people from getting the disease.

When enough of the community is immunized against a contagious disease, most other members are protected from infection because there's little opportunity for the disease to spread.

But the virus spreads so easily that, when the first new measles case since 2000 was reported in 2005, it was linked to one unvaccinated U.S. resident. He was infected during a visit to Europe, which has lower vaccination rates.

The returning traveler infected American children who hadn't been vaccinated.

Luckily, a major outbreak didn't happen at that time. That's because enough people in surrounding communities were vaccinated against measles.

This type of protection is known as "community immunity" or "herd immunity."

When enough people in a community are protected against a contagious disease, it's hard for it to spread.

Newborns, pregnant women, or people with weak immune systems aren't able to get certain vaccines. Yet, with community immunity, even they will get some protection because the spread of a contagious disease is contained.

NIH MedlinePlus magazine talked to Anthony Fauci, M.D., director of NIH's National Institute of Allergy and Infectious Diseases. Dr. Fauci explained how vaccines work to protect us.

What is a vaccine?

Most vaccines are made from either a killed (inactivated) or live attenuated virus. The measles mumps rubella (MMR) vaccine is a live attenuated virus vaccine. This means the three viruses in the vaccine are not completely killed, but they are no longer able to cause the disease. They can alert your immune system to develop a response that can fight off these three diseases.

How is a vaccine developed?

After working on a potential vaccine in the lab, researchers start with a small Phase 1 clinical trial in healthy people to determine if the candidate vaccine is safe. If it proves to be safe, they then can move it into a larger Phase 2 clinical trial, which tests safety in more people and determines whether the candidate vaccine induces an immune response that likely will protect people from the virus, and helps us learn what the proper vaccine dosage is. After that, the potential vaccine can be moved into a large Phase 3 clinical trial, which tests safety in more people and determines whether the candidate vaccine can protect people against the disease.

Why do we wait to give the MMR vaccine to babies?

You generally do not want to vaccinate children before 1 year of age because their immune systems are not fully developed. However, if children between 6 and 11 months old are going to be traveling abroad or to an area in the United States with a measles outbreak, they should talk to their pediatrician about getting a dose of the vaccine before they travel.

You usually want to start somewhere between 12 and 15 months for the first injection. Even then, not all babies respond, so you want to boost it with a second injection when they are between 4 and 6 years old. This will make sure they are fully protected against these three viruses.

MMR is one of the most effective vaccines available. It is about 97% effective when you give two doses.

What advice do you have about vaccine safety?

My advice is that if you look at the science, as opposed to rumors or misinformation, it shows that the vaccines that are used now are very safe and highly effective, particularly the MMR vaccine.

Because measles is one of the most contagious viruses around and can sometimes have very bad outcomes, my message for parents is to get their children vaccinated against it and other vaccine-preventable diseases.

Immunity to a disease is achieved through the presence of antibodies to that disease in a person’s system. Antibodies are proteins produced by the body to neutralize or destroy toxins or disease-carrying organisms. Antibodies are disease-specific. For example, measles antibody will protect a person who is exposed to measles disease, but will have no effect if he or she is exposed to mumps.

There are two types of immunity: active and passive.

Active Immunity

Active immunity results when exposure to a disease organism triggers the immune system to produce antibodies to that disease. Exposure to the disease organism can occur through infection with the actual disease (resulting in natural immunity), or introduction of a killed or weakened form of the disease organism through vaccination (vaccine-induced immunity). Either way, if an immune person comes into contact with that disease in the future, their immune system will recognize it and immediately produce the antibodies needed to fight it.

Active immunity is long-lasting, and sometimes life-long.

Passive Immunity

Passive immunity is provided when a person is given antibodies to a disease rather than producing them through his or her own immune system.

A newborn baby acquires passive immunity from its mother through the placenta. A person can also get passive immunity through antibody-containing blood products such as immune globulin, which may be given when immediate protection from a specific disease is needed. This is the major advantage to passive immunity; protection is immediate, whereas active immunity takes time (usually several weeks) to develop.

However, passive immunity lasts only for a few weeks or months. Only active immunity is long-lasting.

Vaccines do an incredible job of protecting you from serious diseases like whooping cough and measles.

Have you ever wondered how vaccines actually work? Vaccines help your immune system do its job better and faster. And that protects you from serious diseases.

What is the immune system?

The immune system is a network of cells, tissues, and organs that work together to defend the body from harmful germs. When bacteria, viruses, and other germs invade your body, they multiply and attack. This invasion is called an infection. Infections cause the diseases that make you sick.

Your immune system protects you from the disease by fighting off the invading germs.

How does the immune system work?

Your immune system is always on patrol in your body. When it comes across an invading germ, it attacks that germ. This is called an immune response.

Here’s how an immune response works:

• Your immune system sounds the alarm so your body knows there’s an infection.
• It begins releasing antibodies to fight the germ — think of antibodies as soldiers designed to fight off the specific germ you have. This process can take a few days.
• The antibodies work to attack, weaken, and destroy the germ.
• Afterwards, your immune system remembers the germ. If the germ invades again, your body can recognize it and quickly send out the right antibodies so you don’t get sick!
• This protection against a certain disease is called immunity. In many cases, immunity lasts your whole life.

How do vaccines work?

Vaccines help your immune system fight infections faster and more effectively. When you get a vaccine, it sparks your immune response, helping your body fight off and remember the germ so it can attack it if the germ ever invades again. And since vaccines are made of very small amounts of weak or dead germs, they won’t make you sick.

Vaccines often provide long-lasting immunity to serious diseases without the risk of serious illness.

Which is safer: Natural immunity or immunity after getting a vaccine?

Vaccines are much safer. Natural immunity happens after you get sick with a disease. But diseases can be serious — and even deadly. A vaccine protects you from a disease before it makes you sick.

Vaccines don’t just protect you — they also protect the people around you

Did you know that some people — like infants and people with weak or failing immune systems (like people with HIV/AIDS or cancer) — may not be able to get many of the vaccines that protect us from serious diseases?

The good news is that when you get vaccinated, you’re also protecting the unvaccinated people around you. This is called community immunity. Learn more about community immunity.

Get vaccinated

Getting vaccinated is easy. Vaccines are available at the doctor’s office and many pharmacies — and most are covered by insurance.

Did you know that when you get vaccinated, you’re protecting yourself and your community?

This concept is called community immunity , or herd immunity. And it’s an important reason for you and your family to get vaccinated — so you can help keep yourselves and your community healthy.

How does community immunity work?

Germs can travel quickly through a community and make a lot of people sick. If enough people get sick, it can lead to an outbreak. But when enough people are vaccinated against a certain disease, the germs can’t travel as easily from person to person — and the entire community is less likely to get the disease.

That means even people who can’t get vaccinated will have some protection from getting sick. And if a person does get sick, there’s less chance of an outbreak because it’s harder for the disease to spread. Eventually, the disease becomes rare — and sometimes, it’s wiped out altogether.

Who does community immunity protect?

Community immunity protects everyone. But it’s especially important because some people can’t get vaccinated for certain diseases — such as people with some serious allergies and those with weakened or failing immune systems (like people who have cancer, HIV/AIDS, type 1 diabetes, or other health conditions).

Community immunity is also important for the very small group of people who don’t have a strong immune response from vaccines.

If vaccines have wiped out some diseases in the United States, can we stop getting vaccinated for them?

No. Many vaccine-preventable diseases that we don’t see much in the United States still make people sick in other countries. So it’s possible for travelers to bring these diseases back to the United States, where they could then spread. If we stop getting vaccinated, we won’t be protected from these diseases — community immunity protects us only if enough people continue to get vaccinated.

If you’re traveling outside of the United States, you may need to get vaccines to keep you healthy and safe.

Community immunity at work: Pneumococcal vaccines

Pneumococcal disease can cause serious infections of the ears, lungs, blood, and brain. Although it’s common in young children, older adults are most at risk for serious pneumococcal infections.

Since the pneumococcal vaccine was approved for use in children, the number of older adults hospitalized for pneumococcal disease has gone way down. This tells us that vaccinating infants protected older adults from the spread of serious pneumococcal infections before a vaccine for older adults was available.

Get vaccinated

Getting vaccinated is easy. Vaccines are available at the doctor’s office and many pharmacies — and most are covered by insurance.

Parents know that kids are vulnerable to a host of infectious diseases. Research supported by NIH and others proves that the benefits of vaccines in preventing illness and death greatly outweigh the risks.

The list of childhood diseases can be overwhelming: measles, mumps, rubella, diphtheria, pertussis, polio, meningitis, influenza and rotavirus. In the era before vaccines, many children in the U.S. died or became disabled from these diseases. Many still do in countries and regions with lower vaccination rates.

With all the international travel in the world these days, it’s important to keep vaccines, or immunizations, up to date. Here’s just one example of what might happen if you don’t. By 2000, immunization had practically wiped out measles in the U.S. But a measles outbreak in 2005 was traced to one unvaccinated U.S. resident infected during a visit to Europe. The returning traveler infected American children who hadn’t been vaccinated because of safety concerns—despite study after study showing that childhood vaccines are safe and effective.

A major epidemic didn’t emerge that time. That’s because enough people in the surrounding communities had already been vaccinated against measles.

“The important concept,” says Dr. Marc Lipsitch of the Harvard School of Public Health, “is that vaccinating people protects not only them, but others in the community. If I’m protected, I can protect others.”

This type of protection is known as “community immunity” or “herd immunity.” When enough of the community is immunized against a contagious disease, most other members are protected from infection because there’s little opportunity for the disease to spread.

Newborns, pregnant women or people whose immune systems are weakened may not be eligible for certain vaccines. Yet even they will get some protection because the spread of contagious disease is contained.

“Epidemiologists think of infections as chain reactions, whose speed depends on contagiousness,” says Lipsitch. “The more contagious the disease, the more vaccination is required. The data tells us that herd immunity works.”

Using mathematical formulas and computer programs, NIH-funded scientists like Lipsitch have developed models to determine what proportion of the population has to be vaccinated to eliminate the spread of disease. As one example, a worldwide vaccination campaign completely eliminated, or eradicated, smallpox in the 1970s. So many people were immunized that the virus couldn’t sustain itself.

More recently, infant vaccination against Haemophilus influenzae type b (Hib, which can cause meningitis) lowered the risk of disease in the whole population. Before the vaccine, Hib struck about 1 in 200 children younger than age 5. It killed many and often left survivors with permanent brain damage. After the Hib vaccine was introduced in the mid-1980s, the incidence of Hib dropped by 99%.

“Infectious disease eradication is possible,” says Lipsitch. Even when a disease—such as measles or Hib— hasn’t been completely wiped out, immunizations can reduce disease transmission, so that epidemics become less frequent.

When parents choose to immunize, they’re helping more than their own. Make sure your child’s immunizations are up to date. And talk with your child’s doctor if you have any concerns about vaccine safety.

Why Immunize?

• Diseases are becoming rare thanks to vaccinations. Polio, diphtheria, and other diseases are seldom seen in the U.S. because of immunizations.
• If we stop vaccinating, diseases that are almost unknown would return. Even if there are only a few cases of a disease today, taking away vaccines would lead to more infected people spreading the disease to others.
• Vaccinate to protect our future. Immunization protects not only our children but also our grandchildren and their grandchildren.

Adapted from the U.S. Centers for Disease Control and Prevention

The measles mumps rubella (MMR) vaccine is one of the most effective around. It is 97% effective against the viruses when two doses are given.

But Diane Griffin, M.D., Ph.D., a researcher at Johns Hopkins University's Center for Global Health, is working to improve that already high rate for this and other vaccines.

She's doing that by studying the measles virus, with help from NIH's National Institute of Allergy and Infectious Diseases.

Lifelong immunity

Dr. Griffin and her team want to understand how people who've had measles are protected from the virus for the rest of their lives.

"If you get measles when you're 6, you're still immune to it at 85," Dr. Griffin says. "That is something that a lot of vaccines would like to accomplish."

They're looking at the vaccine and the actual virus side by side to better understand this.

Dr. Griffin cautions that while immunity can lessen over time in vaccines like MMR, it's still high enough to protect most people for life. Her research wants to close that already narrow gap.

All in the antibodies?

As part of their research, Dr. Griffin's team is studying antibody production.

Antibodies are proteins in our blood that fight off infection and disease. More antibody production against a virus like measles means better immunity or protection.

By researching antibody production against the measles virus, Dr. Griffin and her team want to see if they can identify which genes help make the antibodies better.

Once they identify those genes, they can work to improve antibody production by the vaccine to provide similar lifelong immunity.

Reducing complications

One of the main reasons behind Dr. Griffin's work to improve vaccines is to stop potential complications that come with diseases like measles.

"The natural disease is a nasty one," Dr. Griffin notes.

Most people with measles are sick for about a week. But it actually takes months or years for peoples' bodies to clear the virus and completely recover.

While that helps give them strong immunity from getting the virus again, it can also be dangerous. Some patients can have long-term symptoms or complications, leading to serious health issues.

"Other infections like bacterial pneumonia are quite common after measles," Dr. Griffin adds.

"This is why access to medical care makes such a big difference," Dr. Griffin says. "Most people who die of measles die of related infections, usually a month or two after having measles."

A particularly serious complication is postinfectious encephalomyelitis, which is rare, but can cause seizures and brain damage that lasts for years after a case of the measles.

As Dr. Griffin works to improve future vaccine immunity, she says that current vaccines are essential for staying healthy and keeping others healthy.

"We have to keep the rate of immunity greater than 90% in order to prevent outbreaks and protect those who can't get vaccinated," she adds.