How Vaccines Actually Work

Understanding the powerful role vaccines play in disease prevention

Michelle Crouch,

AARP

Medical review by

Merle Myerson, M.D.

Comments

Published September 15, 2025

AARP (Getty Images)

About Vaccines

What is a vaccine?
How do vaccines work?
What happens when you get a vaccine?
How does your body fight back?
Why can you feel side effects?
Why do some vaccines last longer than others?
What is herd immunity?

There’s a lot of noise out there about vaccines. News headlines, social media posts and even conversations with friends can make it difficult to know what’s true and what isn’t.

Maybe you’ve heard that vaccines can “overwhelm” the immune system. Or that vaccines “don’t work” because some people still get sick after a shot. With so much misinformation, it’s no wonder people feel uncertain.

The science, though, is clear. Vaccines are safe, effective and the most powerful tool we have to protect ourselves from getting sick, says Dr. Robert Hopkins, medical director of the National Foundation for Infectious Diseases and a professor of internal medicine and pediatrics at the University of Arkansas for Medical Sciences.

They are especially crucial for older adults, who are at higher risk from illnesses like influenza, COVID-19, RSV and pneumonia.

Vaccines have saved millions of lives — and continue to do so every year.

“When there’s a huge volume of information out there that’s not correct, it’s hard oftentimes for people to filter through that and find the truth,” Hopkins says. “We are in a particularly challenging situation because there are news pundits … that are putting out information that is blatantly false, that is scaring people. It’s causing people not to get vaccinated and will cost lives.”

Here, public health experts share the basics about what happens in your body when you get a vaccine to help clear up confusion and hopefully offer some peace of mind.

What is a vaccine?

A vaccine is a medical treatment that helps your body defend itself against disease, says Dr. William Schaffner, a professor of preventive medicine and infectious diseases at the Vanderbilt University School of Medicine in Nashville, Tennessee.

All vaccines work basically the same way: They introduce a harmless component into your body that “trains” your immune system to recognize and respond to potential invaders such as viruses, bacteria or other pathogens.

“Because it’s trained, it’s much more effective at fighting off that infection after you get exposed,” Schaffner says.

Some vaccines prevent infections entirely. Others reduce your risk of having severe disease that can lead to complications, hospitalization or death. Some also reduce your risk of transmitting the disease to others.

How do vaccines work?

Every infection has a telltale feature, called an antigen, that the body can spot as foreign, says Dr. Paul Thottingal, an infectious disease specialist and senior medical director for communicable diseases at Kaiser Permanente in Seattle.

An antigen is the identifier or “fingerprint” of a germ, he explains.

Vaccines introduce an ingredient that mimics that fingerprint, giving your immune system a chance to learn how to fight the germ. Different types of vaccines use different methods to do this.

What happens in your body after you get a vaccine?

Once you get a vaccine, your body gets its first look at the ingredient that’s designed to mimic the germ. Special cells called antigen-presenting cells live in tissues throughout your body, acting as lookouts for your immune system.

“When one of those cells sees something that doesn’t look right, they suck it up,” Hopkins says.

The cells then display a piece of what they’ve captured on their surface and carry it to other immune cells throughout the body, showing them what the invader looks like.

That’s when your immune system really kicks into gear.

How does your body fight back?

Almost immediately, your body starts producing antibodies, specialized proteins that recognize a germ. The antibodies are highly specific, Thottingal says, “matching” and binding to the antigen.

Once attached, the antibodies basically “change the lock,” making it harder for the germ to infect your cells, Thottingal says. They also mark the intruders so that killer T cells, another component of your immune system, can attack and destroy them.

Antibody levels typically peak within two to four weeks after vaccination, giving you strong, immediate protection.

Over time, antibody levels naturally decline, Schaffner says, but your immunity doesn’t disappear. Some of your cells turn into memory cells that “remember” the antigen and can quickly dispatch new antibodies if you’re exposed again.

“Down the road, if you encounter a germ or you get a booster, it no longer takes 10 days to do things because everything is primed and trained,” Schaffner says.

Why you may feel side effects

The training process is what causes common vaccine side effects such as a sore arm, fatigue, mild fever and chills, though it’s important to note that not everyone experiences side effects after vaccination.

“If I feel that way after the vaccine, I know I’ve gotten something. I know my immune system is responding,” Hopkins says.

The temporary inflammation your body creates is part of building protection. These effects usually last only a day or two and are far milder than what you’d experience with the actual infection.

The Centers for Disease Control and Prevention (CDC) says serious adverse reactions can happen after vaccination, but are rare. If you have concerns about side effects or adverse reactions, talk to your doctor, who can help explain the risks vs. benefits of vaccines.

Why do some vaccines last longer than others?

Some vaccines protect you for a lifetime, while others need a booster or require a new shot every year. The difference comes down to the nature of each virus, Schaffner says.

Measles, for example, is a very stable virus, he says: “Measles is basically the same virus that infects children now around the world as it was 50 years ago.” (AARP: Do you need a measles vaccine?)

That makes it easy for your immune system’s memory cells to recognize it year after year.

Also, measles takes nearly two weeks to make you sick, Schaffner says, giving your body plenty of time to fire up its defenses and shut it down in the bloodstream.

Viruses like flu and COVID-19 are different. They change, or mutate, frequently, and newer variants may not be as susceptible to previous vaccines or immunity.

What’s more, these viruses replicate right on the surface of your nose and throat — places where antibodies don’t work as well. That’s why flu and COVID-19 vaccines may not always prevent the sniffles and coughs of a mild infection, Schaffner says, but they’re very good at keeping you out of the hospital.

What is herd immunity and why is it important?

The more contagious a virus is, the higher the percentage of people who need to be vaccinated to stop its spread, Schaffner says.

Measles, for example, is one of the most contagious viruses, so about 95 percent of people in the community need to be vaccinated to stop its spread.

That matters because some people can’t get vaccines at all or their immune systems don’t respond fully. If everyone else gets vaccinated, it provides “a cocoon of protection” around them, Schaffner explains.

That’s the idea behind herd immunity, he says. “Vaccines protect the individual being vaccinated, but that’s half of what they do. The other half of what vaccines do is that they protect the entire community.”

Cuts in mRNA research

The U.S. Department of Health and Human Services recently announced that it is decreasing funding for the development of mRNA vaccines, citing concerns over safety and efficacy. These vaccines are best known for their role in fighting COVID-19, but are also being studied for the treatment of diseases like cancer and HIV.

Many researchers and public health experts say the cuts are a major scientific setback. Five leading physicians’ groups, including the American Academy of Family Physicians, said in a statement that they are “dismayed and alarmed” by the decision.

“This act stifles scientific innovation and our country’s ability to react swiftly to future pandemics and public health emergencies — putting millions of lives at risk,” the statement said.

“Sustained research funding is essential to developing the next generation of tools that protect Americans from infectious disease. Thanks to decades of rigorous science, testing and monitoring systems, vaccines used in the U.S. continue to be safe, effective and save lives.”

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Michelle Crouch is a contributing writer who has covered health and personal finance for some of the nation’s top consumer publications. Her work has appeared in Reader’s Digest, Real Simple, Prevention, The Washington Post and The New York Times.

Dr. Merle Myerson is a board-certified cardiologist with specialties in sports medicine, lipids, women’s health and prevention of cardiovascular disease.