How do vaccines work? Medical experts explain.

USA Today

06-05-2025

How do vaccines work? Medical experts explain.
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Robert F. Kennedy Jr. posts endorsement of measles MMR vaccine on X
Health and Human Services secretary, and vaccine skeptic, Robert F. Kennedy Jr. supports measles vaccine after visiting West Texas.
Medical experts repeatedly reiterate how vital vaccinations are to global health. As of last spring, the World Health Organization (WHO) estimated that worldwide immunization efforts have saved at least 154 million people's lives over the past 50 years.
But despite vaccines having a "long history of successfully protecting people and communities against infectious diseases," per the Centers for Disease Control and Prevention (CDC), some hesitancy – and sometimes outright opposition – around them remains.
"Since the outbreak of the COVID-19 pandemic, we have seen a shift in the perception of vaccines. Misinformation about health, and especially about vaccines, is prevalent," says Dr. Jennifer Brull, president of the American Academy of Family Physicians (AAFP).
Being vaccinated for a certain illness also decreases a person's chance of contracting that illness. And if they do get it anyway, the CDC says they're "less likely to die or become seriously ill than someone whose immune system is unprepared to fight an infection."
A 2023 survey of local U.S. health departments, conducted by the National Association of County and City Health Officials (NACCHO), found that more than 80% of local departments reported experiencing challenges with patients and/or parents being hesitant toward vaccinations. That's nearly a 40% increase from their 2017 survey results.
"Vaccines are safe, effective and save lives," Brull adds. "They are one of our best tools to prevent severe illness, hospitalizations and death in our communities. ... Being vaccinated against diseases helps build a community of immunity and protects those more vulnerable to disease who may not be able to get vaccinated."
Here's everything medical experts want you to know about how vaccines work.
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What is a vaccine? How do vaccines work?
Vaccines work by imitating an infection to teach the body how to recognize and defend itself from disease without experiencing the dangers of an actual infection, according to the CDC.
"They do so by exposing the body to an active ingredient called an antigen which triggers an immune response," Brull says. Depending on the vaccine, "this immune response can protect you for many years or your entire life."
They exist for all kinds of diseases, including measles and whooping cough, among many others, and have different formulations.
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Are vaccines safe?
Side effects may cause tiredness, fever or a sore arm at the injection site for a day or two, but medical experts want to assure you that it just means the vaccine is working. If you have a more serious reaction, you should call your doctor. But most of the time, reactions are nothing to worry about.
"Instead of a mild reaction deterring you from getting future vaccines, it should encourage you," Brull says. "That's the vaccine working and jumpstarting your body's immune system."
Brull encourages those who have questions or concerns about vaccines to speak with a properly qualified and certified medical expert. They're prepared to have an "honest conversation" and listen to your concerns while providing accurate information and can debunk unfounded claims.
"As trusted messengers in our communities, we can help dispel myths and misconceptions about vaccines," she says. "Your family physician can help answer questions you may have about information you see online or hear from friends and family."