16 Things Doctors Wish You Knew About Vaccines

Vaccines are one of modern medicine’s greatest success stories. According to the World Health Organization (WHO), vaccines saved an estimated 154 million lives over the past 50 years—that’s six lives every minute, every year. 

But this tremendous progress has introduced a new challenge: as a population, we’ve forgotten the threat these diseases once posed. “This dangerous idea has become more common because we’ve eliminated so many diseases with vaccines that people don’t have experience with how traumatic and fatal viruses can be,” says William Schaffner, MD, a professor of preventive medicine at Vanderbilt University School of Medicine and medical director of the National Foundation for Infectious Diseases. 

Public health experts now warn that falling vaccination rates—and confusion over who needs which vaccine and when—could allow rare, preventable diseases to resurge. To help clear up misconceptions and protect community health, here are the key facts leading infectious disease doctors want you to know about vaccines.

1. Good hygiene can’t replace vaccines

Increased medical care, better nutrition, and improved sanitation have helped reduce the spread of infectious diseases. But experts say good hygiene—while important—cannot replace the protection vaccines provide. Research published in 2020 in the journal Frontiers in Microbiology sums it up: “The impact of vaccination on the health of the world’s peoples is hard to exaggerate. With the exception of safe water, no other modality has had such a major effect on mortality reduction and population growth.”

Washing hands and covering coughs help prevent some germs from spreading, but only after exposure. Vaccines, on the other hand, train your immune system in advance so it’s ready to fight off bacteria before they cause serious illness. Plus, some infections are so contagious that hygiene alone is not enough to stop their spread. That’s why even in places with high sanitation standards, diseases like measles can still spread quickly among unvaccinated people.

2. “Rare” diseases still matter

It’s easy to think that once a disease—such as diphtheria, for example—becomes rare in the U.S., the vaccine for it might no longer be necessary. “This is a dangerous belief,” Dr. Schaffner says. Rarity is actually the result of vaccination success—not a reason to stop. 

Measles was almost eradicated in the Western Hemisphere because nearly everyone was vaccinated, Dr. Shaffner explains. But when vaccination rates drop, once-rare infections can return—as seen in recent measles and whooping cough outbreaks in the U.S.

Global travel makes this risk even greater. “It just takes one unimmunized traveler to bring a disease home from another country,” says Eileen Yamada, MD, MPH, a public health medical officer with the California Department of Public Health Immunization Branch. “If immunization levels drop, the rare cases we have in America could very quickly multiply.”

3. Healthy people need vaccines, too

If you’re in relatively good health, you might feel like your immune system has things under control—but vaccines help keep it that way. Remember: vaccines train your immune system to deal with potentially dangerous germs without having to experience serious illness, explains Dr. Yamada. 

Still, some people assume that if they’re healthy, gaining “natural immunity” by getting a virus is better than vaccination. “That’s a disturbing notion,” says Dr. Schaffner. Vaccines give your body the same defensive knowledge—without the potentially dangerous experience of getting sick first. 

For instance, HPV can cause six types of cancer, and the U.S. Centers for Disease Control and Prevention (CDC) says that there’s no way to know who will develop serious health problems from the virus. In addition, 2022 research published in Advances in Therapy shows that being healthy doesn’t eliminate your disease risk. It found that most young adults hospitalized for meningitis had no identifiable risk factors, and up to 40% of survivors had long-term effects. Even with the flu, 2025 CDC data show that 11% of hospitalized patients had no underlying conditions—highlighting that healthy people can still face serious complications.

4. Herd immunity protects everyone

“We know that if we have a population that is vaccinated, then it reduces the ability of the virus to spread,” says Kathryn M. Edwards, MD, a retired professor of pediatrics at Vanderbilt University and former principal investigator of the CDC-funded Clinical Immunization Safety Assessment Network. “If everyone around us vaccinated, we can really cut down remarkably the spread of the infection.” That’s called herd immunity—and measles is a prime example.  

Because measles is so contagious, high vaccination rates—around 94%—are needed to prevent transmission, according to 2024 research published in Clinical Infectious Diseases. Even a small drop in vaccinations can trigger outbreaks. In 2025, measles vaccination coverage fell to 92.4%, according to the Johns Hopkins Bloomberg School of Public Health, and the U.S. has reported the most measles cases since the vaccine was first introduced. 

Plus, some people, such as cancer patients and newborns, can’t get vaccines, so getting yourself immunized protects not only you but others in your community, says Jennifer Lighter Fisher, MD, an infectious disease physician and former epidemiologist at NYU Langone Medical Center.

5. Adult vaccines offer important protection

Adults need to keep up-to-date on their vaccines, too. Research from Vaccines published in 2025 notes that immunity from childhood shots can fade over time, and longer lifespans have increased rates of chronic diseases—which raise the risk of complications from vaccine-preventable diseases. And some vaccines, like tetanus-diphtheria (Td) or Tdap, require a booster every 10 years to maintain protection. 

Older adults are also at a greater risk of serious illness from diseases like the flu, RSV, whooping cough, and shingles. Make sure you ask your doctor about these vaccines for adults that you might need. 

6. Pregnant women have special vaccine needs

The CDC recommends that women who are pregnant—or will be pregnant during flu season—get a flu shot because they are more susceptible to serious complications and hospitalizations. (The flu may also be harmful for the developing baby.) And you don’t need a doctor’s approval. “The flu shot is fine for pregnant women because it doesn’t contain live virus,” says Richard Kuhn, PhD, the Trent and Judith Anderson distinguished professor in science at Purdue University. 

Because infants don’t start receiving most vaccinations until they’re 2 months old, pregnant women should also receive certain vaccines to pass immunity to their baby. These include:

• Whooping cough (pertussis): between 27 to 36 weeks of pregnancy. 
• RSV (respiratory syncytial virus): between 32 to 36 weeks of pregnancy. 
• COVID-19 vaccine.

7. Don’t skip boosters

It’s easy to lose track of booster schedules, but staying up to date is essential to prevent disease. The CDC explains that a single dose of most vaccines provides only partial protection. Some, like chickenpox and MMR (measles, mumps, and rubella), need a second dose for lasting immunity. 

Protection from other vaccines can fade over time, so additional doses are necessary to maximize and maintain protection. For example, the DTaP vaccine (diphtheria, tetanus, and pertussis) requires three or more doses for children to reach full immunity. Even so, protection wanes with age, which is why adults require a tetanus booster (which also protects against pertussis and diphtheria) every 10 years. 

8. Vaccines aren’t 100% effective—but they do work

“No vaccine is 100% effective,” says Dr. Edwards, although some (like measles) come very close. But even if you do get sick, vaccination can make the illness much less severe.

Take the flu shot, for example. Its effectiveness can vary year to year depending on circulating strains, but vaccinated people are far less likely to be hospitalized or die. “The vaccine doesn’t always prevent infection,” says Dr. Schaffner, “but it dramatically reduces how sick you get.” That same principle applies to other vaccines—protection may not be perfect, but it’s powerful.

9. Vaccines don’t cause illness

The World Health Organization (WHO) explains that most vaccines contain either:

• Weakened or inactive parts of the disease-causing bacteria.
• A harmless “blueprint” that teaches your body how to fight the real bacteria. 

In either case, the vaccine will not cause the disease. “I hear some people say ‘I got the flu shot and I got the flu,’ but the flu vaccine is by definition incapable of causing disease,” Dr. Kuhn says. 

Even if you do feel a bit sick, your sniffles are probably totally unrelated to your shot, adds Dr. Fisher. “Often when people get the flu shot, it happens to be in the season when respiratory viruses are starting to circulate in the community,” she says. “So if they get a flu shot and then one of the other common respiratory viruses, they presume it’s from the shot when they probably got a different virus.” 

You may experience side effects after getting a vaccine, but that’s a good sign, says Dr. Kuhn. “It reflects that you’re getting a strong immune response.” According to the CDC, common side effects from vaccines can include soreness at the injection site, low-grade fever, headache, or tiredness. 

10. Vaccine safety is heavily monitored

Vaccines go through rigorous testing before they become available. While there’s a chance you could have mild side effects like bruising or a day of feeling sniffly, a serious side effect is extremely rare, Dr. Fisher says. “Vaccines are tested in more children over a longer period of time than any other drug before they’re approved by the FDA,”  she says.

Even after approval, vaccines continue to be monitored through systems like the CDC’s Vaccine Adverse Event Reporting System (VAERS). “People need to understand that there is careful recording after each dose is administered,” Dr. Edwards explains. “When doctors and nurses give vaccines, they are required to record the lot, the kind of vaccine, and where it was given, so if there were any clusters of reactions, those could be looked at.” There’s also a large system of HMOs (health maintenance organizations) that look for reactions after vaccines, she says. “We take this very, very seriously.”

11. Here’s what’s in a vaccine—and why

The CDC notes that some vaccines contain trace amounts of preservatives and other ingredients, like formaldehyde and aluminum. These additives help our body create specific disease-fighting antibodies, keep the vaccine safe and long-lasting, and boost effectiveness. 

And there’s no need to fear these chemicals. The CDC says that we encounter them in everyday life—and at much higher levels—in fruit, drinking water, household furnishings (such as carpets), and over-the-counter medications like antacids. 

The CDC also emphasizes that most vaccines no longer contain thimerosal, a type of ethylmercury that, importantly, does not cause mercury poisoning or other health issues. Even so, ethylmercury was taken out of childhood vaccines in 2001 to ease any concerns.

12. The flu shot: it’s still effective late into the season

Flu cases typically peak from December to February, which is why experts recommend getting the vaccine by the end of October. But the flu can circulate into May, so the CDC says that if you missed this ideal window, vaccination later in the season still helps. 

This is true even if you’ve already had the flu. Several strains circulate each year, and the vaccine protects against multiple types. “Sometimes at the beginning of the year, we will have one strain then, as the year goes on, another comes on,” Dr. Edwards says. “If you haven’t been vaccinated, it can still be recommended because another strain may come later.” 

13. The HPV vaccine: it’s not just for girls

The HPV vaccine protects against the human papillomavirus (HPV), the most common sexually transmitted disease in the U.S. Some types are harmless or can cause genital warts, while other strains can lead to cervical and other cancers. 

“The vaccine was originally touted as preventing cancer of the cervix, and while that is wonderfully true, it also prevents head, neck, and anal cancers,” which are not gender-specific, says Dr. Schaffner. That’s why the CDC recommends that both boys and girls get the HPV vaccine around age 11 or 12. The vaccine requires two doses, spaced six months to a year apart.

If you miss this window, vaccination is still recommended up to age 26—and up to age 45 for certain individuals. This is true if you’ve had HPV before, too, says Dr. Schaffner. However, even after vaccination, women should get screened for cervical cancer from ages 21 to 65. 

14. Vaccine combination shots are safe

The CDC assures us that combo vaccines—and receiving multiple vaccines at the same time—have no adverse health effects. “Children are exposed to hundreds of viruses and bacteria during normal activities like eating and playing,” says Dr. Yamada. “Getting vaccines is no extra burden on the immune system, even for babies.” 

Some studies even suggest that the immune system might get stronger when multiple vaccines are given together, likely because they call for different arms of the immune system that then work together, Dr. Fisher says.

15. Vaccines do not cause autism

“This has been totally discredited,” says Dr. Schaffner. The supposed link stems from a small, flawed 1998 study that only examined 12 children and didn’t include a control group to pinpoint the cause of their symptoms. An investigation by The British Medical Journal found that the lead author had altered the kids’ medical histories, and the study has since been retracted. 

According to the CDC, there is no link between vaccines and autism. Nine separate CDC-funded or conducted studies since 2003 have disproven any connection between thimerosal—found in trace amounts in some vaccines—and autism. Vaccines for measles, mumps, and rubella have also all been cleared of any connection. The National Foundation for Infectious Diseases explains that “no environmental factor has been better studied as a potential cause of autism than vaccines,” going on to say that research has found no link between the two. “This is consistent across multiple studies, repeated in different countries around the world, with different individuals, and using different model systems,” the foundation concludes.

16. Vaccines may make it possible to not need vaccines in the future

When mass vaccination programs work the way they’re supposed to—everyone gets the shot—it can lead to the eradication of a disease. “When we eradicated smallpox, we stopped vaccinating against smallpox,” explains Dr. Schaffner. Before the vaccine, smallpox killed three out of every 10 people, according to the CDC. 

While smallpox dates back at least 3,000 years, an Intensified Eradication Program led by the WHO (which included mass vaccination) took the deadly disease down within a decade. The last case of naturally acquired smallpox came and went in 1977 when a hospital cook in Somalia contracted (and recovered from) the virus. 

Experts say that polio could be the second disease ever eradicated. The CDC reports that it has been eliminated in the U.S. due to widespread vaccination since 1955, but a few dozen cases pop up every year worldwide, according to the WHO. As global vaccination coverage improves, diseases like polio have fewer opportunities to spread—so every shot contributes to a future where an illness may no longer exist. 

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