If you’re confused about whether vaccines are safe, you’re not alone. Experts say that vaccines have saved hundreds of millions of lives and pose little risk, especially in light of their benefits. But maybe your social media feed has you wondering if those experts aren’t telling you the full story. Here’s what to know about how vaccines are approved and tracked for safety, the evidence on additives like mercury, and the rare risks of some vaccines.

How do vaccines work?

“The goal of a vaccine is to induce the immunity that typically follows a natural infection without having to pay the price of a natural infection,” explains Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia. “There are a variety of ways to do that.” 

• Live, weakened vaccines. “Unlike the natural virus, which can reproduce millions and even billions of times, a weakened virus can only reproduce itself a limited number of times,” says Offit. “It can’t cause the disease but can still induce an immune response.” Examples of weakened viral vaccines include ones for measles, mumps, rubella, chicken pox, and rotavirus.  
• Inactivated vaccines use a killed version of a virus that can trigger an immune response but can’t reproduce itself. Polio, hepatitis A, and flu vaccines are all made with inactivated viruses.
• Component vaccines use a protein, sugar, or other component of a virus or bacteria to teach the immune system to mount a response against that part of the germ. Component vaccines protect against hepatitis B, shingles, human papillomavirus, and whooping cough.
• mRNA vaccines use messenger RNA (mRNA) to tell the body to make viral proteins that trigger an immune response. mRNA was used to make both the Pfizer and Moderna Covid-19 vaccines. mRNA vaccines take far less time to manufacture than other vaccines. That’s one reason why the Covid-19 vaccines were developed so much faster than any other vaccine in history. 

No matter what type of vaccine you get, its job is the same: to trigger your immune system’s “memory” cells and antibodies to retain information about the virus or bacteria. And that’s what helps protect you from future infections.

How effective are vaccines? 

All FDA-approved vaccines are effective, though efficacy varies between diseases. Best-case scenario: “The smallpox vaccine essentially eliminated smallpox from the face of the earth by the 1980s,” says Offit. And thanks to vaccines, diseases like rubella and polio have been eliminated from the United States. 

“The measles vaccine is also a superb vaccine,” adds Offit. Before the vaccine, experts estimate that in the US, 3 to 4 million people were infected, 48,000 were hospitalized, and 400 to 500 died of the disease each year. About 1 in 20 children with measles will get pneumonia and 1 in 1,000 will develop brain swelling that can leave a child deaf or with an intellectual disability. “We had eliminated measles, far and away the most contagious infectious disease, from this country by the year 2000,” says Offit. “But it’s come back because people have chosen not to vaccinate their children.” 

Because measles is so easily transmitted—it can linger in the air for up to two hours after an infected person has left the room—about 95 percent of children need to be vaccinated in order to prevent the virus from spreading in the general population. But the U.S. has fallen below that level in recent years. (If you’re unsure if you’ve been vaccinated against measles, ask your healthcare provider to check with a blood test. If your antibodies fall below the immune threshold, you can get a booster shot.) 

Vaccines for diseases like measles, polio, and rubella typically provide lifelong protection, whereas protection against infections like the flu or Covid is shorter-lived and less hardy, and it requires boosters. Why?  

• The infection is local. Diseases like measles and polio are systemwide infections that result in robust immune responses. Viruses like the flu and Covid, on the other hand, infect mucous membranes like those lining the nose, throat, and lungs. The type of immune response to combat those viruses wanes faster than the response that combats systemic infections. “When you get a yearly Covid or flu vaccine, your protection will last for months and then fade,” says Offit.
• The virus changes rapidly. Viruses like measles and polio don’t change much over time. But flu and Covid viruses mutate often, which means that the antibodies that your body makes against one strain or variant may not work against another.
• Symptoms appear quickly. The flu and Covid viruses have a short incubation period, meaning that symptoms appear only a couple of days after you’ve been infected. Those symptoms act like a surprise assault to the body, and your immune system doesn’t have much time to mount a response. Diseases with a longer incubation time (measles symptoms, for example, typically appear 11 to 12 days after infection) allow your immune system to call its memory cells out of reserve and fortify the defense system so that it can strategically fight off the illness.  

“Even if the entire world were vaccinated, viruses like the flu would still circulate,” Offit explains. “The goal of those vaccines is not to eliminate the virus; it’s to make you less likely to be hospitalized or die. And since that’s the goal, they’re all highly effective.” 

How are vaccines approved? 

Before a vaccine reaches the market—and your arm—scientists do several rounds of studies to test how well it works and how safe it is. In most cases, vaccines are tested first in animals, then in small human trials, scaling up to large studies in thousands of people. It can take 10 to 15 years for a vaccine to get through the entire testing and approval process. 

One exception: the Covid-19 vaccine. When Covid began circulating in 2019, the government’s Operation Warp Speed enabled scientists to get to work on vaccines immediately, conducting multiple studies at the same time instead of one after another. That approach allowed scientists to do longer-term safety and effectiveness studies while vaccine makers were simultaneously churning out millions of doses of the vaccines. Once the vaccines were approved, the supply was ready to be distributed across the country. In other words, the Covid vaccines underwent the same safety protocols as other vaccines…just a lot faster.  

“In order for a vaccine to be initially licensed by the Food and Drug Administration, you have to show that it works and is safe in a large placebo-controlled clinical trial involving thousands or tens of thousands of people,” notes Offit. (There are some rare exceptions to that rule. For example, if a vaccine already exists for a disease, a new vaccine may be compared to the old vaccine instead of a placebo. That's because it would be unethical to not give some people a vaccine when we already have one that works. In other cases, researchers might determine that a vaccine is effective by looking at the immune response to a new version of a vaccine compared to the immune response seen in previous trials.)

But even with trials that large, it’s still possible to miss a rare, serious side effect. “That’s why we have additional safety systems in place,” says Offit.

How is vaccine safety tracked? 

Several systems monitor vaccine safety. “The most important is the Vaccine Safety Datalink,” says Offit. The VSD is a partnership between the Centers for Disease Control and Prevention and healthcare organizations across the country that serve roughly 12 million people.  

The VSD relies on electronic health records that include data on when someone was vaccinated, the type of vaccine given, and other vaccines given on the same day. The VSD data is evaluated weekly to study the short-term side effects of a vaccine, as well as complications that can take years to develop. 

In addition, the CDC’s Advisory Committee on Immunization Practices meets at least three times a year to determine who should receive vaccines, to set vaccine schedules, to evaluate vaccines’ safety and effectiveness, and to update guidelines when necessary. The voting members include medical and public health experts (but not government employees) who follow strict guidelines to avoid any conflicts of interest. 

Vaccines are held to a much higher standard of safety and effectiveness than prescription drugs. 

For one, “vaccines are given to healthy people,” says Offit. That means there’s a higher bar to clear. In addition, vaccines must be effective across a wide range of ages and underlying health risk factors. Drugs, on the other hand, are typically targeted to patients with a known disease or condition. And it’s harder to show that a vaccine can prevent an infection (or severe illness from an infection) than to show that a drug can treat an already existing condition. 

What’s more, drugs aren’t monitored for safety as rigorously as vaccines are. “Vaccines are held to the highest standard of safety,” says Offit. “It’s too bad we don’t have a drug safety Datalink. I think if there were one, we would have found that the prescription painkiller Vioxx was a rare cause of heart attacks much more quickly than we did.” 

What are the risks of vaccines? 

Nearly every decision involves risks and benefits. For example, going for a jog could result in a twisted ankle. But most joggers are willing to accept that rare risk because the jog may improve their mood or cardiovascular health. The same risk-benefit analysis exists for vaccines.  

“Vaccines are considered to be safe if the benefits clearly and definitively outweigh the risks,” says Offit. “It doesn’t mean that there are no risks.” 

And our monitoring systems have flagged rare risks. 

“For example, the mRNA Covid vaccines have the rare side effect of myocarditis, or inflammation of the heart muscle,” says Offit. Myocarditis can cause fatigue, chest pain, shortness of breath, and heart palpitations. Most cases tend to be mild and don’t require hospitalization. According to one estimate, roughly 1 in every 38,000 young men developed myocarditis after receiving their second dose of an mRNA vaccine. (Very few girls, women, or older men developed the condition.) 

Ultimately, the Advisory Committee on Immunization Practices decided that the benefits of the mRNA Covid vaccines vastly outweighed the risk of myocarditis, even in young men. (It’s worth noting that people are far more likely to get myocarditis from a natural Covid infection than from a Covid vaccine.) 

“Covid is a perfect example of that risk-benefit analysis and the safety monitoring system working as it should,” says Offit. “The Covid vaccines probably saved 3 million American lives. But the Johnson & Johnson vaccine was found to cause a rare, severe clotting that happened in about one in 250,000 people. It took about nine lives, which is unacceptable, especially when there were safer alternatives. That’s why that vaccine was ultimately taken off the market.” 

Other vaccines also carry rare risks. “Flu vaccines have a risk of Guillain-Barré syndrome, which is a paralysis that often starts in the legs and spreads upward,” says Offit. “It occurs in about one in a million people who receive the flu vaccine.” But fear of Guillain-Barré shouldn’t keep you from getting a flu shot. You’re roughly 16 times more likely to develop Guillain-Barré after getting the flu than you are after getting the flu vaccine. (Most people recover from Guillain-Barré, though the recovery can take months or even years.) 

“And there are people who can be allergic to a component of a vaccine,” says Offit. That could include allergic reactions to components like the egg protein that’s in the flu, yellow fever, and rabies vaccines; the gelatin that’s used in the MMR (measles-mumps-rubella) and chickenpox vaccines; certain preservatives; or the trace amounts of antimicrobials that are used in most vaccines to prevent bacterial or fungal growth. 

Allergic reactions can range from mild—like redness, swelling, itching, or hives—to severe anaphylaxis, which is a life-threatening reaction that can cause swelling of the tongue or throat, making it difficult to breathe. But anaphylactic reactions are incredibly rare. About 1 in 1.3 million vaccinations result in anaphylaxis.  

The most common risks? “All injectable vaccines could cause pain, redness, or tenderness at the site of injection,” says Offit. Or you may feel fatigued, feverish, or have head or body aches for a day or two after getting a vaccine. Most of those side effects are simply a sign that your immune system is mounting a response against the disease. Said another way, they mean the vaccine is working.

Kids and vaccines 

Plenty of people who question the safety of vaccines are worried about their own health. But the lion’s share of vaccine hesitancy is focused on vaccines given to children. 

Too many vaccines? 

“The concern I hear the most often is that kids are getting too many vaccines too soon, and they just can’t handle that; that the number of vaccines somehow weakens or overwhelms their immune system,” says Offit, who adds that he understands what leads to those concerns. 

“We give vaccines to prevent 14 different diseases in the first few years of life, which can mean as many as 26 injections,” he notes. “It can be as many as five shots at one time to prevent diseases most people don’t see, using biological fluids that most people don’t understand. It makes sense that there’s pushback.” 

“But it’s not the number of vaccines that matters,” Offit explains. “What matters is the number of immunological components of vaccines.” (He’s talking about the proteins, sugars, or other pieces of the bacteria or virus in the vaccine that the immune system responds to.)  

“If you add up all the immunological components used in all the vaccines recommended in the U.S. today, you get around 160. But if you look at the only vaccine that we had 100 years ago, the smallpox vaccine, that had about 200 immunological components.” 

Thanks to huge advances in vaccine technology, far fewer components are needed to make vaccines than in the past. “The vaccines we make now are the safest we’ve ever made,” says Offit. 

What’s more, “If children’s immune systems couldn’t handle vaccines, we’d be in trouble as a species. When you enter the birth canal and make your way into the world, you are colonized with trillions of bacteria, to which you make an immune response.”

“Plus, the food you eat, the dust you inhale, the water you drink isn’t sterile,” Offit adds. “You’re constantly exposed to immunological challenges, which you meet. Billions of new immune cells pour from your bone marrow into your circulation every day to meet that need. Vaccines really are a drop in the ocean of what your immune system encounters and manages every day.”

Aluminum in vaccines 

Aluminum has been used in some vaccines—like those for hepatitis A, hepatitis B, diphtheria-tetanus, and pneumococcus—as adjuvants since the 1930s. “Adjuvants enhance the immune response, which means you need either fewer doses or a smaller amount of the viral protein or bacterial protein,” Offit explains. In other words, adjuvants help vaccines work better. 

Why are some people concerned about aluminum’s presence in vaccines? “Aluminum is a light metal, but it is a metal,” says Offit. “I think people think of metals as potentially harmful. But you’re exposed to aluminum all the time.”  

Babies are exposed to aluminum from either breast milk or infant formula. And blood levels of aluminum in infants who receive aluminum-containing vaccines remain far below the toxic threshold.  

The rest of us are likely consuming tiny amounts of aluminum from cookware, water, aluminum cans, baking powder, seafood, antacids, and more. “So what you get from vaccines is nothing compared to the aluminum you ingest over time,” says Offit. 

Aluminum toxicity can cause disorders of the brain and nervous system, such as impaired coordination, weakness, and seizures.  

But there’s no cause for concern about aluminum in vaccines. For one, the safety of vaccines that contain aluminum is continually monitored. They’ve been used for decades and given to over a billion people without problems. Second, we store very little aluminum in our bodies. Half of the aluminum we absorb—either through our diet, vaccines, or other sources—is eliminated within 24 hours. 

Third, there’s just no good evidence that aluminum from vaccines harms developing children. In a study of 9-to-13-month-old infants, researchers found no link between blood aluminum levels and vaccination history or cognitive, language, or motor development. 

The only (minor) downsides: Aluminum-containing vaccines are more likely to cause irritation at the injection site and reactions like fever, chills, and body aches than vaccines without aluminum.

Vaccines don’t cause autism 

The most pervasive, damaging, and false claim about vaccines is that they cause autism.  

That concern was first raised in a report published in 1998 that zeroed in on the MMR vaccine. The evidence? Parents or doctors reported worsening behavior shortly after 8 of the 12 children in the study received the MMR vaccine. That shockingly small study—which included no control group—kicked off an unfounded health scare that’s lasted for more than a quarter century.  

A follow-up investigation uncovered a string of falsified claims and discrepancies in the study. For example, several of the children with developmental issues, including autism, had shown symptoms before receiving the MMR vaccine. On top of that, the lead author, Andrew Wakefield, failed to disclose that he was being paid by a law firm that was working on a lawsuit against MMR vaccine makers…or that he had filed a patent for his own competing vaccine. Because of that fraudulent paper, Wakefield had his medical license revoked in the UK for “serious professional misconduct.” 

The study was officially retracted in 2010, but by then the damage was done. 

Why the focus on autism? It’s true that autism diagnoses have become more common over the last 25 years. But that’s not because of vaccination. Instead, doctors’ ability to diagnose autism has vastly improved, and autism awareness has increased (which means that parents may be more likely to share that their child has autism). Some people may have suspected that vaccines—and the MMR vaccine, in particular—caused autism simply because children often start showing symptoms around the age they receive their first dose of the MMR vaccine. But that’s simply a coincidence. It doesn’t mean that vaccines have anything to do with autism. 

Studies conducted around the world looking for an increased risk of autism following MMR vaccines continue to come up empty. One analysis of data from studies on roughly 1.2 million children found absolutely no link. 

And it’s not just MMR. According to the rumor mill, thimerosal, a mercury-containing preservative used in some vaccines, can also cause autism.  

Thimerosal (pronounced thy-MEHR-uh-sol) is used in vaccines that are drawn from vials that contain multiple doses. Why do they need a preservative? “You’re repeatedly violating the rubber stopper with your needle and syringe,” Offit explains, “so you may inadvertently introduce bacteria into the vial.” 

One form of mercury, methylmercury, can cause neurological harm at high doses. But the type and minuscule amount of mercury in thimerosal are considered safe, says Offit. “Thimerosal is a different form of mercury, called ethylmercury, that’s excreted from the body much more quickly. There was never any evidence that that quantity and type of mercury was ever going to be harmful.” 

Not surprisingly, study after study has found no link between exposure to thimerosal-containing vaccines and autism or childhood development markers like fine motor skills, attention, executive function, or language. What’s more, a study of children in Sweden and Denmark reported that rates of autism diagnoses continued to rise even after thimerosal was phased out in vaccines used in the two countries. That’s the opposite of what you’d expect if thimerosal caused autism. 

Nevertheless, thimerosal was removed from childhood vaccines in the United States in 2001 as a precautionary measure. After the removal, the American Academy of Pediatrics issued a press release in which it said that “The current levels of thimerosal will not hurt children, but reducing those levels will make safe vaccines even safer.” 

Offit isn’t buying that logic. “If thimerosal is safe, which it is, then taking it out doesn’t make vaccines safer. It just makes them perceived to be safer, which was what this was all about.” (Offit was on the Advisory Committee for Immunization Practices “when that all came crashing down.”) 

And taking thimerosal out of childhood vaccines wasn’t harmless. “For many vaccines, that meant going from multi-dose vials to single-dose vials, which made those vaccines much more expensive and much less available in the developing world,” says Offit. 

The controversy around removing thimerosal may have also shaken parents’ trust in vaccines, Offit argues. Even for those parents who don’t believe that vaccines cause autism but who question other aspects of vaccine safety, the thimerosal incident may have helped sow the distrust in vaccines that is still harming children. 

“I think we’re seeing that in West Texas now,” says Offit. “We’ve just had our first child death from measles in this country since 2003.” As of April 11, 711 measles cases had been reported nationwide and two children had died. And 97 percent of those cases were in people who were unvaccinated or whose vaccination status was unknown. Both children who died were unvaccinated.