Confused About Vaccines?

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About 92% of Americans vaccinate their children, and the results have been substantial. According to a 2024 CDC report, routine childhood vaccination for those born between 1994 and 2023 has prevented more than 32 million hospitalizations and 1.1 million deaths in the United States.

For decades, U.S. medical societies and the federal government were largely consistent on vaccine recommendations. Today, that alignment is fractured.

Political appointees running federal health agencies are, for the first time, being shaped by voices that elevate ideology and individual preference over decades of rigorous scientific evidence and public health practice. This shift is not a minor recalibration, but rather a fundamental break from evidence-based policymaking.

This has sown doubt and confusion among American families, undermining confidence in the most effective public health tools we have. Many Americans may be wondering who to believe and what is true. I can’t say I blame them.

Here, I tackle some of the most common questions about vaccines and what the evidence tells us. My hope is that it’ll arm you with information that’ll clear up any confusion for you and arm you with the information you need to have productive conversations with those in your world who may be hesitant or resistant to vaccines.

Don’t too many vaccines overwhelm our immune system?

Our immune system is remarkably strong. Over a lifetime we encounter countless germs, and the immune system recognizes and responds to an enormous range of them—likely more than a billion different viral and bacterial targets.

When it comes to vaccinating children, many parents wonder how many vaccines are too many to administer at once. Can they overwhelm kids’ immune systems? Because today’s childhood vaccination schedule as recommended by AAP includes more vaccines than it did 20 years ago, it’s easy to assume that this means children are exposed to more immune “stimuli” than ever before.

In fact, the opposite is true.

Modern vaccines are much more refined than they once were. Scientists have learned how to include only the key pieces—specific proteins or sugars—needed to train the immune system. As a result, even though there are more vaccines today, the total number of these components has dropped sharply. For example, in the 1980s, the routine schedule included 7 vaccines that together exposed children to over 3,000 such proteins and sugars. Today, about 15 vaccines expose children to fewer than 175 proteins and sugars in total. A major reason is the shift from the older “whole-cell” pertussis vaccine (which contained thousands of components) to today’s “acellular” version (which contains just 2-5 key components).

Isn’t natural immunity better than vaccine immunity?

Thanks to our robust immune system, protection against many diseases can be long-lasting—sometimes even lifelong—whether it comes from natural infection or vaccination. But which is safer and more reliable?

Take measles, for example. In otherwise healthy children, a natural measles infection typically provides strong lifelong immunity. Vaccination with two doses of MMR offers about 97 percent lifelong protection, essentially matching natural immunity.

The difference in durability is trivial compared with the risks of infection. Natural measles carries roughly a 1 in 1,000 risk of death, a 1 in 1,000 risk of encephalitis (severe brain swelling), and a 1 in 20 risk of pneumonia.

By contrast, the MMR vaccine’s main serious risk are seizures related to high fevers (i.e., febrile seizures), occurring in about 1 in 3,000 children. While febrile seizures are an alarming event (and can occur in up to 5 percent of children under age 2, regardless of vaccination), they almost never cause long-term harm.

Do vaccines contain dangerous toxins?

Historically, two additives were frequently associated with vaccines: thimerosal and aluminum.

Until 1999, thimerosal was used in some multi-dose vaccine vials to prevent bacterial contamination. Many U.S. vaccines, such as MMR and varicella, never contained thimerosal. By 2001, thimerosal had been removed from nearly all routine vaccines in the U.S., with the exception of some multi-dose influenza formulations. It’s important to note that the removal of thimerosal was precautionary, driven by public concern rather than any evidence of harm.

Thimerosal contains ethylmercury, which is distinct from methylmercury. Methylmercury can be found in certain fish and can be toxic at high levels and is therefore regulated. By contrast, ethylmercury is processed differently by the body than methylmercury and has not been shown to accumulate or cause harm at the levels previously used in vaccines. Large-scale studies involving hundreds of thousands of children across multiple countries over several decades have consistently found no link between thimerosal exposure and adverse outcomes related to brain development. An independent, comprehensive review by the Institute of Medicine (now the National Academy of Medicine) found that the evidence does not support a link between vaccines that contain thimerosal and brain development disorders, including autism.

Aluminum is ubiquitous, making up about 9 percent of the Earth’s crust. It is present in soil, water, plants, and food and has been safely used for decades in everyday products such as antiperspirants, antacids, and food packaging. In vaccines, aluminum serves as what’s known as an adjuvant, which is an ingredient that helps to strengthen the body’s immune response and improve vaccine effectiveness.

Here’s a fact that surprises a lot of people: Exposure to aluminum from vaccines is extremely small compared to everyday dietary aluminum intake. For example, by age 18, individuals are exposed to approximately 73–438 mg of aluminum through food, compared with less than 8 mg from all recommended vaccines combined. Claims that injected vs. ingested aluminum is handled differently by the body have not been supported; regardless of the source, aluminum follows similar biological processing pathways. Similar to thimerosal, large-scale evidence reinforces the safety of aluminum: a nationwide Danish study of more than 1.2 million children found no association between cumulative aluminum exposure from vaccines and over 50 chronic conditions, including autism.

Of course, proving a negative (i.e., no association) is inherently challenging. While evidence can accumulate to support a positive claim, the absence of evidence is often questioned as a limitation of how a study was designed rather than accepted as reassurance. Claims that rising autism diagnoses are caused by increased vaccination confuse correlation, meaning when two things happen at roughly the same time, with causation. By that logic, one could just as easily blame the rise on the growth of ride-sharing apps or social media use over the same period. After decades of research involving millions of children worldwide, the consistent lack of association between vaccine-related thimerosal and aluminum exposure and adverse health outcomes provides strong and reassuring evidence of safety.

Other countries do not vaccinate as much as we do in the U.S. and have better health outcomes. Why do we need so many vaccines? 

In January 2026, with little explanation or evidence to support its actions, HHS officials overhauled U.S. childhood vaccine policy, copying a model based after Denmark, which reduced the number of recommended vaccines from 17 to 11. These actions were recently blocked and invalidated—at least temporarily—by a federal judge.

While it is true that the U.S. sits on the slightly higher end of vaccine recommendations among wealthy nations and Denmark on the slightly lower end, mirroring Denmark’s approach makes little sense. Denmark is a small homogeneous country (its population is smaller than that of Philadelphia) and operates a high-investment universal welfare model, including 52 weeks of paid parental leave (compared with the limited unpaid leave in the U.S.), universal healthcare coverage, and very low infant mortality (roughly half that of the U.S.). These robust social supports and preventive care measures—not vaccine policies—drive Denmark’s low rates of poverty, obesity, and cardiovascular disease, which in turn contribute to the nearly four-year higher life expectancy enjoyed by Danes.

Where should I go to now for the best vaccine information?

Research shows that patients continue to trust their healthcare providers as their primary source of reliable vaccine information. These providers, who are members of national medical professional associations such as the American Medical Association, American Academy of Pediatrics, American College of Obstetricians and Gynecologists, and the Infectious Diseases Society of America, remain aligned on evidence-based vaccine guidance.

While federal guidance may be in flux, the Vaccine Integrity Project is collaborating with these professional societies to provide up-to-date, transparent resources, references, and data to support informed vaccine decisions.

Rochelle Walensky, MD, MPH served as the 19th Director of the Centers for Disease Control and Prevention, professor of medicine at Harvard Medical School (2012–2020, 2025–present), and as a board-certified infectious diseases clinician. She is mother of three boys (her proudest title), and wife.