In consulting rooms across America, physicians face a challenge that no medical school prepared them for. A parent arrives with a list of concerns gathered from social media, podcasts, and well-meaning friends. The questions sound scientific. The language borrows from immunology. The citations reference real studies. And yet the conclusions are wrong.
These parents are not ignorant. Many are educated, thoughtful, and deeply invested in their children’s health. They have encountered a sophisticated ecosystem of misinformation that exploits legitimate parental instincts: protect your child, question authority, demand evidence. The problem is not that these parents are asking questions. The problem is that they are receiving false answers.
The current moment has made this worse. The head of the Department of Health and Human Services has spent decades promoting vaccine skepticism and has now revamped the US childhood immunization schedule to have one-third fewer recommended vaccines. The Advisory Committee on Immunization Practices, which guides the Centers for Disease Control and Prevention (CDC), has reversed long-standing recommendations without presenting new safety data. Measles, a disease eliminated from the United States in 2000, has returned at levels not seen in decades.
The information environment has never been more polluted, and the stakes have never been higher.
The most persistent myths about childhood vaccines dissected below and in part 2, to be published tomorrow, are not fringe theories. They are the claims that appear most frequently in my clinical practice and in the broader public discourse.
Each has been studied extensively. Each has been refuted. And yet each persists, because misinformation travels faster than correction and because these myths tap into fears that are genuinely human.
Myth #1: ‘Vaccines were never properly tested’
The claim takes several forms. Vaccines were never tested against placebos. Vaccinated children have never been compared to unvaccinated children. Safety was monitored for only a few days. Each version relies on the same sleight of hand: narrowing the definition of “evidence” until only one impossible standard remains, then declaring that standard unmet.
The trick works by conflating regulatory paperwork with the totality of scientific evidence. When critics claim a vaccine was “never tested against a placebo,” they often mean that a specific Food and Drug Administration (FDA) filing for a specific commercial approval used an active comparator rather than saline as a placebo. They systematically exclude the foundational trials that established safety of these antigens (the substance in a vaccine that produces a response by your body’s immune system): trials conducted before the current product’s licensure, trials run in other countries, trials published in peer-reviewed journals but not listed in a particular regulatory submission.
The primary literature tells a different story. The hepatitis B vaccine, for instance, has been tested in numerous placebo-controlled trials. One early example: a randomized, double-blind trial published in the New England Journal of Medicine in 1980. Researchers enrolled 1,083 subjects and randomized them to receive the vaccine or placebo.
The placebo contained only the aluminum adjuvant and was packaged in vials visually indistinguishable from the vaccine. Using the adjuvant alone as the control allowed researchers to attribute any differences specifically to the hepatitis B antigen rather than to the aluminum salt that both groups received. They followed up with participants for 24 to 30 months. Side effects were similar between groups.
Each version relies on the same sleight of hand: narrowing the definition of ‘evidence’ until only one impossible standard remains, then declaring that standard unmet.
Critics often assume that only saline counts as a “real” placebo, but this reflects a misunderstanding of trial design.
The goal of a control is to isolate the variable being tested. If you want to know whether the active ingredient causes side effects, you compare it against everything else in the shot minus that ingredient. This design reflects a common approach in vaccine trials: placebos often contain the adjuvant or other vaccine components minus the antigen, allowing researchers to isolate the effect of the immunizing agent itself. A 1984 trial in Hong Kong newborns used a similar approach for the hepatitis B vaccine, with a placebo of aluminum phosphate in saline, and assessed infants for 12 to 24 months.
Other trials have used saline placebos. A 1968 double-blind trial of the mumps vaccine—now a component of the measles, mumps, and rubella (MMR) vaccine—randomized schoolchildren to vaccine or saline and demonstrated 80% efficacy with a reassuring safety profile. Saline-placebo-controlled trials also exist for varicella; rotavirus; Haemophilus influenzae type B (Hib); tetanus, diphtheria, and acellular pertussis (Tdap); pneumococcal; meningococcal; human papillomavirus (HPV); influenza; and COVID-19 vaccines—representing hundreds of thousands of participants across dozens of studies.
The polio vaccine was tested in the 1954 Francis Field Trial, which enrolled more than 400,000 children and used a placebo consisting of the culture medium, antibiotics, and formalin—everything in the vaccine except the poliovirus antigen and monkey kidney protein. It remains one of the largest randomized controlled trials in medical history.
Our research team has systematically reviewed the randomized controlled trial literature and identified hundreds of childhood vaccine trials using inert placebos such as saline, water, or buffer without antigen. (Full results are in preparation for publication.)
Myth #2: ‘Vaccinated and unvaccinated kids haven’t been compared’
The claim that vaccinated and unvaccinated children have never been compared collapses just as quickly.
A Danish study in 2002 of 537,303 children directly compared autism rates between children vaccinated or not with the MMR vaccine. A follow-up study of 657,461 children did the same and included a sub-analysis of children who had received no early childhood vaccinations at all. Germany’s Robert Koch Institute analyzed 17,641 children, finding no meaningful differences in allergies, atopic diseases (like asthma, allergies, and eczema), attention deficit hyperactivity disorder (ADHD), or epilepsy between vaccinated and unvaccinated groups. The only notable difference was that unvaccinated children had higher rates of vaccine-preventable diseases.
By demanding one trial do everything, they construct a standard no evidence can satisfy.
When shown this evidence, critics often shift their demands. They want a single trial with a saline placebo, years of follow-up, and statistical power to detect rare events. No single trial can do all three simultaneously. Power comes from large samples. Long-term safety comes from post-market surveillance. Placebo controls come from pre-licensure efficacy trials.
By demanding one trial do everything, they construct a standard no evidence can satisfy. If the goalposts move every time evidence is provided, the goal is not safety. It is constructing a position that cannot be challenged.
Myth #3: ‘The ingredients are toxic’
Aluminum. Thimerosal. Formaldehyde. The names sound alarming in isolation. But how toxic something is depends on dose, and the doses in vaccines bear no relationship to the scenarios that frighten parents.
Aluminum salts have enhanced vaccine effectiveness since the 1930s, with doses tightly regulated by the FDA at no more than 0.85 milligrams (mg) per shot. Children receive approximately 4.4 mg of aluminum from vaccines by six months of age. For context, formula-fed infants ingest about 38 mg of aluminum in their first six months from formula alone, and breast-fed infants receive smaller but still measurable amounts.
The body processes aluminum the same way regardless of how it enters: it binds to proteins like transferrin and is filtered by the kidneys. Studies show that routine vaccination does not increase total blood aluminum levels.
The largest study ever conducted on this question—and published just last year—analyzed data on more than 1.2 million Danish children, examining whether cumulative aluminum exposure from vaccines was associated with 50 different conditions including autism, ADHD, asthma, and autoimmune disorders. The results were unequivocal: It found no association between aluminum exposure from vaccines and an increased risk for any of these conditions.
[A recent large study] found no association between aluminum exposure from vaccines and an increased risk for any of these conditions.
Thimerosal, a mercury-containing preservative, was removed from childhood vaccines in the United States starting in 1999 as a precautionary measure. If thimerosal caused autism, rates should have fallen after its removal. They did not. In Denmark, which eliminated thimerosal in 1991, autism rates actually increased in subsequent years.
Currently, only multi-dose influenza vaccines contain thimerosal, and thimerosal-free alternatives are widely available. The mercury in thimerosal is ethylmercury, which is processed and excreted far more rapidly than the methylmercury found in fish. A meta-analysis involving more than 1.25 million children found no association between thimerosal and autism.
Formaldehyde is used to inactivate viruses and detoxify bacterial toxins in some vaccines. Everyone has formaldehyde detectable in their blood as part of normal metabolism. The level in circulation is approximately 10-fold higher than what any vaccine contains. The quantity in vaccines is at least 600-fold lower than that necessary to induce toxicity in experimental animals.
Many newer vaccines contain none of these ingredients. Rotavirus, chickenpox, meningococcal conjugates, annual influenza, and mRNA COVID-19 vaccines are all aluminum-free. Live-attenuated vaccines like MMR work differently than other vaccines, using weakened viruses that replicate briefly and generate strong immunity without requiring aluminum adjuvants at all.
Myth #4: ‘Too many, too soon’
In 1986, children received vaccines protecting against seven diseases. Today, the schedule protects against 16 to 18 diseases. The number of shots has increased substantially. Parents experience more office visits and naturally assume a greater biological burden.
The biology tells a different story. What actually matters to the immune system is not the number of injections but the number of antigens, the proteins and molecules that immune cells recognize and respond to. A century ago, the single smallpox vaccine contained approximately 200 antigens. In 1986, a single whole-cell pertussis (whooping cough) shot contained roughly 3,000 antigens. Today’s entire pediatric schedule exposes children to about 165 antigens total. That is a 95% reduction in antigenic load despite the increase in diseases prevented.
This reduction reflects advances in vaccine engineering. We switched from whole-cell to acellular pertussis vaccines, which contain only the specific components needed for protection rather than every protein the bacterium produces. Recombinant DNA technology now allows manufacture of single-protein vaccines. The result is far more protection with far less immune burden.
The immune system routinely processes thousands of antigens daily through environmental exposures. A few hundred additional antigens from vaccines present no meaningful challenge.
The immune system has enormous capacity. Researchers have estimated that infants could theoretically respond to thousands of vaccines simultaneously before using even 1% of their circulating immune cells. The real-world schedule barely registers on this capacity. The immune system routinely processes thousands of antigens daily through environmental exposures. A few hundred additional antigens from vaccines present no meaningful challenge.
Multiple studies have tested the “too many, too soon” hypothesis directly. A CDC study compared total cumulative antigen exposure in the first two years of life between children with autism and matched controls without autism. The total amount of antigen from vaccines received was the same between groups. A Danish study of more than 800,000 children found that neither the number of vaccines nor receipt of multiple-antigen vaccines increased the risk of hospitalizations from non-targeted infectious diseases. Children who received combination vaccines were actually more likely to complete their full vaccination series, and studies have found no increase in serious adverse events among them.
Stay tuned
Tomorrow I’ll explore five more top myths, from vaccines causing autism to “natural immunity is better” to mRNA vaccines creating “turbo cancers.” See you then.
Dr. Scott is a clinical associate professor of infectious diseases at Stanford University School of Medicine and co-author of “Updated Evidence for Covid-19, RSV, and Influenza Vaccines for 2025–2026” in the New England Journal of Medicine.
The opinions voiced in CIDRAP Op-Ed pieces are the authors’ own and do not necessarily represent the official position of CIDRAP.
