Dr. Philp McMillan, John McMillan
On January 9th, 2025, something unprecedented happened in American public health: all 17 members of the Advisory Committee for Immunization Practices (ACIP) were fired. Robert F. Kennedy Jr., the newly appointed Secretary of Health and Human Services, justified this sweeping action as necessary to restore public trust in the committee that shapes national vaccine policy. This raises the question: How did a committee tasked with protecting public health deteriorate to the point where its dissolution was the only path forward?
The answer lies in a principle of mathematical probability that shapes all medical decision-making, though it’s often overlooked in public health debates. Thomas Bayes, an 18th-century English Presbyterian minister and mathematician, developed a theorem that revolutionized how we think about evidence and uncertainty. In medicine, this means that the value of any test or intervention depends critically on the baseline risk of the population receiving it. A screening test that works brilliantly in high-risk groups can become almost useless — generating more false positives than true cases — when applied to low-risk populations. When public health authorities ignore this mathematical reality, even well-intentioned policies can produce outcomes that defy common sense and erode public trust.
What We Knew in 2020
Consider what was known in November 2020. Matt Hancock, then UK Secretary of State for Health, articulated the consensus view on BBC Question Time: vaccines would go to “people who are either older, or work in health and social care, or who have a particular vulnerability.” As for children? The position was unequivocal: “not to children. Because this vaccine hasn’t been tested on children. It’s not designed for children. And anyway, children are very, very, very unlikely to get ill with coronavirus.”
This wasn’t one politician’s opinion. CDC COVID-NET data from before widespread pediatric vaccination showed hospitalization rates for children aged 5-17 at 38.9 per 100,000, compared to 676.1 per 100,000 for adults over 65—an 18-fold difference. A systematic review and meta-analysis titled “Risk Factors for Pediatric Critical COVID-19” found that in previously healthy children, the absolute risk of critical COVID-19 was only 4% (95% CI, 1%–10%). This meant that even among children who contracted COVID-19, 96% of previously healthy children did not develop critical illness. CDC provisional death data from the same period confirmed these minimal risks in the 0-18 age group.
The Reversal
Yet within months, the calculus changed dramatically. Countries that had acknowledged children’s low risk began implementing universal pediatric vaccination programs. Some went further, implementing mandates and, as Dr. Ros Jones, a retired pediatrician, observed in early 2022, offering “pizzas, given rollout of vaccines in nightclubs and football stadiums where they were giving free tickets to the first thousand people to turn up.”
Dr. Jones, speaking with the freedom that retirement affords, posed uncomfortable questions. Why were educational materials comparing COVID to polio and smallpox when the risk profiles bore no resemblance? Why did an effective intervention require such aggressive marketing? Most critically, she articulated what Bayes himself might have asked: What happens when you apply an intervention with unknown long-term effects to a population with minimal short-term risk?
The emergence of vaccine-associated myocarditis, particularly in young males, complicated the probability equation further. A peer-reviewed Israeli study published in the “New England Journal of Medicine” in October 2021 found the highest risk among males aged 16-19, at approximately 1 case per 6,600 individuals after the second dose. Nordic countries responded by pausing or restricting certain vaccines for younger populations, as reported by Reuters in October 2021. Different nations, examining the same data, reached divergent conclusions.
This divergence points to a deeper issue than simple scientific disagreement. When CDC Director Rochelle Walensky acknowledged in an August 5, 2021 CNN interview with Wolf Blitzer that vaccines could no longer “prevent transmission,” the primary rationale for vaccinating low-risk populations — protecting others — was substantially weakened. The CDC’s quiet revision of its definition of “vaccination,” shifting from producing “immunity” to producing “protection,” reflected this evolving understanding. Archived versions of the CDC’s “Immunizations: The Basics” webpage from before and after September 2021 document this definitional change.
Trust Lost
The ACIP, tasked with navigating these shifting probabilities, faced an impossible position. Public health committees must maintain trust while adapting to evolving scientific understanding. Yet how can this trust endure when fundamental assumptions underlying recommendations change? How should committees balance known short-term benefits against unknown long-term risks in populations where the disease itself poses minimal danger?
It’s worth noting that the virus itself evolved during this period, with different variants presenting varying risk profiles. However, the fundamental mathematical principle remained: interventions must be evaluated based on the actual risk to the population receiving them, not on fears or assumptions carried over from higher-risk groups.
Robert F. Kennedy Jr.’s decision to dismiss the entire committee suggests these questions had no satisfactory answers within the existing framework. The statement accompanying the dismissal emphasized “unbiased science” and “transparency,” implying these qualities had been compromised. Whether this assessment is fair depends on one’s interpretation of the committee’s actions during a period of unprecedented uncertainty.
What remains clear is that the global approach to pediatric COVID vaccination revealed fundamental tensions in public health policy. When Dr. Jones noted that Austria threatened citizens with fines of 3,000 euros and potential imprisonment for non-vaccination, she highlighted a government system that had departed so far from its original risk assessments that extraordinary measures seemed justified.
The firing of the ACIP represents more than administrative housecleaning. Other nations observing this development face their own reckonings with decisions made when certainty was claimed but uncertainty reigned. The challenge moving forward is not simply about COVID vaccines but about how public health policy incorporates risk stratification and acknowledges uncertainty.
In medicine, as in life, prior probabilities matter. When historical data is ignored — when low-risk populations are treated as high-risk ones, when definitions change to fit evolving narratives, when coercion replaces consent — the foundational trust upon which public health depends erodes. This erosion affects not just COVID policy but the entire vaccination enterprise, potentially undermining decades of progress against genuinely dangerous childhood diseases.
The ACIP’s reconstitution offers an opportunity to rebuild that trust. Success will require renewed commitment to the foundational principle that medical recommendations must flow from evidence, not from what we wish were true or what seems politically expedient. It will require acknowledging that one-size-fits-all approaches rarely fit all, and that respecting individual risk assessments strengthens rather than weakens public health.
Without this foundation, even the most well-intentioned policies risk becoming exercises in what Bayes would have recognized as the most dangerous kind of reasoning: certainty in the face of uncertainty.
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