Evidence, Not Guesswork: The Childhood Vaccine Schedule is Very Intentional

Responding to the claim that “we haven’t tested the full schedule” with the layers of evidence we have

Over the weekend, I (Jess) watched Free Guy with my husband and kids. (My vote was for Miss Congeniality, but I was outnumbered in our household’s democratic process.)

If you haven’t seen it, the premise is that Ryan Reynolds plays Guy, a cheerful bank teller who doesn’t realize he’s a background character in a video game. When he figures it out, he has to “level up” fast—collecting weapons, skills, and power-ups—to take on the greedy game developer trying to destroy his world.

With ACIP meeting this week, it feels like public health is in a perpetual state of leveling up. We don’t know exactly what’s coming, but we know we need to be ready. You may have noticed an uptick in our social media posts and Substacks lately—that’s no accident. The best defense is a good offense, and we’re doing our part to stockpile the arsenal: evidence, context, and clear explanations so that when news breaks, you’re not starting from zero.

This article is one of those power-ups. The childhood vaccine schedule is under unprecedented scrutiny right now—critics are calling it arbitrary, claiming it’s not evidence-based, arguing it hasn’t been properly studied. This piece breaks down how the schedule is actually built, and why those claims don’t hold up.

Parents face an overwhelming amount of conflicting vaccine information, so it’s natural for questions to surface: Why are some vaccines given early? Why are multiple vaccines given at the same visit? Would spacing them out be safer? But what’s often missing from the conversation is how the schedule is built. U.S. vaccine recommendations are not arbitrary, and they don’t rely on assumptions. They’re built on data from clinical trials and ongoing safety monitoring. Let’s discuss…

What Is a Vaccine Schedule?

Child, teen, and adult vaccine schedules are federally-recommended roadmaps that outline the following:

• Which vaccines are recommended, and how many doses are needed

• How far apart doses should be spaced

• At what age(s) each vaccine dose is recommended

• Any exceptions or special circumstances

For the purpose of this article, we’ll focus mostly on the childhood vaccine schedule.

Right Time → Best Protection

The vaccine schedule isn’t just about which vaccines children receive. It’s also about when they receive them. Each dose is timed to protect children before they’re most likely to encounter certain infections and at the ages when their immune system responds most effectively.

It’s also designed to be practical. Many vaccine doses are aligned with routine well-child visits to make staying on track easier for families.

Spacing out vaccines isn’t safer

Spacing out vaccines or delaying them doesn’t provide extra safety or a better immune response.

In fact, a delayed schedule:

• Extends the amount of time a child remains vulnerable to infections. For example, children who miss or delay the diphtheria, tetanus, and pertussis (DTaP) vaccine are significantly more likely to get pertussis.

• Lowers the likelihood of completing the schedule. Spacing out doses makes it less likely that the vaccine schedule will be completed.

• Increases the number of healthcare visits. Spacing out vaccines means more clinic appointments. When vaccines are timed with routine well-child visits, children are more likely to receive all recommended doses, ensuring they’re protected from infections as early as possible.

• Limits protection for vulnerable groups. Delays leave both infants too young for vaccination and immunocompromised children more exposed.

Why some vaccines are timed for early infancy

Young infants have limited immune defenses. Infections like pertussis (whooping cough), Haemophilus influenzae type B (Hib) disease, and pneumococcal disease are more likely to cause severe disease in infants than in older children.

That’s why these vaccines (among others) are typically given at 2 months of age, with follow-up doses at 4, 6, and 12 to 15 months. Each dose builds on the last, and completing the full series provides long-lasting immunity.

Hepatitis B vaccination begins even earlier, within 24 hours of life. This is because hepatitis B virus (HBV) can be transmitted during delivery or in early infancy. Newborns who contract HBV have up to a 90% risk of developing chronic infection, compared with only 5% to 10% of older children and adults. Chronic HBV infection is serious. It can lead to severe liver damage and cancer later in life, and about 1 in 4 children with chronic HBV infection will eventually die from HBV-related complications.

So why does the birth dose matter? Even if you tested negative for hepatitis B during pregnancy, infection can occur after screening but before delivery. And newborns may be exposed to people who aren’t aware they’re infected, including family members and healthcare workers. Because the consequences of HBV infection in infants are so serious, the birth dose serves as a crucial safety net.

Why some vaccines are best given in older babies and children

Not all vaccines work well in infancy. The measles, mumps, and rubella (MMR) vaccine is one example. When given before 12 months, the immune response is weaker. That’s why the first dose is given at 12 to 15 months, which is about 94% effective at preventing measles.

This second dose at 4 to 6 years boosts protection to over 99%, and is timed before children typically enter school when measles exposure risk increases.

Vaccines timed for preteens and teens

Some vaccines are intentionally given to older kids and adolescents because risk increases around these ages.

For example, the human papillomavirus (HPV) vaccine is recommended at 11 to 12 years old, and it can begin as early as age 9. Vaccinating at this age protects preteens before potential exposure to HPV and generates strong immune responses. It’s also a long-term cancer-prevention strategy, protecting against several cancers caused by HPV, including cervical and penile cancer.

Another example is the meningococcal conjugate vaccine (MenACWY), given at 11 to 12 years old and again at 16. This timing strategy provides strong protection when meningococcal disease risk rises, particularly during the teen years, when adolescents spend more time in close contact with peers and later move into college dorms or other shared living environments.

Who Creates the Vaccine Schedule?

A highly unusual shift in 2025

In June 2025, an unprecedented change occurred: HHS Secretary Robert F. Kennedy Jr. removed all 17 sitting voting members of the Advisory Committee on Immunization Practices (ACIP), the federal advisory committee that develops U.S. vaccine recommendations. No previous administration has ever replaced the full ACIP committee at once. This move stands in direct contrast to ACIP’s intentionally staggered 4-year term structure, which was designed to preserve stability and prevent the entire committee from being overturned at once. As of September 2025, RFK had appointed 12 new voting members.

Also in 2025, the American Academy of Pediatrics (AAP) began publishing its own childhood vaccine schedule. Historically, the AAP endorsed the federal schedule rather than producing a separate one. The 2025 shift reflects the AAP’s longstanding role in providing pediatric vaccine guidance, even as federal agencies and advisory committees like ACIP undergo changes. The CDC schedule remains the official federal recommendation, and the current AAP schedule mirrors the CDC’s recommendations closely. But it will be important to monitor whether any differences emerge over time.

How the process has historically worked

Under normal circumstances, U.S. vaccine schedules are developed by ACIP, a volunteer advisory committee housed within the CDC. Traditionally, ACIP reviews clinical trial data, real-world effectiveness studies, and safety information on each vaccine before making its recommendations.

The process typically goes like this:

1. After reviewing the available research and data, ACIP votes on whether a vaccine should be recommended, for whom, and at what ages.

2. Then, the CDC director makes the final decision. Historically, CDC directors have always accepted ACIP’s recommendations for child, teen, and adult vaccine schedules.

ACIP and CDC recommendations directly influence vaccine access. Most private insurance plans, Medicare, and Medicaid are required to cover recommended vaccines at no cost to the consumer. Uninsured or underinsured children can receive all recommended vaccines through the Vaccines for Children Program (VFC) at no cost.

Who sits on ACIP?

ACIP includes up to 19 voting members, all appointed by the Secretary of Health and Human Services (HHS). Members are selected for expertise in a variety of fields, including pediatric, infectious disease, public health, vaccine research, and more. One voting member must be a consumer representative, bringing the perspective of the general public.

Voting members of ACIP can’t be employed by vaccine manufacturers or hold relevant patents, and all potential conflicts of interest should be disclosed publicly. ACIP also includes non-voting members from federal agencies (such as the FDA and NIH) and from professional groups like the American Academy of Pediatrics (AAP).

How members are usually appointed

Individuals may apply or be nominated. When positions open up, the HHS secretary selects new members. Voting members can serve four-year terms and are intentionally staggered. This is meant to prevent the entire committee from being replaced at once, preserving continuity and stability even as political administrations change.

How Vaccines Get Added to the Schedule

Understanding how a vaccine makes it onto the schedule explains why each dose is timed so precisely, and why following that timing matters. Each vaccine goes through a multi-step scientific process, which we’ll describe below.

This is the framework that has historically guided U.S. vaccine policy for decades. But recent changes within ACIP, the CDC, and other federal agencies mean that the consistency with which this process is applied now depends on the stability and independence of these bodies. The process still exists, but its reliability depends on the people and institutions that carry it out.

Step 1: Vaccines gain FDA-licensure, answering “Does the vaccine work and is it safe?”

Before ACIP can officially recommend a vaccine, it must be licensed by the FDA.

FDA vaccine licensure (approval) is typically based on the results of Phase 1 through Phase 3 clinical trials. This means the vaccine has been tested in thousands of people for safety and effectiveness.

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Vaccine manufacturers also have to show the vaccine is safe and effective at the specific age for which FDA approval is sought.

Step 2: ACIP Work Groups review the data, answering “What does the evidence show?”

Anyone can request that ACIP evaluate a vaccine. But most requests come from the CDC, FDA, or ACIP staff. Before a vaccine is brought to the full committee for a vote, it is evaluated in detail by the ACIP Work Groups. These are subcommittees made up of ACIP members, FDA and/or CDC staff, and other experts. Work Groups review:

• Safety and effectiveness of a vaccine at specific ages

• The severity of the disease that the vaccine prevents

• How common the disease would be without vaccination (disease burden)

To evaluate this evidence, ACIP Work Groups use two structured frameworks: GRADE (Grading of Recommendations, Assessment, Development, and Evaluation) and EtR (Evidence-to-Recommendation).

• GRADE helps committee members determine how strong the evidence is. It’s an internationally used system to rate both the quality of research and how confident we can be in the results. Evidence is graded as high, moderate, low, or very low.

• EtR helps answer “Based on the evidence, what should we recommend?” It evaluates information beyond raw data, like how large the disease burden is, who’s at risk, the cost-effectiveness of the vaccine, and equity considerations.

The CDC publishes GRADE tables and any available EtR documents for each vaccine. Because ACIP only introduced the EtR framework in 2018, many older vaccines don’t have EtR documents. However, all ACIP recommendations, past and present, are documented in the CDC’s Morbidity and Mortality Weekly Reports (MMWR), a long-running series of publications that contains timely public health information and guidance.

Step 3: ACIP votes, answering “What does the committee recommend?”

Work Groups present their findings at full ACIP committee meetings, which are held three times a year. Each meeting usually lasts two days. They are open to the public, and anyone can watch them live through a CDC webcast.

During these meetings, ACIP voting members vote on whether to recommend a vaccine and where to place it on the schedule. But ACIP may delay a vote if the vaccine in question hasn’t been FDA-approved yet, or if new data isn’t yet publicly available.

Upcoming meeting dates and agendas are posted here. The public can submit written comments in advance or make oral comments at the meetings during designated times.

Step 4: The CDC director approves the ACIP recommendations

ACIP recommendations become part of the official U.S. immunization schedule only after the CDC director approves them.

Once approved, recommendations are published in the CDC’s MMWR along with the scientific rationale and supporting evidence.

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What to consider when someone says, “We haven’t studied the full vaccine schedule.”

You may hear people say that we haven’t studied the whole vaccine schedule and assume that means we’re operating in the dark. That isn’t the case. While we may not have all the evidence we’d like to see in an ideal world, we have plenty to indicate that the current vaccine schedule is safe.

Why we don’t have that one ‘perfect’ study:

Let’s call it out upfront: we’ll likely never have a giant study comparing the current vaccine schedule to an alternative, for both ethical and practical reasons:

• Ethics of clinical trials: Withholding vaccines or recommending delayed schedules would violate medical ethics–specifically Article 33 of the Declaration of Helsinki, which is the international medical ethics guidelines for human research. Any clinical trial testing an alternative schedule would first require preliminary evidence suggesting it could be equal to or better than the current one–and we don’t have that evidence.

• Observational studies are complicated: The current, approved schedule already gives windows of time in which each dose should be received, so people on a delayed schedule likely end up vaccinating at overlapping times anyway. You can’t isolate the independent effect of a delayed schedule from other factors unless everyone follows the exact same delayed schedule, which doesn’t occur naturally. And comparing different countries’ schedules? This is similarly problematic due to countless unmeasured confounding factors, such as healthcare access and cultural differences, rendering it impossible to isolate the independent effect of the different vaccine schedules.

Understanding the burden of proof:

In reality, we can’t prove that the current vaccination schedule is “best”. Doing so would require us also to test every imaginable alternative schedule, which would be unethical and expensive. The current schedule is the best evidence-supported option, and the burden of proof lies with anyone who is proposing an alternative one.

The evidence we do have:

A 2013 Institute of Medicine (now National Academy of Medicine) report acknowledged the limited schedule-wide research. However, after reviewing 421 candidate articles, the committee found no evidence of harm and concluded the recommended schedule is safe, while also calling for additional high-quality studies. What we do have is substantial evidence about the vaccine schedule as a whole:

• New vaccine trials: When new vaccines are tested, participants receive all other scheduled vaccines too. This means each trial is effectively evaluating the full existing schedule (standard of care) plus the new vaccine.

• Concomitant use studies: Regulators require that vaccines are studied at the ages and combinations in which they are actually used in the real world–alongside other vaccines in the schedule. For example, the MenACWY vaccine has been studied alongside tetanus, diphtheria, and pertussis (Tdap) and HPV, and getting these vaccines at the same visit has been shown to be safe.

• A large 2022 study of nearly six million childhood vaccine doses found that giving routine vaccines at the same visit didn’t lead to more severe side effects in most cases. Some minor short-term effects occurred slightly more or less often with co-administration, but the overall data support the current schedule.

• A 2011 German national health survey of more than 13,000 children and adolescents compared unvaccinated and vaccinated children. The difference? Unvaccinated kids had a far greater incidence of vaccine-preventable diseases. When researchers looked at other measures (like infections that aren’t preventable by vaccines), there were no meaningful differences between the groups. The study was limited by the very small number of unvaccinated participants. Still, its findings are consistent with other research.

• A federal 2021 Agency for Healthcare Research and Quality (AHRQ) review of 338 studies evaluated the safety of all routinely recommended U.S. vaccines across children, adolescents, adults, and pregnant people. It found no evidence of new or unexpected safety concerns, and the risks of serious adverse events remained extremely rare and consistent with long-established data. The authors emphasized inherent limits in detecting extremely rare events but concluded overall that the evidence supports the strong safety profile of the routine U.S. immunization schedule.

Beyond these individual studies, we have large safety-monitoring systems that track the entire schedule in real time across millions of children. So while we don’t have one perfect study, the evidence we do have is broad, layered, and far stronger than the claim that “we haven’t studied the full schedule” suggests.

How Vaccines Are Continually Monitored

Vaccine safety and effectiveness continue to be evaluated long after a vaccine is FDA-approved. Historically, ACIP has reviewed the full schedule annually. And it can update recommendations at any meeting if new data emerge. Below, we’ll discuss a few ways vaccine safety is continually evaluated:

• Post-marketing safety monitoring. After licensure, the FDA can require vaccine manufacturers to conduct Phase 4 studies to evaluate rare side effects or the duration of immunity provided from the vaccine, for example. If significant safety concerns arise, a vaccine can be removed from the schedule.

• Manufacturer reporting requirements. Federal regulation requires manufacturers to report serious or unexpected adverse events within 15 days, report all adverse events quarterly for the first 3 years, and provide annual reports thereafter.

• U.S. vaccine safety surveillance programs. The U.S. relies on several complementary systems to monitor vaccine safety:

  • Vaccine Adverse Event Reporting System (VAERS) is a system managed by the CDC and FDA that accepts reports from clinicians, manufacturers, and the public. VAERS reports capture early warning signs about vaccines and can detect unusual or unexpected patterns. But these reports alone don’t prove that vaccines caused a problem. More on that later.

  • Vaccine Safety Datalink (VSD) is a partnership between the CDC and large health systems such as Kaiser Permanente. Using electronic health records from millions of patients, VSD conducts rapid studies to evaluate safety signals identified in VAERS.

  • Clinical Immunization Safety Assessment Project (CISA) is a network of vaccine safety experts at academic medical centers that provide clinical consultation for complex, patient-specific vaccine safety questions and conduct research on rare adverse events.

• Global safety programs. The CDC and FDA can review vaccine safety information from international organizations, including the World Health Organization (WHO) and the European Medicines Agency (EMA). President Trump signed an executive order withdrawing the US from the WHO, with membership scheduled to end in January 2026. Although the U.S. is no longer a WHO member, WHO and EMA safety findings remain publicly available.

These systems have led to real changes. For example, the first rotavirus vaccine (RotaShield) was withdrawn from the market within a year after post-marketing data identified an increased risk of intussusception, a rare type of bowel obstruction.

Vaccine effectiveness is also continually reviewed. For example, when studies showed that younger adolescents mount a robust immune response to the human papillomavirus (HPV) vaccine, ACIP moved from a three-dose to a two-dose series that allowed vaccination beginning at 9 years old. Starting earlier can also make adolescents more likely to get both vaccine doses in the series.

Another example is the MenACWY vaccine. When evidence showed that immunity from the first dose at 11 to 12 years didn’t last, a booster dose at age 16 was recommended to ensure strong and long-lasting protection.

What else can be done to monitor vaccine safety?

Expanding active surveillance programs and increasing data sharing across health systems could strengthen the ability to monitor vaccine safety. Broader global collaboration could do the same.

Still, it’s important not to confuse the potential for improvement with a lack of rigor in the current processes. The U.S. already relies on multiple vaccine safety monitoring systems like VAERS, VSD, CISA, along with manufacturer reporting and post-marketing studies. These systems have successfully identified safety concerns and prompted action.

While systems can always be enhanced, this doesn’t imply that vaccines aren’t safe now. Rather, it reflects a core public health principle: continuous improvement as new technologies and data become available.

Putting It All Together

The science behind the vaccine schedule shows that every dose has a purpose: to protect children at the ages when each infection is most dangerous and when the immune system responds most effectively. Staying on schedule means aligning with the best available evidence to reduce vaccine-preventable disease.

The system that creates and monitors these recommendations includes rigorous clinical trials, ACIP review, FDA oversight, and post-marketing surveillance. These processes work together to keep guidance evidence-driven.

If you haven’t already, check out our Substack over the weekend that debunks nine common vaccine misconceptions:

9 Vaccine Myths That Won’t Go Away

Stay Curious,

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