A natural experiment found pediatric influenza vaccination reduced influenza diagnoses among children aged 2 to 5 years.
A natural experiment using US insurance claims data suggests seasonal influenza vaccination meaningfully reduces influenza diagnoses among young children, adding evidence for annual pediatric flu vaccination in a population with high transmission and variable vaccine uptake.1
For every 100 children aged 2 to 5 years who received influenza vaccination in a given flu season, researchers estimated 9 to 14 fewer influenza cases, according to findings reported by Harvard Medical School and published June 1, 2026, in JAMA Pediatrics.1
“In the United States, that’s hundreds of thousands, if not a million cases of flu that we can avoid each year,” senior author Anupam Jena, MD, PhD, the Joseph P. Newhouse Professor of Health Care Policy in the Blavatnik Institute at Harvard Medical School, said in a press release.1 “That’s a huge effect size.”
Key Facts
The study used timing of birthdays as a quasi-random source of variation in vaccination. Young children commonly receive annual well-child visits around their birthdays. Children born in the fall are more likely to have those visits when influenza vaccines are available, whereas children born in summer may need a separate visit to be vaccinated. Prior work by the investigators suggested this timing difference affects uptake, creating a natural experiment in which vaccination rates vary for reasons not directly related to underlying health status.1
Investigators compared insurance claims among children aged 2 to 5 years with summer vs fall birthdays across 5 influenza seasons from 2016 through 2023, excluding the 2020-2021 and 2021-2022 seasons because of COVID-19–related confounding. Across seasons, fall-born children had influenza vaccination rates 8.6 to 12.5 percentage points higher than summer-born children. They also had influenza diagnosis rates 1.0 to 1.4 percentage points lower.1
“Across these five seasons, we see that for every hundred kids who are randomly vaccinated because of when their birthday falls, somewhere between nine and 14 of them avoid a case of the flu that they otherwise would have caught,” Jena said.1
The investigators also examined conditions without vaccines, including common cold and gastrointestinal viral illness, and found no comparable differences between summer- and fall-born children. That negative-control finding supports, but does not prove, the interpretation that the influenza difference was related to vaccination rather than broader differences in care-seeking or exposure.1
The findings arrive amid continued debate over childhood vaccine policy and uptake. The Advisory Committee on Immunization Practices has recommended annual influenza vaccination for all persons aged 6 months or older without contraindications, with any age-appropriate licensed influenza vaccine.2 For children, vaccination is intended not only to reduce symptomatic illness but also to decrease missed school, caregiver work disruption, outpatient visits, and severe outcomes among those at elevated risk for complications.
Influenza vaccines used in children include inactivated influenza vaccines and, for eligible children, live attenuated influenza vaccine. Effectiveness varies by season, age group, vaccine product, prior immunity, and antigenic match between vaccine strains and circulating viruses.2 This study does not estimate product-specific effectiveness, nor does it assess hospitalization, intensive care, mortality, or laboratory-confirmed influenza.
Christopher Worsham, MD, MPH, assistant professor of medicine at Harvard Medical School and Massachusetts General Hospital and first author of the study, framed the analysis as evidence generated from existing clinical data. “The federal government cited an absence of evidence that they want to see, and so we have provided that,” Worsham said. “We have randomized data, and it shows that flu vaccines are effective for these young children.”1
Clinically, the analysis is notable because it focuses on real-world vaccine uptake and outcomes rather than trial efficacy under controlled conditions. However, the design remains observational. Birthday timing may approximate random assignment for vaccination opportunity, but claims-based influenza diagnoses can miss undiagnosed infections and may reflect testing patterns, coding practices, or care access. The analysis also cannot establish effectiveness in older children, adolescents, or adults because the birthday–well-child visit relationship weakens after approximately age 5, according to the investigators.1
The next questions are whether similar natural-experiment approaches can quantify effects on more severe pediatric outcomes and whether operational changes—such as reminder systems, vaccine-only visits, school-based programs, or coadministration during other encounters—can close uptake gaps among children whose routine visits occur before seasonal vaccine availability.
References
