The US Centers for Disease Control and Prevention recently removed annual influenza vaccination from its childhood recommendations,1 citing concerns about lack of contemporaneous randomized data to support annual influenza vaccination and historical reliance on observational data to support vaccine efficacy (ie, annual comparisons of influenza vaccination rates among individuals with influenza-like illnesses testing positive vs negative for influenza), which may be confounded.2,3 One way to use observational data to assess annual influenza vaccine efficacy in children that is less susceptible to confounding is to identify instances in which some children are, by chance, vaccinated while others are not—a natural experiment.4 [JAMA Pediatrics]
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Young children tend to have annual well-child visits timed around their birthday. This visit can be a convenient opportunity for influenza vaccination among children with birthdays in the fall, when the vaccine for the upcoming season is available at a well-child visit. In contrast, children with summer birthdays (and summer well-child visits) must make an additional appointment to get vaccinated, leading summer-birthday children to be effectively randomized to a more burdensome pathway to vaccination compared with otherwise similar children with fall birthdays. Prior research has shown that children aged 2 to 5 years who have a fall birthday are both more likely to receive influenza vaccination and less likely to be diagnosed with influenza compared with children with summer birthdays, despite both groups of children being similar on a broad range of observable characteristics—quasi-experimental evidence of vaccine efficacy.4 We used this natural experiment to assess the efficacy of influenza vaccination in several recent individual influenza seasons.
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