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Study Finds Flu Vaccine Reduces Influenza Cases in Young Children

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Study: Flu Vaccine Significantly Reduces Cases in Young Children

For every 100 vaccinated children, there were 9 to 14 fewer influenza cases.

A new study published in JAMA Pediatrics on June 1 confirms that the flu vaccine is highly effective at reducing influenza cases in children aged 2 to 5. The research was conducted by Anupam Jena, Christopher Worsham, and Charles F. Bray of Harvard Medical School.

The Natural Experiment

The researchers analyzed insurance claims data for children aged 2 to 5 over five flu seasons (2016–2023, excluding 2020–2022). They leveraged a unique natural experiment: children born in the fall are more likely to receive flu vaccines during birthday-related doctor visits, while summer-born children are less likely due to scheduling constraints.

This allowed the team to compare vaccination and illness rates between the two groups without the need for a traditional randomized controlled trial.

Key Results

  • Vaccination rates: Fall-born children had rates 8.6 to 12.5 percentage points higher than summer-born children.
  • Influenza diagnosis rates: Fall-born children had rates 1.0 to 1.4 percentage points lower.
  • No difference was found for non-vaccine-preventable illnesses like common colds or gastrointestinal viruses.

This suggests the observed difference in influenza cases is directly attributable to the vaccine, not other factors.

Context and Significance

The study arrives amid heightened scrutiny of childhood vaccines. In January, the CDC removed the influenza vaccine from its recommended childhood schedule—a decision blocked by a U.S. District Court in March.

The authors note that randomized controlled trials are not possible for every question, and similar natural experiments can be found in existing data.

“The natural experiment approach works only for children under 5, because older children's doctor visits are less tied to their birthdays.”

Bottom Line

The findings provide strong, real-world evidence that vaccinating young children against influenza yields meaningful public health benefits—reducing cases by roughly 9 to 14 per 100 vaccinated children.