Study Finds Flu Vaccine Sharply Reduces Infections in Children Ages 2 to 5

A study conducted by researchers at Harvard Medical School (HMS) has found pediatric flu vaccines significantly reduce the number of childhood cases of influenza. Specifically, for every 100 children vaccinated, between nine and 14 fewer children catch the flu. The findings provide additional support for the flu vaccine at a time when childhood vaccines have come under scrutiny in the U.S.

“In the United States, that’s hundreds of thousands, if not a million cases of flu that we can avoid each year,” said senior study author Anupam Jena, the Joseph P. Newhouse professor of Health Care Policy in the Blavatnik Institute at HMS. “That’s a huge effect size.”

In the study, the researchers compared insurance claims data for summer-born and fall-born children between the ages of 2 and 5 over five flu seasons between 2016 and 2023. (The 2020–2021 and 2021–2022 seasons were skipped because of confounding factors from COVID-19.) In each season, fall-born children were more likely to be vaccinated and less likely to catch the flu. The vaccination rates for children with fall birthdays were between 8.6 and 12.5 percentage points higher than those with summer birthdays, and the influenza diagnosis rates were 1.0 to 1.4 percentage points lower. According to the researchers, after about age 5, the influenza diagnosis rates between fall- and summer-born children start to even out.