Kids with autumn birthdays are more likely to get the annual flu shot and therefore less likely to catch the virus compared with those born in the summer, according to a study published today in JAMA Pediatrics.
The Centers for Disease Control and Prevention (CDC) removed the flu vaccine from its list of recommended childhood immunizations in January, citing a lack of evidence that the vaccine prevents serious illness and deaths in children. Now the agency recommends “shared clinical decision-making” between a clinician and a patient or their guardian, though that recommendation is currently on hold following a court ruling in March.
By leveraging the randomness of the month a child is born, the study’s authors say they were able to conduct a “natural experiment” that provides strong evidence that the flu shot is an effective intervention against a disease that resulted in 280 pediatric deaths during the 2024-25 flu season.
Year after year, the flu shot keeps kids healthy
Previous research shows that if a kid sees the doctor earlier in the year, they’re less likely to get the flu vaccine. This is because young kids are more likely to have an annual checkup around their birthdays, said senior author Anupam B. Jena, MD, PhD, a physician and health policy researcher at Harvard University. Therefore, children born in the fall are more likely to get an annual flu shot, which is released in mid-August.
Children with summer birthdays, on the other hand, might need to make an additional appointment to get their flu shot.
Jena and his collaborators built on that knowledge by analyzing data from five typical influenza seasons from 2016 to 2023. The study excluded the 2020-21 and 2021-22 seasons because of COVID-19. They found that for every 100 children aged 2 to 5 who received the shot, there were nine to 14 fewer influenza cases among that age group.
For example, during the 2022-23 season, 50.8% of kids in the 2 to 5 age group born between September and November were vaccinated against influenza, compared with 38.3% of those with birthdays in June, July, and August. In the first group, 7% of kids got the flu, compared with 8.4% in the second group.
Then the authors scaled those results to account for the fact that not all kids are vaccinated. They found that during the 2022-23 flu season, for every 100 kids in the 2 to 5 age group who were vaccinated, there were 10.9 fewer flu cases.
Because the vaccine’s efficacy varies year to year, the study found that the number of cases the flu shot prevented also varied, ranging from 9.3 during the 2016-17 season to 14.3 during the 2019-20 season.
These findings are statistically significant, said Jena.
“I appreciate the need to have the highest quality evidence possible to guide clinical decisions,” he said.
Jena and his colleagues say that by using recent randomized observational data, they were able to show that the flu vaccine protects kids from the virus without needing to conduct an expensive clinical trial every year.
