Flu vaccines significantly reduce the number of illnesses in children, new research from Harvard Medical School shows.
The findings underscore the public benefits of the vaccines at a time when vaccine hesitancy has driven an increase in deadly and preventable diseases, including a resurgence of once-eliminated measles.
Vaccine hesitancy is largely fueled by misinformation and the growing anti-vaccine movement.
For every 100 children aged 2-5 who get the shot or nasal spray, there are between 9 and 14 fewer cases, they found.
“In the United States, that’s hundreds of thousands, if not a million cases of flu that we can avoid each year,” Anupam Jena, the Joseph P. Newhouse Professor of Health Care Policy in the school’s Blavatnik Institute, said in a Monday statement. “That’s a huge effect size.”
The US. Centers for Disease Control and Prevention also removed recommendations for annual vaccines earlier this year — a move that was later blocked by a U.S. District Court.
“The federal government cited an absence of evidence that they want to see, and so we have provided that,” Christopher Worsham, an assistant professor of medicine at Massachusetts General Hospital, said. “We have randomized data, and it shows that flu vaccines are effective for these young children.”
The analysis compared flu vaccination and diagnosis rates in young children born at different times of the year using national insurance claims data. Sorting children by their birth was strategic because young children typically have an annual visit to the doctor at around that time.
The flu vaccines are released in the fall, so that’s convenient for kids with fall birthdays. But kids in the summer will need an additional appointment to get their vaccine. That results in lower flu vaccination rates for summer birthdays, the researchers said.
Over the course of five years between 2016-2023 – skipping 2020-2022 because of the Covid-19 pandemic – they found insurance claims showed fall children were more likely to be vaccinated: with vaccination rates from 8.6-12.5 percent higher than their summer counterparts.
“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 explained.
At about age five, the vaccination rates start to even out because birthdays are less aligned with doctor appointments.
While seasonal vaccine effectiveness changes each year, getting the vaccine reduces the risk of illness and more serious outcomes disproportionately seen in children.
The flu vaccination prevented 10 million illnesses and 12,000 deaths for all age groups during the 2024-2025 flu season, according to the CDC.
“The most recent data indicate that flu vaccination reduced the risk of flu-related death by more than 75 percent among children with underlying, higher risk medical conditions and by more than 85 percent among healthy children,” the agency says.
Still, vaccine hesitancy has contributed to years-long lowered vaccination rates that led to a record number of child flu deaths from 2024-2025.
The 2025-2026 flu season has seen just a fraction of those deaths, but deaths from the previous season were still being reported earlier this year.
Nearly 90 percent of child deaths were in kids who were not fully vaccinated, researchers said in September. That number is 85 percent this year.
