Pediatric influenza vaccination effectively protects young children

Pediatric flu vaccines significantly reduce the number of childhood cases of influenza, new research from Harvard Medical School confirms. The findings, published on June 1 in JAMA Pediatrics, show that for every 100 children vaccinated, between nine and 14 fewer children catch the flu.

In the United States, that's hundreds of thousands, if not a million cases of flu that we can avoid each year. That's a huge effect size."

Anupam Jena, senior study author, the Joseph P. Newhouse Professor of Health Care Policy, Blavatnik Institute, HMS

The findings provide additional support for the flu vaccine at a time when childhood vaccines have come under scrutiny in the United States. This January, the U.S. Centers for Disease Control and Prevention removed the annual influenza vaccine, as well as several others, from its childhood schedule of recommended vaccines. That change, which was widely condemned by medical societies and public health organizations, was blocked by a U.S. District Court in March.

"The federal government cited an absence of evidence that they want to see, and so we have provided that," said Christopher Worsham, HMS assistant professor of medicine at Massachusetts General Hospital and first author on the study. "We have randomized data, and it shows that flu vaccines are effective for these young children."

Sorting by birthday

Young children typically have an annual visit to the doctor scheduled around their birthday. For children born in the fall, those visits are a convenient time to get the flu vaccine. But children born in the summer will likely have appointments before the flu vaccine becomes available - they need an additional appointment to get vaccinated.

In previous research, Jena and Worsham found that the additional burden on caretakers results in lower flu vaccination rates in summer-born children. This creates a natural experiment, randomly sorting children into more- or less-vaccinated groups based on the happenstance of when they were born.

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. (They skipped the 2020-2021 and 2021-2022 seasons 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.

"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," said Jena, who is also a professor of medicine at Mass General. 

For other illnesses that do not have vaccines, such as the common cold or gastrointestinal viruses, there was no difference in the infection rates of the two groups.

"It comes down to: vaccines work," Worsham said.

More natural experiments

As children get older, birthdays and doctor's appointments stop being so closely aligned. The researchers note that after about age five, the influenza diagnosis rates between fall- and summer-born children start to even out.

"The randomized data we have is limited to these very young children because their doctor appointments are tied to their birthday," Worsham said.

That doesn't mean that the vaccine isn't effective in older children, teenagers, or adults - it definitely is, Worsham said. But after a certain age, birthdays are no longer good indicators of whether someone is more or less likely to have received the flu vaccine, so this experiment can't capture vaccine effectiveness in older groups.

This work is just one example of a randomized experiment that can be found in existing data - there are many similar opportunities across different fields of medicine, the researchers said. Jena and Worsham are frequent collaborators and have co-authored a book, Random Acts of Medicine, on the subject.

"It is impossible to do a randomized controlled trial for every single thing that we want to know and understand," Jena said. "But we have an incredible amount of data out there and there are randomized experiments like this sitting in that data, waiting to be uncovered."