Pediatric influenza vaccination effectively protects young children

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

In the United States, there are millions, if not millions, of cases of flu that we could avoid each year. That’s a huge effect size.”

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

These 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 US 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 the U.S. District Court in March.

“The federal government has cited a lack of evidence that they wanted to see, and so we’ve provided that,” said Christopher Worsham, MD, an assistant professor of medicine at HMS Medicine at Massachusetts General Hospital and the study’s first author. “We have randomized data, and it shows that flu vaccines are effective for these young children.”

sort by birthday

Young children usually 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 a flu vaccine. But babies born in the summer are likely to get an appointment before the flu vaccine is available – they will need an additional appointment to get vaccinated.

In previous research, Jenna and Worsham found that flu vaccination rates are lower in summer-born babies as a result of the added burden on caregivers. This is a natural experiment, in which children are randomly sorted into more or less vaccinated groups based on the incidence of the disease at the time of their birth.

Researchers compared insurance claims data from summer-born and fall-born children between the ages of 2 and 5 in five flu seasons between 2016 and 2023. (He skipped the 2020–2021 and 2021–2022 seasons due to factors related to COVID-19.)

Each season, children born in the fall are more likely to be vaccinated and less likely to catch the flu.

Vaccination rates for children with fall birthdays were between 8.6 and 12.5 percentage points higher than children with summer birthdays, and influenza diagnosis rates were 1.0 to 1.4 percentage points lower.

“Over these five seasons, we see that for every 100 children who are randomly vaccinated because their birthday falls on them, nine to 14 of them avoid a case of the flu that they otherwise might have,” said Jenna, MD, professor of medicine at Mass General.

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

“It comes down to this: The vaccines work,” Worsham said.

more natural experiments

As children get older, birthdays and doctor’s appointments stop matching each other. Researchers say that after about five years of age, influenza diagnosis rates begin to equalize between fall and summer-born children.

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

That doesn’t mean the vaccine isn’t effective in older children, teens or adults — it certainly is, Worsham said. But after a certain age, birthday is no longer a good indicator of whether someone is more or less likely to get a flu vaccine, so this experiment may not capture the vaccine’s 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 in different areas of medicine, the researchers said. Jenna and Worsham are frequent collaborators and have co-authored a book, Random Acts of MedicineOn the topic.

“It’s impossible to do randomized controlled trials for every single thing we want to know and understand,” Jenna said. “But we have an incredible amount of data and in that data there are random experiments like this, waiting to be uncovered.”