A Massachusetts scientific study discovers best month for flu shot

Science and Health

The rate of Israelis getting their annual shots against influenza is very low, apparently because they (and others around the world) are tired of the multiple vaccinations against COVID-19. As a result, more than 700 Israelis, including children, are now hospitalized due to flu complications, and some of them are even in intensive care. 

Now, a first large-scale analysis in Boston of optimal timing for flu shots finds October is the best month for children to get vaccinated against the flu. The study of 800,000 visits to pediatricians identified links among children’s birth month, annual physical schedule, and vaccination timing.

Children born in October are most likely to be vaccinated for the flu in October and are least likely to be diagnosed with influenza, according to results of the first large-scale study of optimal timing for the flu shot.

“There are a lot of variables when it comes to the timing and severity of flu season or a person’s risk of getting sick, and many of those are out of our control,” said healthcare policy Prof. Anupam Jena of Massachusetts General Hospital, and senior author of the study and Harvard Medical School and critical care Prof. Christopher Worsham of Massachusetts General Hospital. “One thing we have some control over is the timing of the shot,” Jena said, “and it looks like October is indeed the best month for kids to get vaccinated against the flu.”

Thousands of flu-related hospitalizations and death

Last month, the US Centers for Disease Control and Prevention reported over 150,000 hospitalizations and 9,400 deaths due to flu as of the time of the report and noted that high demand for hospital care for the flu has contributed to strained hospital capacity in some parts of the country. Over the past decade, almost 200 US children have died of influenza each flu season. Across the years, most children who die are not fully vaccinated against the flu.

Doctor giving a vaccine to a patient (illustrative) (credit: INGIMAGE)

Part of the reason the timing of the shot is tricky is the way the immune system responds to a vaccine. If a person gets the shot too early, their immunity may fade by the time flu season peaks. If they wait too long, their body may not have time to build immunity strong enough to protect against the peak level of infections.

Public health recommendations in the US have long promoted September and October flu shots, but Jena said there has never been a randomized clinical trial to test the best timing nor a large-scale effort to see how likely people who get vaccinated in other months are to get sick. When Jena was at a late summer meeting in 2022, he mentioned that his arm was sore from getting his flu shot. A colleague asked whether he was concerned about his immunity waning before flu season.

“It hadn’t occurred to me to check if one month or the other might make a big difference,” Jena recalled. “When we looked at the science, we were surprised that no one had ever looked at the question in a big population.”

Young children in the US tend to get their yearly checkup around their birthday, and that’s also when they get most of their vaccines. Children with spring and summer birthdays often don’t get the flu shot because it’s not available when they go for their annual visit, and many parents don’t make an extra trip for it.

The findings suggest that US public health interventions focused on vaccination of young children in October may yield the best protection in typical flu seasons, the authors insisted.