Flu shots may be most effective in October, but it’s not too late – The Boston Globe

Although the vaccine works for several months, its potency is known to wane over time. So for maximum protection, you dont want to get it too far ahead of flu season. But if you wait too long, your immune system may not have enough time to rev up before it encounters the virus.

A Harvard research team asked the question when is the ideal time to get vaccinated against influenza? and found a clever way to answer it, at least for children. They realized that children tend to visit their pediatricians during their birth month, and that for children with birthdays in August through December, they typically get a flu shot during that visit.

Mining a vast trove of insurance claims, they found that kids who get their flu shot in October were least likely to get the flu. That points to October as possibly the ideal month to get a flu shot.

In one sense, its not surprising, said Dr. Anupam B. Jena, professor of health care policy at Harvard Medical School and senior author of the study, published Wednesday in the BMJ. October is about midway between when each years vaccine becomes available and when flu season typically begins. But through this study, he said, We were able to show, in a more convincing way, that this is actually the optimal time.

Jena thinks the finding probably applies to adults as well, but that question has not been studied.

He cautioned, however, that getting the vaccine at some point should be the first priority. The benefit of vaccination is much larger than the benefit of tinkering around with the timing, he said.

Small studies have suggested that antibody levels rise about two weeks after vaccination, but no one has examined what that means for protection against flu, Jena said. The most important question, he said, is whether the person gets severely ill with the flu and thats what his study looked at.

How fast the vaccines protection wanes depends on the individual and also the specific strain of flu. Most flu vaccines protect against more than one strain, so whether your vaccine remains effective may depend on which strain of flu you happen to encounter.

Dr. Edward Belongia, an infectious disease epidemiologist and vaccine researcher who was not involved with the study, called it well-designed with a robust methodology, and said it added useful new information about the ideal time to vaccinate children.

Its very helpful from a policy and planning perspective. Its not really too relevant in terms of parents decision-making, said Belongia, who is a senior research scientist with the Marshfield Clinic Research Institute in Wisconsin. He advises parents to simply trust their pediatricians advice.

The difference between September and October is going to be very modest. From a practical standpoint, I would not worry about it, he said. The most important thing is to get vaccinated before the flu season starts. That season typically starts around December with peak transmission in February or March, but the timing can vary significantly from year to year.

This year, flu cases nationally and in Massachusetts peaked in late December. The number of cases has fallen sharply since then, but remain plateaued at a high level. Only 40 percent of Massachusetts residents have been vaccinated against the flu this season. But its not too late to benefit from a flu shot even now, because flu is still widely circulating, Jena said.

Dr. Christina R. Hermos, an associate professor of pediatrics at the UMass Chan Medical School who specializes in infectious diseases and immunology, called the report a cool paper whose results are convincing. But she cautioned, If its summarized as a sound bite, it could be dangerous. I dont want people to think vaccines dont work unless given in October.

The study encompassed 819,223 children ages 2 to 5 who were vaccinated against the flu during 2011 through 2018. It focused on children with birthdays between Aug. 1 and Jan. 31 who were continuously enrolled in employer-sponsored health insurance for at least one flu season, defined as September through May.

The analysis found that October was the most common month for children to be vaccinated. Children born in October were more likely to be vaccinated that month, and were least likely to come down with flu during the ensuing season. Some 3 percent of children vaccinated in August got the flu, compared with 2.7 percent of those vaccinated in October.

The study gathered information only on children who had commercial insurance, and did not include uninsured children and children on public insurance such as Medicaid. It also could not account for any vaccinations that were not submitted for insurance reimbursement.

Dr. Caroline J. Kistin, a pediatrician with Hasbro Childrens Hospital in Providence and a health services researcher at the Brown School of Public Health, who was not involved with the study, said the research points to the need to build capacity to administer vaccines at optimal times to think about whether we need to invest more in community-delivered vaccinations.

Primary care practices might set up Saturday flu clinics in October, for example, she said.

Children whose annual pediatrician visit occurs in the spring may not be getting their shots at all, she noted. Theres a huge population of kids who just logistically end up not getting it. We dont make it in health care particularly easy to just pop in and get what you need.

Kistin noted that the worst time to get the shot is after being exposed to the flu. Getting it early is better than that, she said.

If a child came to her for a visit in September, she would tell the parent that its important to get vaccinated, and mention there is some added benefit to getting it in October. If the parent thinks it would be difficult to come back the next month, shed give the shot in September.

I dont want parents to feel stressed about getting it too early, Kistin emphasized. The findings, she said, are more important as a signal to the health care system to increase capacity during that key time.

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her @felicejfreyer.

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Flu shots may be most effective in October, but it's not too late - The Boston Globe