When is the best time for kids to get flu vaccine?

Birth month is predictive of influenza vaccination timing in young children, owing to its impact on the scheduling of preventive care visits, reports a US study. Specifically, children born in October are more likely to be vaccinated in the same month and less likely to be diagnosed with influenza, supporting recommendations that promote October vaccination.

“The findings support current recommendations that children be vaccinated in October preceding a typical influenza season,” the researchers said.

This population-based cohort study examined commercially insured children aged 2‒5 years who were vaccinated against influenza during 2011‒2018. The researchers assessed the optimal timing of influenza vaccination in this population.

A total of 819,223 children received the flu vaccine. Those vaccinated in November and December were least likely to catch influenza, but this finding may be confounded by unmeasured factors influencing the timing of vaccination and risk of influenza. Vaccination was usually administered on days of preventive care visits and during birth months. [BMJ 2024;384:e077076]

October-born children were commonly vaccinated in the same month and were, on average, immunized later than those born in August and earlier than those born in December. Children born in October were also least likely to have an influenza diagnosis (2.7 percent vs 3.0 percent for those born in August; adjusted odds ratio, 0.88, 95 percent confidence interval, 0.85‒0.92).

Flu season

In studies assessing vaccine effectiveness, results showed that immunity against influenza diminishes over the course of the flu season. Moreover, vaccinated patients who have been exposed to the virus before acquiring vaccine-induced immunity were at greater risk of infection. [Vaccine 2015;33:246-251; Vaccine 2016;34:3907-3912; Clin Infect Dis 2017;64:544-550; Clin Infect Dis 2021;73:726-729]

“This suggests that an optimal time may exist for children to be vaccinated before the flu season and how this question could be evaluated using observational data and quasi-experimental empirical methods,” the researchers said.

“Under the assumption that children born in October are otherwise similar to children born in other months, our findings suggest that the specific timing of influenza vaccination among children born in October may lead to lower rates of influenza infection,” they added.

Earlier studies suggested that the convenience of receiving immunization at preventive care visits, usually timed near birthdays, tend to result in higher vaccination rates. [N Engl J Med 2020;383:184-185]

Such approach may also influence the timing of vaccination among children who have been vaccinated, according to the researchers.

“Taking these studies together, children born in October seem to have two advantages when it comes to influenza vaccination that likely lead to lower infection rates: they are more likely to be vaccinated at all because it is more convenient, and, conditional on vaccination, they are more likely to have optimal vaccine induced immunity because of the specific timing of their vaccination,” they added.

“Our 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 researchers said.