Birth Month and Influenza Vaccination in Children

TO THE EDITOR:

Annual influenza vaccination is recommended in young children.¹ Preventive care visits, which often occur near a child’s birthday, are a convenient time to administer vaccines. Children who have their birthday — and thus their recommended² yearly visit — before the vaccine becomes available may, by chance, be less likely to be vaccinated than children whose birthdays are later in the year, when the new vaccine is available. Therefore, influenza may be more likely to develop in children with earlier birthdays, and they may spread the virus to others.

We used the MarketScan database of commercial insurance claims to compare the timing of preventive visits, influenza vaccination, influenza diagnosis, and spread of influenza to family members according to birth month among children 2 to 5 years of age during the 2015–2016 and 2016–2017 influenza seasons. We hypothesized that children who were born before September, when the new vaccine typically becomes available, would be less likely to be vaccinated, more likely to receive a diagnosis of influenza, and more likely to spread the virus to their older family members than children born in the period from September through December. Details of the research methods, including the full list of covariates used in the adjusted analyses, are provided in the Supplementary Appendix, available with the full text of this letter at NEJM.org.

We analyzed data from 1,122,875 children 2 to 5 years of age. Preventive visits frequently occurred near a child’s birthday (Fig. S1 in the Supplementary Appendix). Vaccinations occurred in significantly higher percentages of children who had birthdays in September (52.7% [69,573 of 131,975 children]), October (55.0% [72,358 of 131,473]), November (53.1% [64,416 of 121,204]), and December (50.6% [61,609 of 121,723]) than of those whose birthdays were in earlier months (e.g., July; 41.8% [56,505 of 135,070]). Similar patterns were observed in the analysis with multivariable adjustment (Fig. 1A).

Figure 1. Influenza Vaccination and Diagnosis in Children 2 to 5 Years of Age, According to Birth Month.

Shown are the percentages of children who had insurance claims for receipt of an influenza vaccine (Panel A) or an encounter during which they received a diagnosis of influenza (Panel B). The center of each box represents the point estimate of the adjusted event rate, and I bars represent 95% confidence intervals (which may be very small).

Lower percentages of influenza diagnosis were observed among children who had been born in the period from September through December than among children with earlier birthdays (e.g., September [4.5%; 5901 of 131,975] vs. July [5.2%; 7069 of 135,070]). Findings in the analysis with multivariable adjustment were similar (Fig. 1B). The findings in children older than 5 years of age, for whom preventive visits and vaccination were not necessarily timed around birthdays, showed a gradually diminishing relationship between birth month and influenza vaccination or diagnosis (Figs. S3 and S5). Older family members of children born in the period from September through December were also less likely to receive a diagnosis of influenza than family members of children who had been born in earlier months (Fig. S6).

These findings are consistent with direct benefits of vaccination to children and indirect benefits to close contacts.^3,4 Although confounding is possible despite our quasi-experimental study design, our study adds to research showing that a seemingly arbitrary characteristic, such as a child’s birthday, may have effects on health.⁵ The majority of young children will not have a preventive visit with vaccination because their birthday is outside the September–December window when the influenza vaccine is typically available; returning to the doctor’s office for a separate vaccination visit may be difficult. Vaccinations for young children outside the doctor’s office — such as at preschools, pharmacies, or community centers — may have public health benefits.