Preschooler's Month of Birth Influences Odds of Becoming Ill With the Flu

By Gene Emery

July 09, 2020

(Reuters Health) - Preschoolers have a higher risk of being diagnosed with the flu if they are born before September, an analysis of insurance claims from more than 1.1 million children has concluded.

The reason: children with birthdays in July and August are less likely to receive the annual influenza vaccine when they are 2 to 5 years old because they often have their annual checkup close to their birthday, before the seasonal vaccine is available.

Dr. Anupam Jena of Harvard Medical School, in Boston, and his colleagues used a database of commercial insurance claims to document that the flu vaccination rates were significantly lower for children born before September and their and influenza diagnosis rates were higher.

While 5.2% of children born in July developed the flu during the 2015-2016 and 2016-2017 seasons, the rates were 4.5% for September-born babies and 4.2% among the preschool-age children born in October.

The trend faded among children older than 5 because their preventive visits were less likely to be timed to their birthdays. Sixty one percent of the children age 2 had their preventive care visit during the month of their birthday.

But the impact of timing goes beyond the children themselves, the researchers report in the New England Journal of Medicine.

"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," the researchers write, probably because the preschooler was more vulnerable to infection and spread flu to the household.

Dr. Jena told Reuters Health in a telephone interview that he's seen the problem first-hand because his son was born in August.

After the child's two-year checkup in August, "the nurse said you can come back in a few weeks for the flu vaccine," he recalled. "If my son had been born 2 weeks later, I would have been able to get the vaccine that day."

At that point, arranging for the vaccine became a problem. Online registration to get the shot filled up immediately, with no spots available until November. In addition, "none of the pharmacies and urgent care centers would vaccinate a 2-year old," said Dr. Jena, an associate professor at Harvard. "It's much more difficult to get them vaccinated, even in an area like here where getting medical care is usually easy."

Even when the vaccine was available at the time of a regular checkup, the vaccination rate was low: 52.7% for preschoolers born in September and 55.0% in October. It was 41.8% for July babies.

The trend was seen even after the researchers adjusted for factors such as age, gender and a child's comorbidities.

This is not the first time Dr. Jena and his colleagues have shown that a child's birthday can have significant health consequences.

Two years ago, they used similar data to show that being one of the youngest children in a kindergarten class dramatically increased odds that the youngster would be diagnosed and treated for attention deficit-hyperactivity disorder (ADHD).

That was because the children were less mature than their classmates, some of whom were up to 11 months older, which can make a big difference in behavior.

The researchers found that when the community's cutoff date for being able to attend kindergarten was September 1, the odds of the youngest children - those born in August - being labeled hyperactive were 34% higher than for the oldest children, who were born the previous September.

In states that didn't have the September 1 cutoff date, children born in August were not more likely to be diagnosed with ADHD.

SOURCE: The New England Journal of Medicine, online July 8, 2020.