Children's birthday parties may have fueled COVID-19 spread, study finds

Kids' birthday parties may be partly to blame for increased coronavirus transmission rates, a new study shows.

The study, published in JAMA Internal Medicine on Monday, gathered private health insurance data from 2.9 million U.S. households from Jan. 1 to Nov. 8, 2020. In counties with high rates of transmission, households were 31% more likely to test positive for the virus that causes COVID-19 within two weeks after someone had a birthday.

In households where a child’s birthday occurred, there were 15.8 more positive coronavirus tests per 10,000 people than in households that didn’t. When adults had a birthday, there were just 5.8 more positive tests in the following two weeks.

“There's a natural inclination to not think that your family members or friends are potentially infected or that you could potentially spread to family members or your friends,” said Dr. Chris Whaley, an author of the study and policy researcher at the RAND Corporation.

The United States has reported more than 33.5 million coronavirus cases and more than 601,000 deaths from of COVID-19, according to data from Johns Hopkins University. From March 1 through May 31, 2020, stay-at-home or shelter-in-place orders were issued in 42 states and territories, according to data from the Centers for Disease Control and Prevention.

Households in states with stay-at-home orders were just as likely to test positive after a birthday, the study found, and the correlation between birthdays and positive tests was just as strong in areas that voted for former President Donald Trump in 2016 compared with those that didn’t.

Informal gatherings, like birthday parties, are an important source of transmission, said Dr. Timothy Brewer, a professor of medicine and epidemiology at the University of California, Los Angeles. 

“While people are doing a good job or a better job of social distancing or wearing their masks when they go to a supermarket, when they get home they're more likely to relax and not necessarily wear masks or social-distance,” said Brewer, who was not involved with the study.

Awareness of the potential dangers of informal gatherings may be helpful in combating future pandemics, Whaley said.

“There's just been so much effort around formal times together, and less policy, and attention toward informal types of gatherings, but we actually show that that's a pretty strong source of transmission.”