Not-So-Happy-Birthdays: Parties Helped Spread COVID, Study Finds

Birthday celebrations raised the risk of spreading SARS-CoV-2 by 30% last year in U.S. counties with high rates of COVID-19, according to a new study.

No such surge was seen in places with low rates of infection.

For the study, researchers at Harvard Medical School and the RAND Corporation analyzed health insurance claims data from nearly 3 million U.S. households in the first 45 weeks of 2020.

Overall, households with recent birthdays averaged 8.6 more cases of COVID for every 10,000 people than households in the same counties without a birthday.

"These gatherings are an important part of the social fabric that holds together families and society as a whole," said study senior author Dr. Anupam Jena, associate professor of health care policy at Harvard.

"However, as we show, in high-risk areas, they can also expose households to COVID-19 infections," he said in a Harvard news release.

In households with an adult birthday, there were an additional 5.8 cases of COVID per 10,000, according to the study.

But the risk was much higher in households where a child had a birthday -- 15.8 more cases per 10,000 in the two weeks after the birthday compared to other households.

The researchers suggested that kids' parties might have been more likely to be held than canceled or that social distancing may have been more relaxed.

The findings were published June 21 in the journal JAMA Internal Medicine.

The investigators found no birthday-related increase in infection rates in counties with low COVID-19 rates.

The findings suggest that informal social gatherings such as birthday parties played role in the spread of infection at the height of the pandemic in the United States.

"We were only able to examine a single kind of event that likely leads to social gatherings, but given the magnitude of the increased risk associated with having a birthday in the household, it's clear that informal gatherings of all kinds played a significant role in the spread of COVID-19," said study co-author Christopher Whaley of the RAND Corporation.

Jena said the results could help inform future infection control measures.

"They do underscore the importance of understanding the types of activities that may worsen viral spread during a pandemic and can inform policy and individual decisions based on risk," he said. "The findings also quantify the potential risk of gathering with people that we know."