Birthdays Fueled COVID-19 Spread During Pandemic Peak: Study

Birthdays might have fueled COVID-19 infections during the peak of the pandemic, a new analysis has found. In the counties with high infection rates, households that had a recent birthday were more likely to have cases of COVID-19.

There were many regulations put in place to help control the spread of COVID-19, particularly during the peak months of the pandemic. They included restrictions on work, sporting events and other formal gatherings that required face-to-face interactions. However, the U.S. has still logged over 33 million confirmed cases and over 596,000 deaths since Jan. 3, 2020.

Experts have been speculating that smaller, informal gatherings might have fueled COVID-19 infections, Harvard Medical School (HMS) noted in a news release. But the risk from such smaller events is more difficult to measure.

For their study, published in JAMA Internal Medicine on June 21, a team of researchers from HMS and the RAND Corporation looked at a possible link between COVID-19 infections and household birthdays, with the latter likely corresponding to informal social gatherings.

To do this, they looked at administrative health care data of 2.9 million households in the first 45 weeks of 2020. They didn't look at actual birthday parties but used the birthdays of household members as a proxy for in-person get-togethers.

Interestingly, the researchers found that in the counties where there was a high COVID-19 infection rate, the households with recent birthdays had an average of 8.6 more cases per 100,000 people than the households without a birthday in the two weeks prior. That's "a relative increase of 31% of county-level prevalence," the researchers noted.

Specifically, households had 5.8 more cases per 100,000 people after an adult birthday compared to households with no birthdays and a much higher 15.8 more cases per 100,000 people after child birthdays. According to the researchers, this suggests that child birthdays were more likely to result in social gatherings with more participants and relaxed mask-wearing and social-distancing behaviors.

The researchers did not see the same pattern in counties with low COVID-19 prevalence, HMS noted, and neither did they find differences based on the county's political leanings, precipitation during the birthday week or the shelter-in-place orders at the time.

"This study suggests that events that lead to small and informal social gatherings, such as birthdays, and in particular, children's birthdays, are a potentially important source in SARS-CoV-2 transmission," the researchers wrote.

"These gatherings are an important part of the social fabric that holds together families and society as a whole," Anupam Jena, study senior author and Ruth L. Newhouse Associate Professor of Health Care Policy at Harvard Medical School, said in the HMS news release. "However, as we show, in high-risk areas, they can also expose households to COVID-19 infections."

Although vaccine roll-outs are now underway, the findings can help inform both authorities and the public on the risks that such informal gatherings pose if another surge in cases happens in the future.

"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," study co-author Christopher Whaley of the RAND Corporation said as per HMS.