Birthday Parties as Virus Vector

K.J. Seung, the chief of strategy and policy for Partners in Health’s Massachusetts Covid response, who helped set up the contact tracing system, said it has been hard for contact tracers to clearly demonstrate that people were contracting the virus in small private gatherings.

Public exposures, like at a factory or a wedding, were easier for them to track. Individuals often didn’t share the nights they had a cousin over for dinner or drove a friend home from work, whether out of shame or forgetfulness — and if they did, they were reluctant to name names.

“Small social gatherings are the most difficult locations to trace,” he said. Yet “when we talked to contact tracers around the country, they were like: Yeah, people are getting infected at these small gatherings.”

So much behavior around the pandemic — including mask use and the uptake of vaccines — appears to differ by people’s political party. But the study found that birthdays led to increased Covid infections by similar levels in Republican and Democratic areas of the country. This suggests that although Democratic-leaning households may have been more likely to wear a mask while walking the dog, they may have differed less than Republicans in their comfort having a trusted friend over to visit.

“There was definitely this element of your home is a safe place and therefore when you have your friends and family over in your home, it just doesn’t feel risky,” said Dr. Ashish Jha, the dean of the School of Public Health at Brown University, who described the paper as “creative” for finding an unusual way of capturing disease transmission that is otherwise hard to measure.

For many Americans, birthday parties have gotten much safer in recent months. The Centers for Disease Control and Prevention says it’s safe for fully vaccinated people to gather indoors without wearing face coverings.

But for those who remain unvaccinated, the study is a reminder that even activities that feel the most safe pose a threat of infection. In many parts of the country, unvaccinated people are clustered by region or social group, meaning that birthday parties — and other such festive, private occasions — can still be risky.