As the Covid-19 epidemic spread in 2020, households that recently had a birthday were more likely to test positive for the coronavirus, according to a study in JAMA Internal Medicine­—findings that suggest informal gatherings played a significant role in spreading the virus. 

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How researchers tested the link between birthday parties and coronavirus spread

For the study, Christopher Whaley, a policy researcher at RAND Corporation, and his team analyzed data from health insurance claims for nearly three million households, which together accounted for about 6.5 million privately insured individual members.

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Importantly, the data, which was collected during the first 45 weeks of 2020, included members' dates of birth. That gave researchers an unusual opportunity to "addres[s] the problem of confounding in who does and does not socially gather": They could identify who had recently had a birthday and so might have had a birthday party.

Specifically, the researchers tracked positive Covid-19 diagnoses in the two weeks following a birthday in a household.

What the researchers found

The study found that in areas with low Covid-19 prevalence, there was no difference in risk between households with birthdays and households without birthdays. However, in areas with high Covid-19 prevalence, the risk of transmission spiked significantly following a birthday.

For instance, the researchers found that in the areas where Covid-19 was most prevalent, averaging about 27.8 infections per 10,000 people, households were 31% more likely to test positive for the coronavirus in the two weeks following a birthday.

But the exact risk varied depending on who had celebrated a birthday. Following an adult's birthday, there was an increase of 5.8 cases per 10,000 people in high-prevalence areas. By contrast, after a child's birthday, the infection rate increased much more significantly—by about 15.8 cases per 10,000 people.

The researchers also assessed other factors—such as political leanings in an area, stay-at-home/shelter-in-place mandates, and inclement weather—to determine whether they affected the increased risk from birthdays, but they did not find any significant statistical associations.

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According to the researchers, the study had several limitations. They couldn't determine whether the households with birthdays had actually celebrated those birthdays with in-person gatherings, nor could they track asymptomatic cases of Covid-19. Further, households with public insurance, which have experienced an especially high rate of Covid-19, were not included in the study data.

What does the study imply for future epidemics?

Although the researchers didn't offer any health policy recommendations, their findings do provide further evidence that informal gatherings played an important role in Covid-19 transmission, MedPage Today reports.

The findings also highlight a potential reason why people might break public health restrictions against group gatherings. Because many shelter-in-place orders originally targeted larger, more formal gatherings, people might have believed that they would be safe in smaller gatherings among friends and family, the researchers hypothesized.

"I think it's just natural to not think that your family or friends could transmit a horrible disease to you," Whaley said, "and so maybe you let your guard down a little bit" (Grover, The Guardian, 6/21; Gever, MedPage Today, 6/21).