At the height of the pandemic, it was easy to worry that strangers would transmit the virus to you. But a new study of what happened after people’s birthdays suggests that people we trust were also a common source of virus spreading.
Private gatherings were harder to measure for researchers than large public events – after all, they are private. And there has been a heated debate among epidemiologists for months about how big their impact was on how the coronavirus moved from person to person.
But a team of Harvard researchers used a creative method to find them: Using damage data from health insurers, they examined the Covid rates of families in the two weeks after one of them had a birthday. Overall, her paper, published in Jama Internal Medicine, found that a recent family birthday increased the risk of Covid by almost a third.
Their theory is that the increased risk is almost certainly explained by birthday parties. Although it is not clear from the insurance claims whether an individual Covid patient actually hosted a birthday party, several aspects of the data strongly suggest a connection. When researchers looked at other days of the year by randomly assigning birthdays instead of using actual birthdays, or looking at diagnoses in weeks before birthdays, they found no such pattern. But, perhaps more importantly, they found the greatest risk of infection in the weeks following a child’s birthday.
“My wife and I certainly didn’t see the need to gather indoors on our birthdays,” said Anupam Jena, professor of health policy at Harvard Medical School and one of the co-authors of the paper, who said the study was inspired by Birthday of his own daughter. “Our children could be more disappointed.”
At birthday parties, of course, groups often huddle together in a small space, perhaps to watch a child blowing out candles on a cake.
With birthdays happening across the country and spread out throughout the calendar year, researchers were able to study the effects where Covid was widespread and less common. Other infections followed on a broad front after birthdays.
The study took into account data from last year when Covid was much more common and fewer Americans were vaccinated. But his conclusions are still relevant to Americans who are unvaccinated today – a group that includes all children under 12. That may be especially true since the new, more contagious Delta variant is circulating in more states.
Much political debates on how to deal with the pandemic have focused on what to do with public spaces – such as whether restaurants are allowed to open or whether masks are required. Officials found it harder to monitor people’s behavior at home. They also struggled to measure the impact.
KJ Seung, the chief of strategy and policy for Massachusetts Covid Response from Partners in Health, who helped set up the contact tracing system, said it had been difficult for contact tracing authorities to clearly demonstrate that people were infected with the virus in small private gatherings infected. Public revelations, like those at a factory or a wedding, were easier for them to follow. People often didn’t share the evenings when they had a cousin for dinner or drove a friend home from work, either out of shame or forgetfulness – and when they did, they were reluctant to give names.
“Small social gatherings are the hardest places to keep track of,” he said. But “when we spoke to tracers across the country, they said, yes, people get infected at these little gatherings.”
So much of the behavior related to the pandemic – including using masks and taking vaccines – seems to differ between people’s political party. However, the study found that birthdays in Republican and Democratic areas of the country resulted in similar increases in Covid infections. This suggests that while Democratic households were more likely to wear a mask when going for a walk, they may have been less different than Republicans in visiting a trusted friend.
“This element of your home is definitely a safe place and when you have your friends and family at home it just doesn’t feel risky,” said Dr. Ashish Jha, the dean of the School of Public Health at Brown University, who called the paper “creative” for finding an unusual way to capture disease transmissions that are otherwise difficult to measure.
For many Americans, birthday parties have become a lot safer in the past few months. The Centers for Disease Control and Prevention say it is safe for fully vaccinated individuals to gather indoors without wearing face coverings. But for those who remain unvaccinated, the study is a reminder that even activities that feel safest pose a risk of infection. In many parts of the country, unvaccinated people are grouped by region or social group, which means birthday parties – and other festive, private occasions – can still be risky.