Small Birthday Gatherings Can Be a Big COVID-19 Infection Risk, New Study Shows

New research is showing just how much small gatherings, including birthday parties, can play a role in COVID-19 infections. The good news is that vaccines can reduce the risk dramatically. Here's what else parents need to know.

An image of a woman and her daughter at a birthday party.
Photo: Getty Images.

Despite the enthusiasm for "Hot Vax Summer," the COVID-19 pandemic is still ongoing, and unvaccinated people remain vulnerable to the disease. Parents living with unvaccinated kids know this well. New research suggests that, depending on where you live, continued vigilance about your social life—especially when it comes to celebrations—may be in order.

A paper published in late June in JAMA Internal Medicine examined the role that small social gatherings likely played in driving COVID-19 infections in the first part of the pandemic, before vaccines were available. The authors used data from Castlight, a health insurance navigation program offered by employers to the people on the companies' health plans. The data included the members of a household and their respective birthdays, along with where they lived. Their hypothesis was that some people might have had birthday parties, and those parties likely contributed to COVID-19 transmission. It turns out they did.

To answer the question, the researchers tallied the diagnosed COVID-19 infections for anyone in the household in the two weeks following any household member's birthday. They grouped the infections according to the case rate of the county the household lived in at the time the infection occurred.

In counties with the highest rates of COVID-19 infections—the top 10 percent by prevalence—households with a birthday in the previous two weeks recorded 8.6 more diagnoses per 100,000 people than those without birthdays, the paper stated. This amounted to 31 percent increase in infections compared to the baseline case rate. That rate was 27.8 cases per 100,000 people, a rate that some counties in the U.S. still exceed, according to data from the New York Times' COVID-19 tracking dashboard.

As might be expected, the effect was greatest in the areas where COVID-19 was most prevalent. Socializing in a community with many cases of coronavirus circulating means each person you see has a higher chance of being infected at the time. Having a small gathering in a lower-prevalence area is statistically less risky.

"If you're in an area where COVID cases are high—as we actually see now in many parts of the country—then any type of social gathering or interaction with other people in person just becomes more risky," says Christopher Whaley, Ph.D., lead author of the study.

A few more points: the study covers January 1 (for the control group before the pandemic) until November 8, 2020—before vaccines were authorized. Vaccinations change this picture, as they dramatically lower the risk of infection and transmission.

Notably, the infection increase was larger after kids' birthdays than adults. The authors also speculated that adults who were willing to forgo celebrating their own birthdays might be less willing to skip their kids' celebrations.

The infection increase was also an average, which likely understates the true risk, Dr. Whaley says. There were undoubtedly households that skipped birthday celebrations entirely, meaning that the infections were concentrated in the subset of families who chose to celebrate with others. It's also not clear in the study whether the infections after gatherings were in children or adults.

How Families Can Handle Small Gatherings During COVID

While this study focused on birthday parties, the intent was to evaluate the infection contribution of small gatherings in general. Birthday parties, since they're usually tied to a knowable date, were easier to model.

Dr. Whaley emphasizes the importance of bringing the same COVID-19 vigilance to small social gatherings as you would to more formal settings like an office, gym, or restaurant. The casual setting can lull a person into thinking the risk is also lower. Getting back to "normal" entails complicated personal risk assessments as rates decrease.

"How do we normalize our social behavior in a way that is safe and not being too cautious where we're missing the opportunity to be together?" says Margaret Aldrich, M.D., a pediatric infectious disease specialist at the Children's Hospital at Montefiore. "However, we're also balancing the fact that [the pandemic] is not totally gone yet, and that we do need to continue some modifications."

Moving gatherings outdoors and masking when inside are still recommended. For kids' birthdays choose activities that keep the guests more spaced out instead of clumped together, says Dr. Aldrich, and ensure good ventilation even outside.

Consider COVID-19 cases in your community

The JAMA paper showed an increase in household infections in all communities except those with the lowest case rates (which were used as a control). However, the increase in infections was bigger in areas with higher case rates: 8.6 extra cases per 10,000 people in the tenth decile compared to .9 extra cases per 10,000 people in the fifth decile. Put simply, the more coronavirus in a community, the riskier any social gathering, including a birthday party, will be.

Thus, areas with higher case rates and low vaccination rates may warrant stricter policies than places with fewer cases and more vaccinated residents.

Dr. Aldrich says that she focuses on the test positivity rate to guide her hospital's policies. If it raises toward 5 percent, that's when more masking and limiting interaction needs to happen. Currently, the prevalence rate in New York City, where Dr. Aldrich works, is less than 1 percent, so the risk of any one person having COVID-19 is low.

The Bottom Line

If you're planning a birthday party, or any social gathering that includes unvaccinated people, standard COVID-19 mitigation measures are still important to consider, especially true if you live in an area with a high prevalence rate. Moving activities outside, staying socially distant from other households, wearing masks when indoors, and making the guest list smaller all reduce the risk of contracting COVID-19.

Always check the vaccination rate in your community, and get yourself and your family members vaccinated if they're eligible. "Even with new variants, like the Delta variant, we see less severe disease, we see less transmission of disease within that community or those communities that have higher vaccination rates," Dr. Aldrich says.

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