Birthday parties may have boosted infection rates at the height of the COVID-19 pandemic

Clinical Trials & Research, Covid-19

The link between birthdays and COVID-19 could explain higher infections in counties that did not prevent such gatherings.

Birthday Party. Image Credit: OPOLJA/

Recent household birthday bashes increased the likelihood of infection by 30 percent

Across the US, counties displayed varying approaches to addressing the early signs of mounting COVID-19 infections. Differences in measures were particularly apparent when developing lockdown measures, detailed by the number of people allowed to gather, or whether gatherings were even allowed. Despite these restrictions, aimed at reducing the kind of social interactions that fuel an infectious outbreak, the nation has seen more than 32 million confirmed cases of COVID-19 and nearly 600,000 deaths.

However, children were particularly affected by preventative measures. For over a year, the life of children experienced drastic changes as many schools were closed for in-person learning, large segments of the population worked from home, and many forms of large and formal gatherings were strictly limited, including sporting events, concerts, and funerals.

A new analysis based on national health insurance claims shows that households that held birthday celebrations were more likely to be infected by COVID-19, and this pattern contributed to county-level increases in infection rates.

Researchers at Harvard Medical School (HMS) and the RAND Corporation reported in JAMA Internal Medicine that households with recent birthdays were 30 percent more likely to have a COVID-19 diagnosis, compared with households with no birthdays.

The analysis included 3 million U.S. households with employer-based insurance provided by Castlight Health. The scientists then used the birth dates of household members registered with the insurance to approximate recent birthdays.

These gatherings are an important part of the social fabric that holds together families and society as a whole. However, as we show, in high-risk areas, they can also expose households to COVID-19 infections,”

Anupam Jena, the Ruth L. Newhouse Associate Professor of Health Care Policy at HMS

This was further reflected in the infection patterns. Over the first 45 weeks of 2020 the researchers found that in counties with high COVID-19 transmission, households with recent birthdays averaged 8.6 more cases per 10,000 individuals than households in the same counties without a birthday.

Changes in risk magnitude and its effects on policy management

Further exploring the data showed that the likelihood of infection during such social gatherings as birthdays also varied with age. Households that had a child had a higher risk magnitude of infection, with an increase in COVID-19 cases of 15.8 per 10,000 persons in the two weeks following a child’s birthday compared to cases in families without a birthday.

In contrast, households that held an adult’s birthday showed an increase of 5.8 additional cases per 10,000 persons. Such differences could be due to the fact that fewer households canceled children’s birthdays and that social distancing and hygiene were less enforced during such events.

Variations in risk threshold varied nationally too, as counties with low COVID-19 prevalence had no differences in infection rates weeks following birthdays. Other factors, including political leaning, lockdown measures, or environment (rainfall during the week leading to indoor parties) did not affect results further.

Findings, therefore, provided insight into how social gatherings affected infection rates among American households. Studies can also consider whether infection rates varied across different types of social events, or whether these patterns were affected during the reopening of schools, nurseries, or public spaces.

“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,” said study co-author Christopher Whaley of the RAND Corporation.

This data can also be key to informing ongoing and future policies, as it indicates which counties are more or less likely to increase infection rates, and how social events can potentially accelerate such rates. Such patterns will be particularly valuable as the number of vaccinated individuals is rising and the economy is reopening.

Our results could help inform future measures. They do underscore the importance of understanding the types of activities that may worsen viral spread during a pandemic and can inform policy and individual decisions based on risk. The findings also quantify the potential risk of gathering with people that we know.”

Anupam Jena

Journal reference:
  • Whaley CM, Cantor J, Pera M, Jena AB. Assessing the Association Between Social Gatherings and COVID-19 Risk Using Birthdays. JAMA Intern Med. Published online June 21, 2021. doi:10.1001/jamainternmed.2021.2915

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