Celebrating a birthday could raise your risk of Covid-19 by a third, a new study suggests.

In US counties with high rates of Covid-19, households that recently celebrated birthdays were 30% more likely to be diagnosed with coronavirus, compared with households with no birthdays, according to the study by Harvard Medical School and nonprofit research organisation the RAND Corporation.

It’s no surprise really when you think about it. At gatherings with close friends and family members, it’s not unusual for people to bend social distancing rules with a hug here and a kiss there. Blowing out candles on a birthday cake isn’t the most hygienic of activities either.

Age also seems to matter – in households with children celebrating a birthday, the risk of catching Covid was even higher. Researchers speculated this could be because households with children’s birthdays were less likely to cancel plans in the pandemic, or that social distancing may have been followed less strictly.

For the study, published in the journal JAMA Internal Medicine, researchers analysed a nationwide sample of nearly 3 million US households with data provided by the insurance firm Castlight Health.

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. 

In households in which a child had a birthday, the impact was even greater, with an increase in Covid-19 cases of 15.8 per 10,000 persons in the two weeks following the birthday compared to cases in families without one. In households with an adult birthday, the increase was 5.8 additional cases per 10,000.

Interestingly, they didn’t count actual birthday parties in their analysis. Instead, they used birth dates of household members as a proxy for social gatherings and in-person festivities.

Nonetheless, they said the findings signal that social gatherings, such as birthday parties, may have contributed to infections during the height of the pandemic.

“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, from the RAND Corporation.

Co-author Anupam Jena, associate professor of Health Care Policy at Harvard Medical School, added: “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.”

The findings hold important clues for public health officials and individuals should more waves of Covid occur. “Our results could help inform future measures,” Jena said. “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.”

Commenting on the findings, Professor Christl Donnelly, an expert in statistical epidemiology at Imperial College London, told HuffPost UK: “What makes this analysis clever is that rather than having to ask people about their social contacts, the researchers looked for an association between a birthday of someone in the household and subsequent increased Covid-19 risks. 

“What they found were increased risks in the following two weeks, with bigger effects when the birthday was of a child. This may well reflect children’s birthdays being more likely to be celebrated with a party and/or the party size being larger.” She added that “there is nothing about a birthday party that makes it higher risk than other get-togethers of the same size”.

Those wanting to stay safe and reduce their risk at gatherings, including birthday parties, can use the same measures that protect you elsewhere, says Prof Donnelly. “If possible, stay outdoors, wear masks (when not eating cake) and limit the number of people,” she says. 

“Finally, I’d suggest that if any candles are to be blown out that they are restricted to the birthday girl’s or boy’s piece of cake – not the whole cake to be served to everyone.”