Having a birthday within a household raises the risk of catching Covid by 30 per cent for a period of a fortnight, a study by scientists at Harvard University suggests.

Researchers looked at nearly three million homes in the US for 45 weeks in 2020 and noticed that there was a leap in Covid positivity for householders within two weeks of an individual celebrating a birthday.

The link remained through restrictions that left schools and events closed, and gatherings limited, suggesting people were less inclined to stick to the rules on birthdays.

Although the overall average prevalence of Covid was 27.8 cases per 10,000 people, it rose by 8.6 more cases per 10,000 within two weeks of a birthday. 

In households in which a child had a birthday, the effect was even higher, with an increase in cases of 15.8 per 10,000 persons in the two weeks following a child’s birthday,  compared with an adult birthday, where the increase was 5.8 additional cases per 10,000.

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The researchers speculated that households with child birthdays might have been less likely to cancel plans due to the pandemic, or that social distancing may have been followed less strictly at children’s birthday parties.

“These gatherings are an important part of the social fabric that holds together families and society as a whole,” said study senior author Dr Anupam Jena, of Harvard Medical School. 

“However, as we show, in high-risk areas, they can also expose households to Covid infections.

“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.”

Experts have speculated that small and informal gatherings might have played an important role in the spread of the virus, but the degree of risk associated with various kinds of social activities has been hard to measure, or even estimate. 

However it has been tricky to link infections to informal gatherings. To get round the problem, the team inferred that gatherings often happen around birthdays, and used cross-referenced Covid cases to birth dates to see if there was a correlation.

The research was published in the journal JAMA Internal Medicine.