US researchers have measured how birthdays might contribute to increases in Covid-19 infection through a study whose findings support the cautionary calls in India against unrestricted social gatherings and parties amid the fall in new infections.

The researchers have found that in areas with high Covid-19 transmission, households with recent birthdays averaged about 86 more cases per 100,000 individuals than other households in the same areas without birthdays during the same time.

Their study, based on a sample of nearly three million households across the US, did not count the birthday parties but used birth dates of household members through health insurance records as a proxy for social gatherings or in-person festivities.

The findings were published on Monday in the JAMA Internal Medicine, a medical research journal.

The study’s major contribution is “to not only show that close contact with people we know and trust can lead to disease, but that the risk is substantial”, said Anupam Jena, associate professor of medicine at the Harvard Medical School.

“Many people probably believe that contact with people they don’t know — at a store or a restaurant — pose a risk and that contact with others they are familiar with would be safe,” Jena, the study’s senior author, told The Telegraph via email.

Public health experts, relying on anecdotal accounts, have long believed that social gatherings such as birthdays, weddings and parties may have contributed to the spread of the novel coronavirus. But the risk from such events has not been measured or estimated yet.

“It is a difficult question to study because one needs to know who’s attending a social gathering, whether they develop infection and whether those that gather differ in other ways such as mask-wearing, which may also be correlated with Covid-19 infections,” Jena said.

In their study, Jena, his colleagues and collaborators at the RAND Corporation, a policy think tank, analysed Covid-19 incidence and transmission patterns across different counties across the US and found that the magnitude of the risk appeared linked to the age of the person with a birthday.

In households where a child had a birthday, they observed an increase in Covid-19 cases of 158 per 100,000 persons in the two weeks after the birthday compared with Covid-19 cases in families without birthdays. And in households where an adult had a birthday, the increase was 58 per 100,000.

The researchers have speculated that households with a child’s birthday are less likely to cancel birthday plans because of the epidemic or that social distancing is followed less strictly at children’s birthday parties.

Senior Indian health officials, while flagging the country’s declining numbers of daily new infections, have asserted the need for the public to continue adhering to personal precautions such as wearing masks and avoiding crowds and social gatherings.

“We were able to examine only a single kind of event that likely leads to social gatherings,” Christopher Whaley, a co-author from the RAND Corporation, said in a media release.

“But given the magnitude of the increased risk associated with having a birthday in the household, it is clear that informal gatherings of all kinds played a significant role in the spread of Covid-19.”