Birthday celebrations may up household COVID-19 transmission risk in high prevalence areas

Informal social gatherings such as birthdays may be associated with an increased risk of household COVID-19 transmission in areas with high COVID-19 prevalence, according to a cross-sectional study conducted in the US.

“[W]e found that the presence of a birthday within a household was associated with significantly greater COVID-19 diagnosis rates in those households in the 2 weeks after birthdays,” the researchers said.

The researchers used data from 6,535,987 individuals from 2,926,530 households with private insurance in the US to identify birthdays and COVID-19 diagnoses between January 1 and November 8, 2020. Households with and without a birthday celebration in the prior 2 weeks were compared.

In counties in the top decile of COVID-19 prevalence, households with a birthday in the preceding 2 weeks had 8.6 more diagnoses per 10,000 individuals compared with those without a birthday in the preceding 2 weeks. [JAMA Intern Med 2021;doi:10.1001/jamainternmed.2021.2915]

This corresponded to a 31 percent relative increase in the risk of household COVID-19 infections for those with a birthday 2 weeks prior over the top decile county-level prevalence of 27.8 cases per 10,000 individuals.

“[T]he absolute adjusted rate difference between households with and without a birthday increase[ed] with greater COVID-19 county prevalence,” the researchers said. This supports the concept that transmission risk is higher at gatherings when community transmission rates are higher.

The findings were consistent when COVID-19–related hospitalizations was assessed, which suggested the results were not due to increased testing rates.

“For each decile of COVID-19 prevalence, the change in household COVID-19 infection rates was larger in magnitude for households with a child’s birthday than for households with an adult birthday,” the researchers pointed out.

In the tenth decile of COVID-19 prevalence, there was an increase in COVID-19 diagnosis of 15.8 per 10,000 individuals following a child’s birthday, while in the fifth decile, the increase was 1.0 per 10,000 individuals (p_interaction<0.001).

In counties in the tenth decile, diagnosis increased by 5.8 per 10,000 individuals following an adult’s birthday vs the 15.8 per 10,000 noted following a child’s birthday (p_interaction<0.001).

The reasons for this could be multi-fold and include relaxed mask-wearing and social distancing measures, as well as a higher likelihood of gathering for a child’s than an adult’s birthday, the researchers added.

They noted that COVID-19 household diagnosis risk did not increase after a milestone birthday. “[S]tate policies and political views around social distancing and masking” also did not appear to affect the results.

“Informal social gatherings are a potentially important mode of SARS-CoV-2 transmission,” they said. “This study suggests that events that lead to small and informal social gatherings, such as birthdays, and in particular, children’s birthdays, are a potentially important source in SARS-CoV-2 transmission,” they added.

The researchers noted that asymptomatic infections may have been missed. As such, the results may be representative of the association between household birthdays and moderate-to-severe COVID-19. Residual confounding is also possible due to the observational study design.

The results were also based on a private insurance database and excluded individuals with public insurance or uninsured individuals, populations at higher risk for COVID-19. “If similar gatherings around birthdays occur for these populations, then our study results may understate the association of birthdays with COVID-19 transmission,” the researchers said.