In what setting does COVID-19 spread the most? According to a new study, a club with a single sick person in it could result in more cases of COVID-19 than any other setting.
A study that looked at the site of transmission for more than 13,000 confirmed cases of COVID-19 in Japan in 2020 found that while the majority of cases were contracted in household or health-care settings, nightlife was vastly more likely to be responsible for spreading the virus to numerous people at once.
The study, , looked at 44,054 laboratory-confirmed COVID-19 cases in total, all of which occurred in Tokyo between Jan. 23 and Dec. 5, 2020.
Out of these cases, they were able to narrow down the setting in which transmission occurred for 13,122 cases.
Approximately 6,700 cases were transmitted in households, 2,733 occurred in health-care settings and 1,174 occurred in nightlife settings.
Although households accounted for the highest number of transmissions, these were largely transmission events of one person to another person, or one person to a couple people within the household.
But compared to dining, workplace, household and other settings, transmission that occurred in health-care and nightlife settings was far more likely to result in the virus being transmitted to five or more people.
鈥淐rowded places, inadequate ventilation, on-site eating and drinking with frequent alcohol consumption, and aerosol- and droplet-producing behaviours, such as talking loudly and singing, can set the conditions for large outbreaks,鈥 the study stated.
Researchers also found that among the 30,932 cases for which exact transmission settings couldn鈥檛 be identified, cases with 鈥渁 history of visiting nightlife establishments were more likely to generate onward transmission to non-household settings.鈥
TRACKING THE SPREAD
Zeroing in on settings that might have the largest impact with the introduction of a single person with the virus may give a better picture of how to mitigate COVID-19 spread in future waves, or in the events of future epidemics.
Researchers isolated the spread into three epidemic waves, the first from January to May, the second from May to September, and the third from October to December. The study period only included the beginning of the third wave, which stretched into February 2021 in Japan.
Out of the 44,000 cases looked at, 57 per cent were among men. The proportion of male cases was highest in nightlife settings compared with any other settings.
While nightlife was associated with more outbreaks of five or more cases than other settings, certain nightlife settings were more dangerous than others. Nightlife settings that had hosts or hostesses were more likely to involve five or more transmissions compared to those without hosts or hostesses.
When all transmission settings were counted together, the 18-39 age group was associated with more multi-case transmission than other age groups.
One finding that researchers highlighted was that nightlife cases were more frequent early on in the pandemic, in the earlier phases of wave one and two, while household cases appeared more in later phases of both waves.
Researchers suggested that this points to how nightlife cases went down as waves got worse and people stopped attending clubs and bars as public health countermeasures were implemented. The study period occurred entirely before vaccines were available.
Transmission settings were filed into seven categories: nightlife, dining, workplace, household, health care, imported due to travel history outside of Japan and other. The other category included school, nurseries and universities.
Establishments such as bars and nightclubs were filed under nightlife, as well as some restaurants which operated mostly at night.
An outbreak that involved five or more cases from one incident was considered a large outbreak in the study.
Out of the 13,000 cases where a transmission site could be identified, 582 cases, or 4.4 per cent, directly resulted in further transmission at another setting.
This was least likely with household cases, followed by cases in health-care settings. It was more likely in nightlife cases, but it was those aged 40-64 years in nightlife settings who generated the most onward transmission compared to younger patrons.
The workplace was the transmission setting that resulted in the highest amount of later household transmission.
Transmission between those aged 65 years or older occurred most often within households, and household transmission had the most amount of intergenerational transmission, while nightlife generally was the site of peer-to-peer transmission within an age group.
The study pointed out that narrowing down which sites of transmission may result in more cases is important to know in order to optimize mitigation tactics based around restricting the public鈥檚 movement and access.
鈥淥ne approach for balanced control measures may be targeting high-risk settings of COVID-19 spread,鈥 the study states. 鈥淢ost COVID-19 cases do not generate secondary cases, but a few generate many, suggesting an important role of superspreading events (SSEs) in sustained COVID-19 transmission.鈥
The study added that government campaigns to urge the public to return to eating and drinking at nightlife establishments often preceded COVID-19 spread, citing studies from the U.K. and Japan.
And despite the advent of vaccines, the development of new variants coupled with the fact that vaccine protection wanes over time led researchers to state that the risk of nightlife settings for transmission should still be taken into account.
鈥淲e believe that the association of nightlife settings with COVID-19 spread is still valid,鈥 researchers wrote.
