Transmission of COVID-19 in Nightlife, Household, and Health Care Settings in Tokyo, Japan, in 2020

Abstract

Importance: There have been few studies on the heterogeneous interconnection of COVID-19 outbreaks occurring in different social settings using robust, surveillance epidemiological data.

Objectives: To describe the characteristics of COVID-19 transmission within different social settings and to evaluate settings associated with onward transmission to other settings.

Design, setting, and participants: This is a case series study of laboratory-confirmed COVID-19 cases in Tokyo between January 23 and December 5, 2020, when vaccination was not yet implemented. Using epidemiological investigation data collected by public health centers, epidemiological links were identified and classified into 7 transmission settings: imported, nightlife, dining, workplace, household, health care, and other.

Main outcomes and measures: The number of cases per setting and the likelihood of generating onward transmissions were compared between different transmission settings.

Results: Of the 44 054 confirmed COVID-19 cases in this study, 25 241 (57.3%) were among male patients, and the median (IQR) age of patients was 36 (26-52) years. Transmission settings were identified in 13 122 cases, including 6768 household, 2733 health care, and 1174 nightlife cases. More than 6600 transmission settings were detected, and nightlife (72 of 380 [18.9%]; P < .001) and health care (119 [36.2%]; P < .001) settings were more likely to involve 5 or more cases than dining, workplace, household, and other settings. Nightlife cases appeared in the earlier phase of the epidemic, while household and health care cases appeared later. After adjustment for transmission setting, sex, age group, presence of symptoms, and wave, household and health care cases were less likely to generate onward transmission compared with nightlife cases (household: adjusted odds ratio, 0.03; 95% CI, 0.02-0.05; health care: adjusted odds ratio, 0.57; 95% CI, 0.41-0.79). Household settings were associated with intergenerational transmission, while nonhousehold settings mainly comprised transmission between the same age group. Among 30 932 cases without identified transmission settings, cases with a history of visiting nightlife establishments were more likely to generate onward transmission to nonhousehold settings (adjusted odds ratio, 5.30 [95% CI, 4.64-6.05]; P < .001) than those without such history.

Conclusions and relevance: In this case series study, COVID-19 cases identified in nightlife settings were associated with a higher likelihood of spreading COVID-19 than household and health care cases. Surveillance and interventions targeting nightlife settings should be prioritized to disrupt COVID-19 transmission, especially in the early stage of an epidemic.

Figure 1.. Daily Number of COVID-19 Cases by Different Transmission Settings Based on Date of Onset

The daily number of cases identified from imported, nightlife, dining, workplace, other, household, and health care settings based on the date of onset are shown using bars. The blue dots indicate the daily number of all cases, including unknown cases. Vertical dashed lines indicate the peak of wave 1 (April 1, 2020) and wave 2 (July 29, 2020), which was determined as the date with the largest 7-day moving average daily number of cases in respective waves.

Figure 2.. Comparison of the Interval Between Each Case’s Date of Onset and the Respective Wave’s Peak by Different Transmission Settings

The interval between each case's date of onset and the respective wave's peak (wave 1, April 1, 2020; wave 2, July 29, 2020) was calculated. The distribution of intervals was plotted regarding nightlife, household, and health care settings, as well as all the cases including unknown cases, stratified by wave 1 and wave 2. The distribution of intervals between nightlife, household, and health care cases were compared using the generalized estimating equations with Šidák corrections to account for the within-cluster association. ^aP < .05. ^bP < .001.

Figure 3.. Age Distribution of Offspring Cases and Their Primary Cases

The age distribution of 6672 pairs of primary and offspring cases involved in household settings (A) and 3362 pairs of primary and offspring cases involved in nonhousehold settings (B) was plotted.