Epidemiology and risk of COVID-19 among travelers at airport and port quarantine stations across Japan: a nationwide descriptive analysis and an individually matched case-control study.

Background The epidemiology and risk of COVID-19 among travelers at international borders remain unclear. Methods We conducted descriptive and individually matched case-control studies using a nationwide register for COVID-19 testing of travelers from August 3 to October 31, 2020 at airport/port quarantine stations across Japan. Cases defined as COVID-19-positive travelers on arrival were individually matched with four controls for arrival date and airport/port. We assessed associations between test positivity and traveler characteristics using conditional logistic regression. Results Overall, 157,507 travelers arriving from 146 countries/areas at 17 quarantine stations across Japan were tested for COVID-19. The test percent positivity during the study period was 0.35%. In the case-control study with 536 cases and 2,144 controls, we found evidence of lower test positivity in travelers aged 3-19 years, females, and travel corridor users (adjusted odds ratio [95% confidence interval]: 0.36 [0.22-0.60], 0.71 [0.56-0.89], and 0.48 [0.30-0.77], respectively), whereas higher positivity was associated with arrival from South-East Asia (1.88 [1.33-2.65]) or lower-middle- or low-income countries (2.46 [1.69-3.58] and 7.25 [2.22-23.66], respectively), any symptom (4.08 [1.43-11.65]), and nasopharyngeal compared to saliva sampling (2.75 [1.85-4.09]). A higher 14-day average incidence in the countries of stay was also associated with higher test positivity (1.64 [1.16-2.33] and 3.13 [1.88-5.23]). Conclusions These findings justify travel restrictions based on the epidemic situation in countries of stay, although underestimation of the epidemic in lower income countries should be considered. A strict travel corridor could also reduce COVID-19 importation risk.