SARS-CoV-2 Infection Risk Imposed by Fully-vaccinated Air Travelers Attending an Island-confined Quarantine System Enabling Tourism During the Pandemic: A Retrospective Cohort Study.

Objectives: This study aimed to identify the incidence of SARS-CoV-2 infection among fully vaccinated air travelers participating in an island-confined quarantine system (Phuket Sandbox Program). It also compared the differential risk of SARS-CoV-2 infection across different COVID-19 vaccines and the difference in time-to-detection periods between asymptomatic and symptomatic cases.

Methods: This retrospective cohort study determined the cumulative incidence of SARS-CoV-2 infection among 63,052 air travelers who participated in a quarantine program from July 1 to October 31, 2021. Using Poisson regression with robust standard errors, we estimated the relative risk of SARS-CoV-2 infection across different brands and types of COVID-19 vaccines, adjusting for relevant covariates. We visualized the time-to-detection periods for SARS-CoV-2 infection using Kaplan-Meier failure curves and compared these curves for asymptomatic and symptomatic travelers using the log-rank test.

Results:  The overall incidence of SARS-CoV-2 infection was 0.3%. Individuals vaccinated with Ad26.COV2.S, Gam-COVID-Vac, CoronaVac, and replicating viral vector vaccines faced a significantly higher risk of infection than those who received the BNT162b2 and mRNA vaccines. The time-to-detection periods for asymptomatic and symptomatic cases did not differ significantly.

Conclusions:  Despite the relatively low risk of SARS-CoV-2 infection, a risk of breakthrough cases remained with certain vaccines. Given the high proportion of asymptomatic cases, quarantine and intermittent testing should be implemented. The mandatory quarantine system proved effective in managing positive cases without necessitating a complete shutdown of travel. Implementing an island quarantine could be a viable strategy for reintroducing travel and tourism during a future COVID-19 outbreak or a new pandemic.