Transmission of COVID-19 by vaccinated, unvaccinated individuals

Editor's Note

This South Korean study finds that fully vaccinated individuals have a shorter duration of viable viral shedding and a lower rate of secondary transmission than partially vaccinated or unvaccinated individuals.

This study uses 2 cohorts to separately determine the transmissibility of COVID-19 according to vaccination status (ie, fully vaccinated vs partially or unvaccinated).

Cohort 1 includes healthcare workers and inpatients and their caregivers or guardians who were diagnosed with COVID-19 (during hospitalization or residence in cases of inpatient and caregiver or guardian) from March 1, 2020, to November 6, 2021.

Cohort 2 includes those with mild COVID-19 infection with the Delta variant who were isolated in a community facility between July 20 and August 20, 2021.

In cohort 1, secondary transmission of COVID-19 from fully vaccinated infected individuals was less common than from unvaccinated and partially vaccinated individuals.

In cohort 2, unvaccinated and partially vaccinated individuals shed culturable virus longer than vaccinated individuals. This difference in duration of culturable virus between groups provides a mechanism for patterns in secondary transmission in cohort 1.

Data from this study provide important evidence that despite the possibility of breakthrough infections, vaccinations remain critically useful for controlling the spread of COVID-19, the researchers say.