Introduction:

COVID-19 superspreader events have occurred when symptomatic individuals without wearing face masks boarded buses.

Objective:

To report the risk of superspreader events when presymptomatic individuals boarded buses together with unvaccinated passengers, but with non-pharmacological preventive interventions being maintained.

Methods:

Prospective study of health personnel transported in buses to a COVID-19 vaccination center for two weeks. Open windows, correct use of face masks and exclusion of symptomatic individuals were mandatory. Prospective surveillance identified workers with COVID-19 within 14 days after vaccination. Each asymptomatic passenger of buses where cases were identified was monitored for a similar time period. Voluntary screening results were available for workers who were tested in the month before or after vaccination.

Results:

1,879 workers boarded 65 buses. On-board time ranged from three to eight hours. Twenty-nine cases of COVID-19 and four asymptomatic cases were identified among 613 passengers of 21 buses. Median time between vaccination and COVID-19 symptoms onset was six days. One case of suspected transmission on a bus was identified.

Conclusions:

Strict nonpharmacological preventive interventions substantially reduced the risk of COVID-19 superspreader events in buses boarded by presymptomatic individuals.