Healthcare workers are at the frontline of the coronavirus disease (COVID-19) pandemic. They will be randomized to receive a single dose of BCG vaccine or 0.9% NaCl placebo.

Participants will be followed up for 12 months with notification from a Smartphone application or phone calls (up to daily when ill) and surveys to identify and detail COVID-19 infection. Additional information on severe disease will be obtained from hospital medical records and/or government databases.

Blood samples will be collected prior to randomization and at 3, 6, 9 and 12 months to determine exposure to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Where required, swab/blood samples will be taken at illness episodes to assess SARS-CoV-2 infection.

The trial includes a pre-planned meta-analysis with data from 2834 participants recruited in the first phase of this study, where participants were randomized to receive BCG or no BCG vaccine at the time of receiving influenza vaccination.