Study procedures.
BLOOD
After having obtained informed consent, participants were randomized to BCG or placebo (1:1) using a computer-generated dynamic randomization algorithm in random blocks of two, four, or six sequences, stratified by hospital. They received an intradermal injection in the left upper arm with either 0.1 mL of the Danish strain 1331 (Statens Serum Institut, Denmark), equivalent to 0.075 mg attenuated At the randomization visit, participants completed an online baseline questionnaire (Research Online, Julius Center, UMC Utrecht, the Netherlands) and installed a diary application on their smartphone (Research Follow App, Your Research BV, Huizen, Netherlands). Participants were asked to report symptoms via the app daily, as well as SARS-CoV-2 exposures and test results and health care visits, including hospital admissions, weekly. The daily questionnaire was integrated into the weekly questionnaire after 6 months to improve user convenience, and COVID-19 vaccination questions were added after the start of the Dutch vaccination campaign on 6 January 2021. We used push notifications, e-mails, and phone calls to maximize adherence with app completion, and we terminated the app for all participants on 27 March 2021. Blood sampling continued until mid-June 2021, and we collected data on symptoms, positive SARS-CoV-2 tests, and COVID-19 vaccinations for the period between 27 March 2021 and the participant’s final sampling date via an online questionnaire (Formdesk, Innovero Software Solutions BV, Wassenaar, Netherlands) and e-mail.Blood samples were collected in two sampling rounds, dividing the follow-up time into two potential seroconversion periods. Period 1 was the period between study vaccination and the first sampling round, after about 3 months for the core hospitals and about 6 months for the other hospitals. Period 2 was the period between the first and the second sampling rounds, about 12 months after study vaccination for all hospitals. Core hospital participants donated 10 mL of serum via in-hospital venipuncture, which was processed, frozen, and transported frozen to the Dutch National Institute for Public Health and the Environment (RIVM in Dutch). The other participants, as well as core hospital participants who missed their venipuncture visit, were asked to collect about 300 μL of peripheral capillary blood at home by fingerprick using a sampling kit that was sent to them by mail; they returned the sample by mail to the RIVM laboratory on the day of collection. The RIVM laboratory used an in-house magnetic immunoassay on a Luminex platform to determine the presence and concentrations of SARS-CoV-2-specific antibodies (