Efficacy of BCG Vaccination in the Prevention of COVID19 Via the Strengthening of Innate Immunity in Health Care Workers

Randomized, multicenter phase III controlled trial, in 2 parallel arms: * One group vaccinated with BCG * One group receiving placebo (0.9 % saline) Randomization in a 1: 1 ratio will be stratified on the center. The study will be proposed to all heath care workers eligible. At screening, the investigator will propose the study to the participant and explain the details by providing an information letter. After signature of the consent, evaluations specific for the study will be undertaken, such as clinical examination, blood sampling, nasopharyngeal sampling in case of clinical signs. At the inclusion visit, participants corresponding to the inclusion criteria will be randomized to receive BCG or placebo. Participants will receive a single dose of BCG vaccine (or placebo). The adult dose of BCG vaccine (or placebo) is 0.1 mL injected intradermally over the distal insertion of the deltoid muscle onto the humerus (approximately one third down the upper arm). Follow-up visits will be done respectively at M3 and M6 and phone contacts between these two visits. Blood samples will be collected prior to randomization (V0) and at 3 and 6 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. Participants will be followed for 6 months with regular mobile phone text messages 2 times weekly to collect local and general reactions until 30 days after vaccination and during the study to identify and detail COVID-19 infection. Immunological studies will be conducted at a central Laboratory. Analysis will take place after M6 visits. A nested study will be carried out on blood samples of 72 study subjects in order to evaluate the impact of COVID-19 on innate immunity. These blood samples will be collected at M6 among 6 groups, each containing 12 study subjects: * One group of subjects vaccinated with BCG that have remained sero-negative for SARS CoV2 * One group of subjects having received placebo that have remained sero-negative for SARS CoV2 * One group of symptomatic subjects vaccinated with BCG * One group of symptomatic subjects having received placebo. * One group without symptoms but sero-positive for SARS CoV2 vaccinated with BCG * One group without symptoms but sero-positive for SARS CoV2 vaccinated with placebo