A potential subject who meets any of the following criteria will be excluded from participation in this study: Known allergy to (components of) the BCG vaccine or serious adverse events to prior BCG administration Known active or latent Mycobacterium tuberculosis or with another mycobacterial species. A history with- or a suspicion of M. tuberculosis infection. Fever (>38 C) within the past 24 hours Pregnancy Suspicion of active viral or bacterial infection Vaccination in the past 4 weeks or expected vaccination during the study period, independent of the type of vaccination. Severely immunocompromised subjects. This exclusion category comprises: a) subjects with known infection by the human immunodeficiency virus (HIV-1), b) neutropenic subjects with less than 500 neutrophils/mm3, c) subjects with solid organ transplantation, d) subjects with bone marrow transplantation, e) subjects under chemotherapy, f) subjects with primary immunodeficiency, g) severe lymphopenia with less than 400 lymphocytes/mm3, h) treatment with any anti-cytokine therapies. i) treatment with oral or intravenous steroids defined as daily doses of 10mg prednisone or equivalent for longer than 3 months, or probable use of oral or intravenous steroids in the following four weeks Active solid or non-solid malignancy or lymphoma within the prior two years Direct involvement in the design or the execution of the BCG-CORONA study Expected absence from work of ≥4 of the following 12 weeks due to any reason (holidays, maternity leave, retirement, planned surgery etc) Employed to the hospital < 22 hours per week Not in possession of a smartphone

A potential subject who meets any of the following criteria will be excluded from participation in this study: Known allergy to (components of) the BCG vaccine or serious adverse events to prior BCG administration Known active or latent Mycobacterium tuberculosis or with another mycobacterial species. A history with- or a suspicion of M. tuberculosis infection. Fever (>38 C) within the past 24 hours Pregnancy Suspicion of active viral or bacterial infection Vaccination in the past 4 weeks or expected vaccination during the study period, independent of the type of vaccination. Severely immunocompromised subjects. This exclusion category comprises: a) subjects with known infection by the human immunodeficiency virus (HIV-1), b) neutropenic subjects with less than 500 neutrophils/mm3, c) subjects with solid organ transplantation, d) subjects with bone marrow transplantation, e) subjects under chemotherapy, f) subjects with primary immunodeficiency, g) severe lymphopenia with less than 400 lymphocytes/mm3, h) treatment with any anti-cytokine therapies. i) treatment with oral or intravenous steroids defined as daily doses of 10mg prednisone or equivalent for longer than 3 months, or probable use of oral or intravenous steroids in the following four weeks Active solid or non-solid malignancy or lymphoma within the prior two years Direct involvement in the design or the execution of the BCG-CORONA study Expected absence from work of ≥4 of the following 12 weeks due to any reason (holidays, maternity leave, retirement, planned surgery etc) Employed to the hospital < 22 hours per week Not in possession of a smartphone