The COVID-19 pandemic challenges available hospital capacity. Strategies to protect health care workers (HCW) are desperately needed. Bacille Calmette- Guérin (BCG) has protective non-specific effects against other infections; a plausible immunological mechanism has been identified in terms of "trained innate immunity".

The primary objective of the study is to evaluate whether BCG can reduce unplanned absenteeism due to illness among HCW during the COVID-19 pandemic. Secondary objectives are to reduce the number of HCW that are infected with COVID-19, reduce hospital admissions for HCW and to improve the capacity for clinical research.

Design: Single-blind, parallel-group placebo-controlled multi-centre block randomized trial including a total of 1050 HCW. The study sites will be the Manhiça hospital in Mozambique, Central Hospital Dr. Agostinho Neto and Central Hospital Dr. Baptista de Sousa in Cape Verde and Hospital Nacional Simão Mendes and other hospitals in the capital Bissau in Guinea-Bissau. Population: HCW (nurses/physicians/others) ≥18 years.

Intervention: Block randomization 1:1 to intradermal standard dose (0.1 ml) of BCG vaccine or placebo (saline). Endpoints: Primary: Days of unplanned absenteeism due to illness. Secondary: Days of absenteeism because of documented COVID-19; cumulative incidence of infectious disease hospitalizations.

Follow-up: mobile phone interviews every second week, regarding symptoms, absenteeism and causes, COVID-19 testing (if done) and their results.

Perspectives: If BCG can reduce HCW absenteeism it has global implications. The intervention can quickly be scaled up all over the world.

The main hypothesis for this study is that BCG vaccination of HCW reduces unplanned absenteeism due to illness by 20%.

Trial design Single-blind, parallel-group placebo-controlled adaptive multicentre trial, with block-randomisation 1:1 (treatment:placebo) in blocks of 20 within strata defined by gender (male/female) and occupational group (doctors/ nurses/other) to an intradermal standard 0.1 ml dose of BCG vaccine or placebo (saline) including a total of 1050 HCW (BCG arm and placebo arm).

Study population:

Health Care Workers defined as a person who delivers care and services to the sick and ailing either directly as doctors and nurses or indirectly as aides, helpers, laboratory technicians, or even medical waste handlers.

Intervention:

The intervention will consist of the administration of an intradermal injection of a standard 0.1 ml adult dose of attenuated Mycobacterium bovis BCG (Bacillus Calmette-Guerin), Danish strain 1331, 2-8 × 10*5 CFU or the placebo comparator: intradermal standard 0.1 ml saline solution (NaCl 0.9%). Both BCG and saline solution will be injected in the skin over the upper deltoid muscle.

Blinding:

The BCG vaccine will be administered by study physicians/nurses, who are not blinded but also not involved in the data collection. Participants, data collectors and data entry clerks will be blinded to the treatment allocation.

In case of serious adverse events, the participant can be unblinded after consultation with the investigator or the vaccinating physician/nurse. When the study has ended, all participants will receive information about the intervention that they received.

Follow-up:

Follow-up will last for 6 months (182 days). Every second week, participants will be contacted over telephone and interviewed for symptoms and absenteeism from work. By the end of follow-up, participants will be invited for another POC test for COVID-19 serology.

Sample size:

The sample size was calculated on the basis of the primary hypothesis. A total of 1050 HCW randomized with an estimated loss to follow-up of 5%, and a mean number of days off work due to illness in the control group of 5 days (SD=5) over a 6-month period will demonstrate a reduction among BCG vaccinated of 20% for a mean absence of 4 days (80% power and alpha 0.05). The estimated loss to follow-up (5%) was based on past telephone-based surveys conducted in Mozambique and Bissau.

Inclusion Criteria:

• health care worker (physician, nurse, or other) based at one of the research hospitals.
• age ≥18 years

Exclusion Criteria:

• known allergy to (components of) BCG or serious adverse events to prior BCG administration;
• known previous, active or latent infection with Mycobacterium tuberculosis or other mycobacterial species
• fever (>38 C) within past 24 hours;
• previous confirmed COVID-19 (positive test - PCR or antibody);
• suspicion of active viral or bacterial infection
• severely immunocompromised subjects
• self-reported HIV infection
• self-reported pregnancy;
• active solid or non-solid malignancy or lymphoma within the prior two years;
• contraindications for live attenuated vaccine administration.
• not having a mobile phone.

Non-identifiable individual data can be shared on the basis of a data sharing proposal sent to professor Inês Fronteira ifronteira@ihmt.unl.pt

When follow-up has been completed and the dataset have been closed.

Non-identifiable individual data can be shared on the basis of a data sharing proposal sent to ifronteira@ihmt.unl.pt

Participants will be blinded to treatment. The physicians administering the BCG vaccine or placebo will not be blinded. Those assessing outcomes will be blinded to the randomization allocation. In case of serious adverse events, the participant can be unblinded after consultation with the coordinating PI or sponsor.