Phase III Placebo-controlled adaptive multi-centre randomized controlled trial Interventional (Clinical Trial). The study will include nine hundred healthcare workers in the isolation hospitals for COVID-19 cases; they will be randomly assigned to receive either BCG vaccine or normal saline.

Nine hundred healthcare workers will be randomly assigned to receive intradermal injection of either BCG vaccine or normal saline.

During the study:

1. Mantoux Tuberculin skin testing (will be done prior to administration of the BCG vaccine in the first group by intradermally injecting 0.1 mL of liquid containing 5 TU (tuberculin units) of purified protein derivative (PPD) in the volar aspect of the left forearm. The test is being read after 48-72 hrs. A positive tuberculin test is defined as induration ≥10 mm.
2. Participants will be assessed clinically on daily basis for COVID 19 infection in the form of: fever, dry cough, fatigue, & dyspnea.
3. PCR sampling for COVID-19 will be withdrawn for all participants at the end of the study.
4. The duration of follow-up for each participant depends on the interim results of the primary endpoint and the probability of obtaining a result, with a maximum of 6 months.

Randomization method:

A block-randomization scheme will be generated by computer software. Participants will be randomized between 0.10 mL intradermal injection of BCG vaccine or placebo normal saline in a 2:1 ratio (600 will receive BCG & 300 will receive placebo saline). In Egypt, the available BCG vaccine is the Copenhagen (Danish strain).

End point of the study:

• Primary endpoint: Confirmed COVID-19 infection
• Secondary endpoint: number of days of (unplanned) absenteeism for any reason.
• Tertiary endpoints: number of days of (unplanned) absenteeism because of documented COVID-19 infection, and the cumulative incidence of hospital admission, ICU admission, and mortality.

Inclusion Criteria:

• Healthy healthcare workers defined as nurses and physicians working at emergency rooms, ICUs, and wards of isolation hospitals

Exclusion Criteria:

• Immunosuppression
• Pregnancy & lactation
• Chronic underlying medical illness
• 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.
• Fever (>38 C) within the past 24 hours
• Current bacterial infection
• Recent viral infection
• Refusal of doing tuberculin test in the first group