The possible puzzles of BCG vaccine in protection against COVID-19 infection

BACKGROUND: The paper aimed to analyze and evaluate the present literature data on the clinical effectiveness of using the bacillus Calmette–Guérin (BCG) vaccine in protecting against the novel coronavirus disease 2019 (COVID-19). MAIN BODY: Several novel clinical data have shown a relationship between the vaccinated population with the bacillus Calmette–Guérin (BCG) vaccine and the severity and mortality rate from coronavirus disease 2019 (COVID-19). However, the linkage between the BCG vaccine and COVID-19 infection mortality and morbidity rate is still ambiguous. The BCG has been protected previously from many other respiratory viral infections. The efficacy of the BCG vaccine in the protection against COVID-19 depends on various factors including social, economic, cultural norms, mitigation efforts, health infrastructure, and demographic differences between countries. CONCLUSION: Thus, the literature analyses show a noticed difference between the countries that follow national vaccination programs than in countries that do not follow such programs (Italy, Netherlands, USA). However, there are not any recommendations for using BCG in the protection against severe cases of COVID-19. The severity of COVID-19 maybe depends on the age, immune state of the patient, and the level of vaccine coverage. The possible reason for BCG protection is trained immunity in both diseases.