BCG Against SARS-CoV-2: Second Youth of an Old Age Vaccine?

The sudden outbreak of the COVID-19 pandemic, caused by SARS-CoV-2, has put the whole world into a difficult situation, asking for the immediate development of therapeutics and vaccines against the disease. Bacillus Calmette–Guérin (BCG), an attenuated strain of Mycobacterium bovis, has been administered for decades in many countries against tuberculosis. Today, when a solution against SARS-CoV-2 is urgently needed, the BCG vaccine has again come into the limelight owing to its earlier prevention of non-specific diseases. Data suggest a higher mortality rate of COVID-19 in non-BCG vaccinated countries, whereas the nations opting for BCG immunization have a comparatively lower mortality rate. The BCG vaccine is known to induce ‘trained immunity’ and generate ‘non-specific’ heterologous immune responses. It can confer anti-viral immunity by eliciting the production of pro-inflammatory cytokines, IL-6, TNF-α, IFN-γ, and IL-1β. Though the initial results look promising, a long trail still needs to be followed to avoid false promises. The accuracy of nationwide data, the role of an already activated immune system against ‘cytokine storms’, optimization and timing of vaccine dosage, and balancing demand-supply are some of the relevant issues that must be resolved before reaching a final conclusion.