COVID-19 AND BCG VACCINE: IS THERE A LINK?

The spread of the novel coronavirus infection (COVID-19) makes the search for new approaches to prevent the infection of great importance. As one of the relevant approaches, the vaccination of risk groups with BCG vaccine has recently been suggested. BCG (Mycobacterium bovis, Bacillus Calmette- Guerin) is a live vaccine for tuberculosis, which is used in many countries with a high tuberculosis prevalence and helps preventing childhood tuberculosis, primarily, military disease and tuberculosis meningitis. Whether BCG may be used to increase the protection against COVID-19 is currently a question of debates. The review considers scientific background underlying possible impact of BCG in increased protection against COVID-19. BCG is able of inducing the heterologous and trained immunity, and its capacity to stimulate antiviral immune response has been demonstrated in experimental animals and humans. Our comparison of the dynamics of COVID-19 morbidity and mortality in countries with different BCG vaccination policies has demonstrated a milder course of COVID-19 (i.e., a slower increase in disease cases and mortality) in countries where BCG vaccination is mandatory for all children. However, an association between BCG vaccination and a milder COVID-19 course is not obligatory direct. Other factors that may affect the association, such as the level of virus testing, the rigidity and the speed of quarantine implementation and others are discussed. An important argument against a role of BCG in the protection against COVID-19 is that BCG is given in childhood and may hardly induce long-lasting immunity. Because mandatory BCG vaccination is implemented in countries with high TB burden and because in these countries latent tuberculosis infection is widely spread, we suggest a hypothesis that latent tuberculosis infection may contribute to the maintenance of heterologous/trained antiviral immunity in countries with mandatory BCG vaccination. Four countries have recently initiated clinical trials to investigate whether BCG vaccination can increase the level of protection against COVID-19 in risk groups. The results of these studies, as well as COVID-19 epidemiological modeling will help understanding the impact of BCG in the level of the protection against COVID-19. Performing analogous clinical trials in Russia seems appropriate and scientifically sound.