The coronavirus disease-2019 (COVID-19) pandemic is one of the most devastating things that happened in the world which has taken the lives of millions of people and has brutally shattered the world economy. This pandemic has instigated an urgent need for a vaccine to reduce the ongoing morbidity and mortality. Bacillus Calmette-Guerin (BCG) apart from being used as an effective and old vaccine against tuberculosis has some known off-target protection effect and is getting more attention in this scenario. BCG confers nonspecific innate immune-boosting effects called trained immunity against secondary infection. Various recent publications have proposed the inverse relationship between the COVID-19 morbidity and mortality with that of BCG coverage of that country on the basis of epidemiological studies. However, these studies have not considered the confounding factors, and a lot of recent articles are contradicting these epidemiological and observational data. Several random control trials for BCG on health-care workers and elderly people are ongoing worldwide and could depict the actual relation between COVID-19 and BCG protection. Although a recent trial has found a protective function of BCG against COVID-19 in health-care workers, more results of the trials can only give approval on this. There has been a shortage of BCG worldwide due to its use in bladder cancer and vaccination in neonates, and hence, its use should be carefully regulated. In this review, we have tried to summarize the various issue and conflicts on BCG to be used as a temporary solution to COVID-19.
Keywords: Bacillus Calmette-Guerin; clinical trials; coronavirus disease-2019; severe acute respiratory syndrome coronavirus 2; trained immunity; vaccine.