[Progress in possible effects of BCG vaccine on the prevention of SARS-CoV-2 infection: An update].

The research and development of the coronavirus disease 2019 (COVID-19) vaccine is being carried out globally. Although vaccine research and development technology has made great progress, the possibility of obtaining a safe and effective vaccine that can control the global epidemic in a short period of time is still low due to the antibody-dependent enhancement effect (ADE) of the vaccine and the mutation of the virus. In the absence of specific treatment for COVID-19, finding other alternative protection schemes has become another treatment idea. Epidemiological studies have found that, in this COVID-19 epidemic, countries with long-term Bacillus Calmette-Guerin (BCG) vaccination policies have relatively less cases and lower mortality rates than countries without relevant policies. This phenomenon may be related to the "training immunity" effect of BCG. In order to further clarify the preventive and protective effects of BCG vaccine on SARS-CoV-2 infection, a number of clinical trials are underway.