relationship between bacillus calmette guerin vaccination policy and coronavirus disease-2019 (covid-19) incidence

Bacillus calmette guerin (bcg) was designed for protecting children against tuberculosis. also, it can protect against other infectious diseases through the induction of trained immunity. due to its heterologous protective effects, the bcg vaccine has been proposed as a treatment option for coronavirus disease-2019 (covid-19). epidemiological studies have found that countries without bcg vaccination policy have experienced higher mortality rates related to covid-19 infection than those with bcg vaccination policy. however, there are some confounding factors such as age, population intensity, immigration, the pandemic phase, and data accuracy that may affect these results. therefore, this hypothesis should be evaluated by clinical trial studies. large-scale clinical trials are in progress to investigate if the bcg vaccine could be used as a useful tool for protection against covid-19 infection.