Researchers tracking COVID-19 mortality early in the pandemic noticed that some countries had a significantly lower mortality rate than others. After accounting for factors like population demographics and density, some researchers suggested that lower mortality correlated with higher rates of bacillus Calmette–Guérin (BCG) vaccination.
The BCG vaccine is given to babies in countries where tuberculosis is common — to protect them from this bacterial infection. One theory, published in the Proceedings of the National Academy of Sciences (PNAS), was that BCG vaccination might train the innate immune system — the body's first line of defense—to also better respond to the SARS-CoV-2 virus. The study authors, researchers at Virginia Tech and the National Institute of Allergy and Infectious Diseases, emphasized that more research was needed.
In a new Letter, published Sept. 29 in PNAS, researchers at La Jolla Institute for Immunology (LJI) re-examine the correlation between BCG vaccination and COVID-19 case severity. Their updated analysis, based on data from the Johns Hopkins University Coronavirus Resource Center, compares mortality data in 51 countries. The original study tracked cases through April, but the new analysis follows cases through August.
The scientists, led by LJI Professor Bjoern Peters, Ph.D., point out that differences in case severity that could be attributed to BCG vaccination were no longer significant as the pandemic continued into August. The researchers show that Bolivia, Panama, Columbia, Peru, Brazil, Mexico and South Africa had the largest increases in COVID-19 deaths as of Aug. 1. These countries also have high BCG coverage.
For the BCG vaccine to provide protection, the immune response to the vaccine would also have needed to last for many years after infancy. The LJI scientists didn't find evidence to support this idea.