Study population and setting
Data from 57 countries were selected from Our World in Data as of April 20, 2020. The countries had to have a population of at least 1 million, and have at least a 10-day observation period. The baseline data for each country was defined as the date when there were at least 10 deaths in that particular country. In order to exclude differing start times for social distancing interventions in the 20 countries that had them, the death toll data for each country was trimmed by 30 days post-baseline.
Summary of Main Findings
Countries with national BCG vaccination programs had a statistically significantly longer doubling time of death toll compared to countries without one. Within countries with national BCG vaccination programs, strain of BCG used played a statistically significant role, with countries using the Tokyo 172-1 strain having a longer doubling time than countries using other strains.
This study builds on another study by Miller et. al. by using death toll instead of deaths per million inhabitants to account for differences in detection rates and epidemic stages in different countries.
The data in this study only includes 57 countries, and there were many more countries with national BCG vaccination programs (42) than countries without them (15) in this data set. The national BCG vaccination data is only reliable for up to the past few decades, and this might have important implications as older individuals who are more vulnerable to COVID-19 may or may not have been vaccinated prior to there being reliable records on this.
It is possible that the BCG vaccine may grant some protective effects against COVID-19, slowing the doubling time of the death toll in countries with national BCG vaccination programs.