Study population and setting
BCG vaccination policies across countries were collated, along with country-level COVID-19 cases and deaths. Data were gathered on March 21st, 2020. This is an ecological study. No individual data were examined.
Summary of Main Findings
Most of the countries with low-income levels reported zero deaths due to COVID-19 and have universal BCG policies in place; as this phenomenon may reflect more limited surveillance systems and underreporting, these countries were excluded. When assessing the number of reported cases to-date, among middle-high and high-income countries only, the number of cases was significantly lower among countries that have a universal BCG policy in place. There was no significant correlation between the year that vaccination started and the total number of COVID-19 cases. Similarly, among middle-high and high-income countries, there was a significantly lower mortality rate among countries that have universal BCG policies compared with those that do not. There was a positive significant correlation between the year of the establishment of universal BCG vaccination and the mortality rate (consistent with the idea that the earlier the policy was established, the larger fraction of the elderly population would be protected). The longer the policy was in effect, the lower the mortality rate.
The range of analyses conducted helps bolster the finding of the protective role of BCG vaccination particularly on mortality.
The authors acknowledge that both mortality rates and the number of cases are in part due to levels of testing and reporting. They attempt to account for this by excluding the low-income countries from their analyses, in which they expect testing and reporting to be low. Arguably, other middle- or high-income countries may also have low testing rates and mortality data lag. Additionally, no adjustment was made for other variables that may explain differences between countries that did and did not establish universal BCG vaccination (potential confounding).
This has important implications for future studies on the role of BCG vaccines in stemming SARS-CoV-2 spread. The authors provide enough evidence to warrant more detailed and careful attention to this potential prevention measure, though caution should be applied to data from ecological studies, particularly when other confounders in health care systems or populations which may account for BCG policies and mortality are not addressed.