Study population and setting
The study is based on global data collected from the Worldometer website, data about BCG vaccination and coverage, incidence of TB and latency from the Institute for Health Metrics and Evaluation, 2018 Global Burden of Disease Collaborative Network, Global Burden of Disease Study 2017, Seattle and WHO data from https://ourworldindata.org/coronavirus-data. Countries reporting more than 1000 cases COVID-19 were included and divided into two groups based on highest number of COVID-19 deaths and total number of cases as the top 20% and bottom 80%.
Summary of Main Findings
The analysis of the incidence data, case fatality rates (CFR) and the interim infection mortality rates (i-IFR) of COVID-19 in the bottom 80% countries shows that on average lower CFR and i-IFR are associated with a higher proportion of latent tuberculosis infection (LTBI). However, this correlation is lost when the top 20% of countries in COVID-19 deaths are considered.
The authors highlight that choosing an interim-infection mortality rate might be a better measure to understand and estimate the true mortality rate as opposed to using the case fatality rate and incidence.
The author used the proportion with LTBI and BCG vaccine coverage interchangeably. This may lead to erroneous assumptions, for example, in the case of a top 20% country with high mortality and incidence such as Italy, which doesn’t have national BCG coverage, but still has a high proportion with LTBI (12.89%).
Further, the author has made an assumption that having no national BCG vaccination coverage would be associated with higher mortality rates. This does not take into account other factors such as higher proportions of patients with comorbidities. The analysis also does not take into account age group, which is of particular interest in countries such as Iran (BCG implemented in 1984), and China (BCG implemented in 1966-76), where older age groups may remain unvaccinated.
The author shows that BCG vaccine coverage alone cannot account for the large variability in the case fatality rate between countries which have a national BCG immunization policy and those which do not. The underlying causes of the differences in the fatality rates needs to be scrutinized further based on other factors such as genetics, gender, age, underlying comorbidities or virulence of the strains currently circulating in different regions.