| Author/year | #Countries | Confounders or predictors/dependent variable | Method | Findings |
|---|
| Escobar et al. 2020 [13] | 22 countries | At least one death per million inhabitants, ≥ 15% of population with an age of 65 years or more, > 60% of population living in urban areas, < 300 inhabitants km2, and an HDI of > 0.7/mean, median, and maximum deaths per million | ANOVA, t test, linear regression | BCG could have a protective effect |
| Wickramasinghe et al. 2020 [39] | 175 countries | Demographic variables, BCG coverage and policy, age-specific TB incidence, and income level/morbidity and mortality of COVID-19 | Linear regression analysis | Immunity from BCG as a likely explanation for the variation of COVID-19 cases and deaths across countries |
| Kumar et al. 2020 [40] | 67 countries | Temperature and BCG vaccination/number of new cases and mortality per day | Multivariable two-level negative binomial regression analysis | High temperature might not be associated with low transmissibility and BCG vaccination had a low fatality rate of COVID-19 |
| Klinger et al. 2020 [41] | 55 countries | Economic, demographic, health-related, and pandemic restriction related quantitative properties/death per million, positively validated cases per million, hospitalization with serious and critical conditions, and recovered | Multivariable regression | BCG immunization coverage, esp. among the most recently vaccinated population, contribute to attenuation of the spread and severity of the COVID-19 |
| Berg et al. 2020 [42] | Confirmed cases (134 countries) and deaths (135 countries) | Median age, gross domestic product per capita, population density, population size, net migration rate, and various cultural dimensions/the number of cases and deaths | Linear mixed effect models with restricted maximum likelihood estimation | Mandated BCG vaccination can be effective against COVID-19 |
| Joy et al. 2020 [43] | 160 countries | Population density, income group, latitude, and percentage of the total population under age groups 15–64 and above 65 years of each country/the difference in the incidence of COVID-19 cases | Meta-regression | BCG is associated with reduced COVID-19 infections if the BCG vaccine coverage > 70% |
| Miller et al. 2020 [44] | 60 countries | Age distribution, income per capita, stage of the epidemic of a country, and quality of the medical care country's response to COVID-19/morbidity and mortality for COVID-19 | Multivariate linear analyses | BCG vaccination is correlated with reduced mortality (from May 5th, there is no significant correlation between start year of the vaccination and mortality per million people < 65 years) |
| Szigeti et al, 2020 [45] | The top 68 countries for number of cases | BCG vaccination status before 1980, historic colonization status, median age, urban population percentage, population density, tests per million (additional factor for May), and air passengers/deaths per million | Multiple regression modeling | No significant association between BCG vaccination and COVID-19 mortality |
| Wassenaar et al. 2020 [46] | 18 countries | Countries that are well into the developing pandemic and compared the BCG vaccination programs in place since the 1950s | | There is currently no compelling evidence of protective effect |
| Hensel et al. [47] | 74 countries | GDP, median age, percent urban population, hospital beds per 1000 inhabitants, smoking, population density, diabetes and CVD-related deaths, vaccine strain, and testing rates/spread and mortality of CoV-2 | Univariate and multivariate regression | epidemiological findings do not provide evidence to correlate overall BCG vaccination with the spread and mortality of CoV-2 |
| Matsuura et al. 2020 [48] | 17 countries | | Regression discontinuity analysis (on some countries) and difference-in-difference analysis (all countries) | The results do not support this hypothesis that BCG can be effective |
| Arlehamn et al. 2020 [14] | 51 countries, | Human Development
Index of > 0.7, > 60% urban population, < 300
Inhabitants per km2, and > 1 COVID-19 death per million/death report | Pearson correlation coefficient | Current mortality rates of the COVID-19 do not support a clear negative correlation with BCG |