Worldwide inverse correlation between Bacille Calmette-Guérin immunization and COVID-19 morbidity and mortality

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The COVID-19 pandemic has spread to all countries in the world after more than six months since it was first reported in late 2019, and different countries have been impacted differently. Correlation analysis between COVID-19 death numbers and different demographic and socioeconomic factors for all world countries (n=210) as of June 1, 2020, reveals that COVID-19 deaths per million population in a country significantly correlates with the country's median age (r=0.48, p=4.8e-4) and per capita gross domestic product (GDP) (r=0.55, p=4.14e-5), and inversely correlates with the country's Bacille Calmette-Guérin (BCG) vaccination rate (r=-0.63, p=9.9e-7). COVID-19 death is found not significantly associated, however, with a country's policy stringency index, population density, extreme poverty rate, hospital beds availability per thousand people, and diphtheria-tetanus-pertussis (DTP3) immunization. Old age is likely a confounding factor for the correlation between COVID-19 and per capita GDP (r=0.66, p=2.3e-7). To control for possible confounding effects of age, countries with similar median age were grouped and analyzed. The inverse correlation between BCG vaccination rates and COVID-19 case (r=-0.338, p=0.0082) and death (r=-0.411, p=0.0011) remained significant among the top 61 countries with highest median age. The current study suggests that BCG might be protective against SARS-CoV-2 infection.