Introduction
The epidemic caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) in Wuhan, China in December 2019 was identified in February 2020 by World Health Organization (WHO) as corona virus disease 2019 (COVID-19).1,2 SARS-CoV-2 infection may appear asymptomatic or may present as mild upper respiratory tract disease, however viral pneumonia may also lead to respiratory failure.3 While the treatment research against coronavirus pandemic continues all over the world, the group of patients with high probability of severe disease is tried to be determined.
COVID-19 mortality and morbidity rates differ disproportionately between countries to the level of measures and restrictions taken by them. It is possible that the sociocultural or lifestyle differences of citizens and the differences in the health policies of countries make surprising differences in COVID-19 epidemiology in different countries. Accordingly, factors underlying the severe epidemic in some countries are tried to be clarified. It has recently been suggested that countries’ Bacillus Calmette-Guérin (BCG) vaccination policies can affect the epidemiology of COVID-19.4 This hypothesis is mainly based on the fact that countries such as Italy, who do not have a BCG-vaccination policy, are severely affected by the COVID-19 outbreak despite implementing high-level of social isolation measures, whereas countries with immunization policy, such as Japan, were slightly affected by the outbreak, even though they did not take high level of measures.4
In the present study, the severity of COVID-19 pneumonia in BCG vaccinated and unvaccinated people is compared and factors associated with the severity of the disease is investigated in a country that has been running national BCG vaccination program for nearly 70 years.