The study collected data on year of introduction and termination of the BCG vaccination and types of strains used from the BCG World Atlas (Zwerling et al., 2011). The data were then matched with country-age-level COVID-19 conﬁrmed case statistics from information available from each government’s website. This resulted in a data set over 17 countries with relevant information: Australia, Colombia, the Czech Republic, Denmark, Finland, India, Japan, Korea, Latvia, New Zealand, Romania, Singapore, Spain, Sweden, Switzerland, Thailand, and Vietnam. Exact age-level case statistics were available in Colombia, the Czech Republic, Thailand, Vietnam, and Singapore. In remaining countries, only a 10-year age-group level data were available. Of these 17 countries, the Japan strain or Russia/Bulgaria strain have been used in Japan, Thailand, Colombia, and Latvia. In Colombia, case statistics at the nationality level are available. Hence, the presented paper focuses on people with Colombian nationality in Colombia data, whereas all residents not diﬀerentiated by nationality are included in the other countries’ data. The immunization rate of infants at each year is available from the World Health Organization’s website (WHO, 2020). We refer to this in the ﬁgure, even though this does not necessarily correspond to the immunization rate at each age as of 2020.
This paper first conducted a regression discontinuity analysis using data from Colombia, the Czech Republic, Thailand, Vietnam, and Singapore. The basic assumption of the analysis is that factors such as indoor air hygiene that can aﬀect the COVID-19 infection rate do not discontinuously change around the age at which the BCG vaccination was introduced. If a comprehensive health care reform is implemented simultaneously with the introduction of a universal BCG policy, then this assumption is violated. However, any bias should occur in the same direction with the expected BCG eﬀects, producing an upward bias. The presented paper demonstrates that even with this potential upward bias, any improvement in the COVID-19 infection rate was observed at the age of policy change. The study exploited the following timing of policy changes. In Colombia, the BCG vaccination was introduced in 1960 as a mass campaign targeted at young people under 15 years old (Arbeláez et al., 2000). In 1978, the strain changed from English/Japan to French. Thus, the eﬀects at three distinct time boundaries, 1945 (i.e. 1960 minus 15) when people covered by the campaign were born, 1960 when the vaccination was introduced, and 1978 when the strain changed, were exploited. In the Czech republic, BCG vaccination was introduced in 1953 for people under 18 (Tu et al., 2012), and stopped universal vaccination in 2010 (Vašáková, 2013). They changed from the Plague to the Russia strain in 1981 and changed from the Russia to the Danish strain in 1994. Therefore, the eﬀects at 1935 (i.e. 1953 minus 18), 1953, 1981, 1994, and 2010 were investigated. In Thailand, the BCG vaccination was introduced in 1977 for infants. In 1987, they changed from the Danish to the Japan strain. Between 1987 and 1991, they revaccinated at age 7. Therefore, the eﬀects at 1977 and 1991 were investigated. In Vietnam, the BCG vaccination was introduced in 1985 for infants (Jit et al., 2015). Older people were not vaccinated, because they do not have a revaccination policy. In Singapore, the BCG vaccination started in 1957 (Goh, 1985). Because they revaccinate at ages 6, 11, and 15, the eﬀect at 1942 (i.e. 1957 minus 15) was studied. Vietnam and Singapore do not use the Japan or Russia/Bulgaria strains.
Second, a diﬀerence-in-diﬀerence analysis was conducted using all 17 countries with 10-year-age-group level case statistics. The identiﬁcation assumption in the current model is that without the BCG vaccination the expected log diﬀerence of the infection rates across age-groups are the same across countries. This paper constructed a treatment variable that indicates the age ratio within age-group t covered by the BCC vaccination ( ) in country i. The paper also constructed a treatment variable that indicates the vaccination using the Japan or Russia/Bulgaria strain ( ) and another treatment variable that indicates the vaccination using other strains ( ). The paper then regressed the log of the number of cases per thousand on these treatment variables, controlling for country and age-group dummies as: