BCG Vaccine in Relation To COVID-19 Morbidity and Mortality

The COVID-19 pandemic caused by severe acute respiratory syndrome coronavirus-2 (SARS CoV-2) significantly interrupts daily life activities leading to global social and economic disruption. The infected cases and deaths due to COVID-19 are continuously rising. The lower incidence of COVID-19-cases among countries with Bacillus-Calmette-Guérin (BCG) vaccination background raises the possibility that BCG vaccine could be a protective against COVID-19. Here, we studied the prevalence of COVID-19-cases in relation to population received BCG vaccination. The collected data were compared at two different time points for proper validation and execution of appropriate association. Our investigations showed a correlative association between BCG vaccination and the number of COVID-19-cases. The association obtained can be supported by the fact that BCG vaccination is associated with an increase in trained immunity. This condition has been used previously to establish a protective effect by BCG vaccine against infections due to pathogens other than Tuberculosis (TB). The data obtained from this study could encourage the ongoing clinical studies regarding the use of BCG vaccine against COVID-19.

Most patients with COVID-19 exhibit mild to moderate symptoms, while 5% eventually develop acute respiratory distress syndrome (ARDS) and/or multiple organ failure leading to death [1,2]. There's currently no treatment approved for COVID-19, although vaccines and drugs are currently under study. One of the options under trial is the use of Bacille-Calmette-Guérin (BCG) as potential protection against COVID-19.
BCG, a live attenuated Mycobacterium bovis introduced in 1921, is mainly used against tuberculosis, leprosy and other non-tuberculous mycobacteria such as buruli ulcer [3][4][5]. Several studies have suggested the use of BCG vaccine as an effective treatment for acute respiratory tract infections [6,7]. This suggestion is attributed to the findings that BCG can decrease the level of virus in infected patients. A recent study demonstrated that countries with a mass BCG vaccination policy exhibits less COVID-19 cases than non-BCG vaccinated countries. This has raised the hope that vaccination with BCG may confer protective effects against COVID-19 infection caused by SARS-COV-2. However, the data supporting this hypothesis is still lacking. To test the validity of such hypothesis, a correlation of the epidemiological data from BCG vaccinated versus non-vaccinated populations with the prevalence of SARS-CoV-2 infection and mortality rates is needed. Interestingly, WHO is currently studying this hypothesis including several clinical trials on selected cohorts.
Randomized controlled trials conducted earlier have provided evidence that BCG may protect against respiratory infections [3].
The beneficial effects of the BCG vaccine can include the induction of metabolic and epigenetic changes that enhance the non-specific activation of innate immune cells [8,9]. The mechanism of immune activation induced in response to BCG vaccination can be classified as antigen-dependent and antigen-independent mechanisms. In the case of antigen-dependent mechanism, BCG vaccine enhanced both innate and adaptive immune responses by activating macrophages, dendritic cells, neutrophils, memory B and T cells [9,10].
Since the bacterial antigens are different from viral antigens, this mechanism is unlikely in the case of SARS-CoV-2. In the case of antigen-independent mechanism, the effect of BCG vaccine is due to the non-specific effect on the innate immunity. The bystander B and T cells are activated, and this activation can lead to long-term activation of innate immune cells, termed as trained immunity [9].
The non-specific effect of trained immunity by BCG vaccine is more likely the one with potential role against SARS-CoV-2 infection. Important features of the trained immunity are that it occurs through epigenetic reprogramming of monocytes at the site of infection. These monocytes undergo histone modification at promoter sites of genes encoding inflammatory cytokines, leading to long-term changes in their ability to respond to novel stimuli and resulting in an increasingly active immune response when they are re-activated [11]. It has been reported that BCG immunized population exhibits an increase in the quantities of cytokines including IL-1β, IL-6, IFNγ and TNF, compared to non-BCG immunized population [12,13].
It has been observed that SARS-CoV-2 infection triggers a local immune response, recruiting macrophages and monocytes, releasing cytokines, and priming adaptive T and B cell immune responses.
In most cases, the infection resolves on its own, however, in some cases, a dysfunctional immune reaction occurs [14]. In the case of BCG vaccination, the human body seems to adapt to accept excessive levels of cytokines due to enhanced trained immunity to fight against pathogens, in particular those affecting the respiratory tract such as SARS-CoV-2 [15,16]. Therefore, we assume that an appropriate dose and controlled delivery of BCG vaccine can give some form of protection against COVID-19. To achieve this aim, currently, four clinical trials are ongoing in Australia, Netherland and USA to determine if BCG vaccination can potentially protect healthcare workers during the COVID-19 pandemic [17][18][19].

Association Between BCG Vaccination and Prevalence Of COVID-19 Infection and Mortality Rates
In this study, we hypothesized that countries which adopted a mass BCG vaccination policy are protected from infection with SARS-CoV-2 contrary to countries that did not adopt the same policy or stopped it. We specifically compared the death rates between the countries that have employed mass BCG vaccination and the countries which stopped or did not employ mass BCG vaccination policy. Data were collected for 71 countries that adopted mass BCG vaccination and 22 countries did not employ or stopped mass BCG vaccination ( Table 1,2) using the BCG Atlas published in 2011 [20].
We excluded the African countries from our study due to lack of accourate information about COVID-19 cases. The data regarding   To reveal the efficacy of BCG vaccination in protection against

SARS-CoV-2 infection, we analysed the global percentage of BCG
vaccinated population and the current COVID-19 death rates. (Figure 1A) showed that approximately 70% of the total population, included in this investigation, were adopted BCG vaccination, whereas ~11% of the total population were not or stopped BCG vaccination.
Approximately 19% of the total population remains unidentified and excluded from the analysis due to lack of data or other factors.
Interestingly, a correlative association was noticed since 64% of COVID-19 cases were documented in the non-BCG vaccinated population, while only 33% of COVID 19 cases were documented in BCG vaccinated population ( Figure 1B). Another stark association was observed when mortality cases in both groups were considered. In the case of non-BCG vaccinated population, the mortality rate was 82%, whereas 17% mortality was observed in BCG-vaccinated population ( Figure 1C). Similarly, a comparable trend was ob-served when the same measures were taken 42 days later on June 24, 2020. Within the same population ( Figure 1D), 1.2 ( Figure 1E) and 0.5 ( Figure 1F) times the COVID 19 infected and dead cases were documented in BCG vaccinated population, respectively.
The infection rate within the countries that did not adopt mass BCG vaccination was ~13-fold those adopted mass BCG vaccination (Figure 2A). Similarly, the death rate in the countries that did not adopt BCG vaccination policy was 2.3 times those adopted mass BCG vaccination (Figure 2A). Data collected on June 24, 2020 showed that the infection and death rates within the countries did not adopt BCG vaccination were 5.3 and 2.3-fold those adopted BCG vaccination, respectively ( Figure 2B). Although the infection rate was reduced, the death rate still similar to the data collected on May 12, 2020 and the overall trend indicating a correlative association between the BCG vaccination and reduction in COVID-19 cases.

BCG Vaccination Shall be an Important Factor In COVID-19 Incidence
During the current COVID-19 pandemic, suspected correlation has emerged between BCG vaccination and the spread of COVID-19.

Conclusion and Future perspectives
Our data analysis presented here is clearly indicated a potential association between BCG vaccination and COVID-19 prevalence.
This pattern was further supported by several studies and the innate immunity mechanisms conferred by BCG vaccination. Despite these promising correlations, it is still early to draw a definitive conclusion and more experiments are required for proper valida- Countries like Australia and Germany which have no mass BCG immunization program showed lower morbidity and mortality rate due to SARS-CoV-2 infections. However, it could be due to imposition of early lockdown restrictions, young age of the infected cases, public adherence to social distancing procedure and most importantly the vigorous testing and contact tracing policies implemented by these countries. The innate immunity of people in low-income countries exposed to the high level of environmental pollutants compared to the developed world. Though not proved, climate and weather conditions could also play a part in the spread of the disease. Geographic location of countries may be a deciding factor too due to climate conditions. Therefore, it is essential to devise models keeping in mind these variables to investigate the impact of BCG vaccination in relation to protection against COVID-19. It would be interesting to observe if the protection against COVID-19 provided by BCG could be long lasting or will fade away with time and the age for vaccination.
These can also help to devise new plans especially for countries who have abandoned BCG vaccination in the last couple of decades.
Lastely, immunological and epidemiological studies supporting or opposing these correlations are necessary to be conducted to present a global consensus in the fight against COVID-19.

Authors Contribution
Rauf Bhat, Sameh Soliman, Ahmed Fahmy and Mohamed Husseiny: Conceptualization, Methodology, Data collection and interpretation, Drafting, reviewing and approval of the final version for submission, all authors attest they meet the ICMJE criteria for authorship.