BCG is a live attenuated TB vaccine developed and bred by Albert Calmette and Camille Guerin about 100 years ago [62], and is the only vaccine approved for TB prevention. BCG has a certain protective effect on TB pleurisy and miliary TB in children [63]. At present, 154 countries and territories have introduced neonatal BCG vaccination policies, of which 53 countries report vaccination coverage of at least 95% [64]. In addition to its application in preventing TB, BCG has also been proven to prevent and treat infections caused by other bacteria and viruses and even some tumors. Many studies have found that the prevention and treatment effect of BCG on bacteria other than M. tuberculosis or tumors is related to the non-specific immune response induced by the BCG, which is called “trained immunity” [65, 66, 67].

Trained immunity refers to the non-specific immune response of the self-immune system to the secondary stimulation of the same pathogen or different pathogens after the initial challenge caused by some pathogen infection or vaccination. The trained immune response is usually faster and more active (Fig. 3) [68, 69]. Previous studies have shown that when the ligand of some pathogenic microorganisms binds to specific receptors on innate immune cells, it induces long-lasting enhanced metabolic activity and epigenetic changes in innate immune cells [70]. The increased metabolic activity provides faster energy support and necessary metabolites for the immune response. Epigenetic changes increased the susceptibility of transcription factors to promoter and enhancer regions of inflammatory genes. When immune cells are repeatedly stimulated by the same pathogen or infected with other pathogens, they accelerate the process of gene expression. The combination of these metabolic and epigenetic effects up-regulates the production of pro-inflammatory cytokines (IL-1$\beta$, TNF$\alpha$, and IL-6), promotes bone marrow proliferation, and promotes a faster and more robust immune response to secondary stimuli from the same or different pathogens, and may even prevent subsequent secondary infections (Fig. 4) [68, 70].

Fig. 3.

The curve of trained immunity over time. After the initial immunization, the host’s innate immune response was gradually enhanced and reached the first immune peak followed by a steadily decreased trend. Subsequently, the second infection with another pathogen will rapidly induce a more intense immune response and then reach a second immune peak followed by a gradually decreased trend.

Fig. 4.

The mechanism of trained immunity induced by the BCG vaccine. Trained immunity is an enhanced innate immune response to different pathogens after an initial challenge, such as vaccination or infection. When the ligand of some pathogenic microorganisms binds to specific receptors on innate immune cells, it induces durable enhancement of metabolic activity and epigenetic changes in innate immune cells. Epigenetic changes increase the susceptibility of transcription factors to promoter and enhancer regions of inflammatory genes when immune cells undergo repeated stimulation or other pathogens, accelerating the gene expression process. In addition, the increased metabolic activity provides faster energy support and necessary metabolites for the immune response. This combination of metabolism and epigenetics prompts the immune system to respond faster and more vigorously to secondary stimuli caused by the same or different pathogens.

Interestingly, there is growing evidence that BCG vaccination induces trained immunity against multiple respiratory viruses, such as RSV and influenza viruses [71]. Therefore, in the face of the COVID-19 pandemic, some researchers have proposed a hypothesis that BCG vaccination may be one of the ways to prevent or control the spread of COVID-19 and reduce the morbidity and mortality of COVID-19 [2, 72].

To explore the impact of BCG vaccination on the incidence, mortality and severity of COVID-19, many studies have been conducted in many countries and organizations around the world. Statistics show a correlation between the morbidity and mortality of COVID-19 and BCG vaccination rates in different countries and regions at the beginning of the global pandemic (Table 2, Ref. [12, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89]). For example, the number of confirmed COVID-19 cases in developed countries such as North America and Western Europe is significantly higher than that in developing countries such as South America, Asia, and Africa, where BCG vaccination policies for newborns are widely adopted. Observing this phenomenon, Madan et al. [73] published a cross-sectional study that included data collected from 174 countries. The 174 countries were divided into four groups based on TB incidence and BCG vaccination coverage, and the incidence and mortality of COVID-19 in each group were statistically analyzed. The results showed that the incidence and mortality of COVID-19 in countries with a high incidence of TB were lower than those in countries with low TB incidence. Similarly, the incidence and mortality of COVID-19 were lower in countries with high BCG vaccination coverage [73]. Furthermore, a large-scale observational study involved in 175 countries found similar results, the results show that countries with a universal BCG vaccination program (n = 138) have significantly lower incidence of COVID-19 than countries without a universal BCG vaccination program (n = 37) [0.0147 $\pm$ 0.027 vs 0.1892 $\pm$ 0.244, respectively, p 0.0001] [74]. Countries with BCG neonatal vaccination programmes also had significantly lower per capita mortality than countries without BCG neonatal vaccination programmes [0.0004 $\pm$ 0.001 vs 0.0113 $\pm$ 0.020, respectively, p 0.0001] [74]. The findings of both studies indicate specific protective mechanisms against COVID-19 in TB endemic areas. These findings also promote research on the pathogenesis and immune response process of COVID-19 in various countries.

To further explore the relationship between BCG vaccination coverage and COVID-19, Klinger D et al. [75] conducted a meta-regression analysis based on the data from 160 countries. They divided countries into three groups based on BCG vaccination coverage $\le$70% group, $>$70% group, and the unvaccinated group. The study found that countries with BCG coverage $\le$70% had 6.5 fewer COVID-19 cases per 10,000 people than unvaccinated countries [75]. Therefore, it suggests that countries with higher BCG vaccination coverage have lower morbidity and mortality of COVID-19. Klinger D ‘s study data took BCG vaccination coverage rate of 70% as the grouping limit. In China, Gong et al. [12] conducted a larger analysis of COVID-19 data obtained from 225 countries released by WHO on 12 August 2020. According to BCG vaccination coverage, Gong et al. [12] divided these countries into three groups: countries with BCG vaccination coverage $\ge$90%, countries that recommended BCG vaccination but BCG vaccination coverage 90%, and countries that have never introduced BCG vaccination. Results showed that the incidence of COVID-19 was significantly lower in countries where BCG was recommended than in countries where BCG was not recommended, and the mortality of COVID-19 was considerably lower in countries with BCG coverage $\ge$90% [12]. Data from these two studies suggest that increased BCG coverage may have a role in reducing morbidity and mortality of COVID-19. However, when Gong conducted statistics, it was found that countries with high BCG coverage were mainly developing countries with low human development index (HDI). Therefore, Gong et al. [12] further analyzed the relationship between HDI and COVID-19 in these 225 countries. They divided these countries into four levels based on the 2019 Human Development Report and found that countries with low HDI had lower incidence and mortality rates of COVID-19 than countries with high HDI. In contrast, countries with low HDI are mostly developing countries, and BCG vaccination policy is widely adopted. Once again, expanding BCG vaccination coverage may reduce the incidence and mortality of COVID-19.

Those studies indicate a negative correlation between BCG vaccination and the incidence and mortality of COVID-19. However, many studies have shown that in the early stage of epidemiological studies, there are often a large number of confounding factors, which have a significant impact on epidemiological data and often lead to distortion of research results. Confounding factors such as differences in health systems, levels of economic development, income, education, population, rural and urban environments, time lag between COVID-19 outbreaks, number and standard of diagnostic tests, and national control strategies are also present in the above studies [90]. To minimize the impact of these confounding factors on epidemiological results, some researchers analyzed the relationship between the BCG index (the level of universal BCG vaccination coverage in a country) and COVID-19 mortality after controlling confounding factors. For example, Escobar and his colleagues performed a major epidemiological study among European countries [76]. A negative correlation was found between the BCG index and COVID-19 mortality in European countries. On the other hand, a 10% increase in the BCG index was associated with a 10.4% decrease in COVID-19 mortality, suggesting that BCG may have a protective effect during the COVID-19 epidemic. With the outbreak of COVID-19 in the Americas, this study analyzed COVID-19-related mortality rates in STATES without BCG vaccination policies and states with BCG vaccination policies located at major entry points in Mexico and Brazil and found that COVID-19 mortality rates in states without BCG vaccine were significantly higher than in countries with BCG vaccination [76]. However, this study did not consider the significantly higher population density in South America than in North America, so the results are controversial. Another study performed multiple regression analyses on data from 171 countries, adjusting for socioeconomic and climatic covariates to avoid potential confounding factors [77]. The results showed a 30-fold reduction in COVID-19 deaths in countries with universal BCG vaccination policies compared with countries without universal BCG vaccination policies, further supporting the role of BCG vaccination in reducing COVID-19 deaths [77].

Although the above ecological or epidemiological data to some extent support the negative association between BCG vaccination rates and COVID-19 morbidity and mortality, other studies have questioned this. Some epidemiological studies have shown no clear association between BCG vaccination policies and morbidity and mortality of COVID-19 (Table 3, Ref. [24, 29, 91, 92, 93, 94, 95, 96, 97]). For example, Szigeti et al. [91] conducted a comprehensive epidemiological study using data from public databases in 68 countries. The relationship between BCG prevalence and mortality in COVID-19 cases found no significant correlation [91]. In addition, several previous epidemiological analyses suggested that although some ecological studies showed that BCG vaccination had a protective effect on COVID-19, the results of these studies were significantly affected by confounding factors. Their literature review found that, after adjusting for confounding variables, there was no longer a clear association between BCG vaccination policy and COVID-19 morbidity and mortality, especially after adjusting for detection rates [24].

The above studies indicate that confounding factors significantly impact the analysis and evaluation of BCG vaccination’s protective effect against COVID-19, leading to the opposite conclusion of the study. Therefore, several cohort studies or longitudinal retrospective observational studies have further analyzed the relationship between BCG vaccination rates and COVID-19 morbidity and mortality. The advantage of these studies over previous epidemiological and ecological studies is that they eliminate confounding factors as much as possible. In a longitudinal, retrospective observational study performed in 6201 HCWs, 29.6% of the participants in the experimental group reported a history of BCG vaccination, and 68.9% of the participants in the control group had never received BCG vaccination [78]. Clinical symptoms associated with COVID-19 and serum anti-SARS-CoV-2 IgG concentrations were compared. The results showed that the sero-positive rate of SARS-CoV-2 and clinical symptoms related to COVID-19 were significantly lower in the BCG vaccinated group than those in the non-BCG vaccinated group [78].

Similarly, a Rhode Island study analyzed the hospitalization rates of 120 patients with COVID-19 and found that those vaccinated with BCG had lower hospitalization rates than those who had not been vaccinated with BCG [79]. These findings suggest that BCG may have some value in reducing the severity of COVID-19 disease. Though the above two studies in order to reduce the influence of confounding factors, choose the same country of the people in the same area as the research object, the application of unified testing means to control confounding variables as much as possible. However, there are still unavoidable potential confounding factors, such as the research object of the age, BCG vaccination time, growing environment and condition. To further reduce the influence of these variables on the results, a cohort study in Israel compared 3064 patients with BCG vaccination and 2869 patients without BCG vaccination. The positive rate of the COVID-19 test in the two groups was analyzed, and no statistically significant difference was found between the two groups [29]. Since Israel abolished its mandatory BCG neonatal vaccination program in 1982, the study looked at individuals born three years before and three years after the BCG vaccination program stopped. This study minimised the impacts of confounding factors such as health systems, national levels of economic development, education, rural and urban environments, time lag between COVID-19 outbreaks, number and standard of diagnostic tests, and national control strategies. In addition, Aksu K et al. [92] conducted a natural experiment in Sweden to evaluate the effect of BCG vaccination on COVID-19 related outcomes by using the regression discontinuities method. Two groups of volunteers were recruited, including 1026,304 people born before 1975 and 1018,544 people born after 1975. The impact of BCG vaccination on COVID-19 related outcomes was evaluated between two groups. Analysis results showed that although BCG vaccination at birth was not protective against COVID-19 in middle-aged individuals, universal BCG vaccination reduced the number of COVID-19 cases by 19% and the number of hospitalizations by 25% [92]. The rigorous experimental design and sufficiently large sample size of this study allow its reliability and precision to exceed that of a randomized controlled trial.

In summary, there is pronounced heterogeneity in the results of these studies. Therefore, it is impossible to answer whether BCG vaccination is meaningful to control the spread of COVID-19, decrease the morbidity and mortality of COVID-19, and reduce the severity of COVID-19 disease. Furthermore, although the above studies have minimized the interference of confounding factors, there are still many potential confounding factors that may affect the study results. Therefore, many clinical trials are needed to assess the significance of BCG vaccination in controlling the COVID-19 pandemic.

As of March 24, 2022, there have been 7,626,640 confirmed cases of COVID-19 with 21,836 deaths, and 33,943,469 doses of the COVID-19 vaccine have been administered in the Netherlands (https://covid19.who.int). As a developed country, the incidence and mortality of tuberculosis are very low in the Netherlands, and the BCG vaccination program has not been widely promoted [3]. Therefore, it is admirable that Switzerlands is at the forefront of research in evaluating the preventive effect of BCG against COVID-19. Currently, 11 clinical trials have been conducted in the Netherlands to assess the protective effect of BCG against COVID-19, including seven clinical trials in phase IV (EUCTR2021-000182-33-NL, EUCTR2020-003470-47-NL, NCT04537663, EUCTR2020-002456-21-NL, NCT04417335, EUCTR2020-000919-69-NL and EUCTR2020-001591-15-NL), one clinical trial in phase III (NCT04328441), and three clinical trials in unknown phase (NL8609, NL8547, and NL8477). Interestingly, the BCG strains used in these clinical trials are all BCG Danish strand 1331 strain. The objectives of these clinical trials are to assess the effect of BCG immunization in reducing the hospitalization rate and COVID-19 disease severity in the elderly ($\ge$60 years old) and the work absenteeism rate as well as clinical symptoms of healthcare workers (HCWs). Furthermore, several clinical trials have analyzed the prime-booster strategy of the BCG and COVID-19 vaccines.

The elderly have become a vulnerable group in the COVID-19 epidemic due to a weakened immune system and primary diseases like hypertension and diabetes. Three clinical trials (NCT04537663, NCT04417335, and NL8547) were performed to evaluate the effect of BCG vaccination in reducing the hospitalization rate and the severity of COVID-19 in 5200, 2014, and 1600 older adults ($\ge$60 years old) in the Netherlands, respectively. The results of these three trials have not been reported, and they will provide data support for assessing the impact of BCG on COVID-19 in the elderly. Additionally, HCWs are a vital force in the fight against COVID-19, and reducing HCWs absenteeism is a critical factor in defeating the pandemic. Two clinical trials (EUCTR2020-000919-69-NL and NL8477) were conducted by the University Medical Center and the University Medical Center Utrecht in 1000 and 1500 HCWs, respectively. Both clinical trials focused on assessing the impact of BCG vaccination on reducing the absenteeism of HCWs. Previous studies have shown that the prime-booster strategy of BCG and specific vaccines has a good effect on improving their protective efficiency. Still, this strategy was rarely used by COVID-19 vaccines under emergency use authorization (EUA). Surprisingly, we found that the clinical trial EUCTR2021-000182-33-NL carried out by Radboudumc made a bold attempt to this immune strategy. This clinical trial aimed to analyze whether BCG vaccination before mRNA1273 vaccination could improve the immune effect of the COVID-19 vaccine. The results of this trial may provide data support for exploring new strategies to fight COVID-19. No experimental results have been announced yet, and follow-up continuous attention is required. Nevertheless, this is another way to explore the potential impact of the joint application of the BCG vaccine and COVID-19 vaccine in COVID-19 prevention and control.

India is a populous country second only to China, but the level of medical security is relatively backward. According to the population data released by the WHO, India has a population of 1.4 billion (https://population.un.org/wpp/Download/Standard/Population/). By March 14, 2022, India had 1.8 billion vaccinations, which means that only one dose of COVID-19 vaccine per person is average. Based on these disadvantages, India has 43,016,372 confirmed COVID-19 cases, ranking second only to the United States, and the cumulative number of deaths is as high as 516,755, ranking third after the United States and Brazil (https://covid19.who.int/table). Unlike Switzerland, India has universal BCG vaccination, and BCG coverage has reached 92% in 2019 (http://www.bcgatlas.org/). As far as India’s challenges are concerned, if BCG proves to be an excellent preventative against COVID-19, then India has one more powerful weapon against COVID-19. To provide sufficient evidence for this hypothesis, eight clinical trials have been conducted in India to evaluate the preventive and protective effects of BCG against COVID-19, including one Phase IV clinical trial (CTRI/2020/06/06/025798), three Phase III clinical trials (NCT04475302, CTRI/2020/07/026668 and CTRI/2020/04/024749), one Phase II clinical trial (CTRI/2020/05/05/0205013), and three clinical trials with unknown phase (CTRI/2020/06/025854, CTRI/2020/09/027684 and CTRI/2020/04/04/024833). Seven of these clinical trials were used to evaluate the preventive effect of BCG on COVID-19, but one was used to assess the therapeutic effect of BCG immunization in patients with COVID-19. Clinical trial CTRI/2020/05/025013 was carried out to evaluate the therapeutic effect of BCG as a therapeutic agent on 60 hospitalized patients aged 20–40 years with COVID-19 and explore the relationship between BCG and COVID-19. These patients were divided into two groups, BCG group with 0.1 mL BCG (2–8 $\times$ 10${}^6$ CFUs) and placebo control group with 0.9% sodium chloride via intradermal injection. The clinical trial is ongoing, and experimental data have not yet been released. The strength of this clinical trial is that it evaluated the therapeutic rather than preventive effect of BCG immunization in COVID-19 patients. At the same time, its disadvantage is that the number of cases recruited is relatively small, which may reduce the reliability and stability of data obtained from this clinical trial.

Overall, the eight clinical trials conducted in India cover a wide range of age groups (20–40, 18–50, 18–60, 18–65, 18–80, 18–99, 60–80, and 60–95 years old, respectively) and high-risk groups such as HCWs, the elderly, and other high-risk workers. Therefore, if the final data from these clinical trials support the hypothesis that the BCG vaccine can prevent and treat COVID-19, widespread BCG vaccination in a populous and relatively underdeveloped country like India may be an effective and inexpensive way to combat the COVID-19 pandemic.

COVID-19 pandemic quickly disrupted health services, including Brazil’s Unified Health System (SUS). As of March 25, 2022, Brazil has confirmed 29,729,991 COVID-19 cases, ranking third only to the United States and India; The total number of deaths is 657,998, second to the United States. As of March 10, 2022, 383,413,147 doses of the COVID-19 vaccine had been administered, and total vaccine doses administered per 100 population and persons fully vaccinated per 100 population are 180.38 and 71.35, respectively (https://covid19.who.int/table). From these data, it can be seen that despite the high COVID-19 vaccination rate in Brazil, the COVID-19 morbidity and mortality remain high. To reverse this unfavourable situation, some scientists in Brazil turned their attention to the BCG vaccine. At present, five clinical trials have been conducted in Brazil, including one Phase IV clinical trial (NCT04369794), two Phase II clinical trials (NCT04659941 and RBR-4kjqtg), and two clinical trials with unknown phase (ISRCTN47802196 and RBR-5ysj54). NCT04369794 is a multicenter, randomized, and double-blind clinical trial conducted in Brazil on April 30, 2020, enrolling 1000 adults over 18 years. These participants will be divided into the BCG vaccine group with 0.1 mL BCG (2–8 $\times$ 10${}^6$ CFUs) and the placebo control group with 0.1 mL 0.9% sodium chloride. This study explores the effects of BCG on promoting the production of cytokines, inducing trained immunity, slowing disease progression, and decreasing the severity of COVID-19 patients. Furthermore, another clinical trial, ISRCTN47802196, was launched on May 19, 2021, which randomly recruited 200,805 COVID-19 patients with or without BCG vaccination from 1996 to 1998. The purpose was to evaluate the impact of BCG vaccination and re-vaccination on the occurrence and severity of COVID-19 in Brazil. The significant advantage of this clinical trial is that the sample size exceeds 200,000, which lays a solid foundation for proving the effectiveness of BCG in preventing COVID-19.

As of March 25, 2022, Denmark had a total of 3,029,228 confirmed cases and 5518 deaths, ranking 36th and 85th in the world, respectively. As of March 5, 2022, as many as 13,178,651 vaccine doses have been administered in Denmark. Total vaccine doses administered per 100 population and persons fully vaccinated per 100 population were 226.3 and 82.26, respectively, higher than the global average. Fortunately, although the number of confirmed cases is high in Denmark, the mortality rate is relatively low, which may be inseparable from the COVID-19 vaccination strategy actively pursued by the Danish government and the strong support of the health system. In addition, Denmark is actively exploring new ways to fight COVID-19, such as the BCG vaccine. At present, four clinical trials were performed in Denmark to evaluate the potential effect of BCG vaccination on preventing and controlling the COVID-19 pandemic. Like other countries, these clinical trials in Denmark have focused on groups at high risk for COVID-19, such as the elderly and HCWs. The clinical trial EUCTR2020-003904-15-DK conducted by the University of Southern Denmark and the clinical trial NCT04542330 conducted by Bandim Health Project intend to recruit 1900 and 1300 participants, respectively. All participants will be randomly divided into two groups, the intervention group will receive BCG Danish strain 1331, and the control group will receive normal saline. The objective of both clinical trials is to evaluate the protective effect of BCG immunization against COVID-19 infection in the elderly over 65 years. In addition to the elderly, HCWs in Denmark are also a high-risk group, accounting for more than 20% of the cumulative number of confirmed cases. A growing number of HCWs with COVID-19 will cause large-scale absenteeism, further leading to a shortage of medical resources. Therefore, there is an urgent need to develop strategies to prevent HCWs absenteeism. Two clinical trials, EUCTR2020-001888-90-DK (Estimated enrollment 1500) and NCT04373291 (Estimated enrollment 1293), have been conducted to evaluate the potential beneficial effects of BCG vaccination by reducing the work absenteeism rate of HCWs or alleviating the clinical process of COVID-19 infection. Both studies use the BCG vaccine to strengthen the non-specific protection of HCWs during the COVID-19 pandemic.

By analysing the available public data, we found an interesting thing: four clinical trials in Denmark were conducted independently by two different institutions, and each institution conducted a clinical trial in the elderly and HCWs. The rational layout of these clinical trials provides a model for a country to systematically evaluate BCG vaccines on the prevention and treatment of COVID-19. Furthermore, by comprehensively analyzing these experimental data, researchers can eliminate some confounding factors and improve the reliability and authenticity of the data.

As a developed country, the incidence of TB in Germany in 2020 is estimated to be 5.5 per 100,000 population, and the mortality rate is calculated to be 0.35 per 100,000 population (https://www.who.int/teams/global-tuberculosis-programme/data). Germany’s BCG vaccination policy has changed several times in its history, and the BCG policy was different between former East Germany and former West Germany. Former West Germany stopped recommending the BCG vaccine (BCG Denmark strain) in 1974 except for infants at risk, but former East Germany continued a mandatory BCG vaccine (Boehring-Hoechst/Jenapharm) until the German reunification in 1990 (http://www.bcgatlas.org/). Therefore, there is a significant difference in BCG coverage between former East Germany and former West Germany, but confounding factors such as geography, genetic background, and current epidemic prevention policy are similar in Germany. These unique advantages provide natural conditions for assessing the protective effect of BCG against COVID-19.

Currently, four clinical trials of the VPM1002 vaccine (a BCG replacement vaccine) have been performed in Germany. Another feature of the four clinical trials conducted in Germany is that they all used the VPM1002 vaccine instead of the BCG vaccine. In addition, the Max Planck Institute genetically modified the VPM1002 vaccine to improve its immune activity. Previous studies have demonstrated that the VPM1002 vaccine showed superior efficacy and safety than BCG in fighting against Mycobacterium tuberculosis infection in mice, guinea pigs, rabbits, nonhuman primates, HIV-unexposed newborn infants, and young adults [35, 38, 87, 88]. Thus, based on the experience of trained immunity induced by the BCG vaccine, some scientists speculate that the VPM1002 vaccine may also (partially) have potential preventive and therapeutic effects on COVID-19.

In Germany, Vakzine Projekt Management GmbH conducted four Phase III clinical trials to evaluate the potential of the VPM1002 vaccine to prevent COVID-19. Briefly, two clinical trials (NCT04435379 and EUCTR2020-001675-33-DE) were performed to assess the efficacy and safety of the VPM1002 vaccine in reducing the hospitalization rate of COVID-19 and severe respiratory infectious diseases in adults. The two clinical trials have the same sample size of 2038, and the difference is the vaccination dose and the age of the population (Table 4). The former used a dose of 2–8 $\times$ 10${}^5$ CFUs to vaccinate adults over 18 years, while the latter used a dose of 2–8 $\times$ 10${}^6$ CFUs to vaccinate the elderly over 60 years, which will lay the foundation for subsequent data analysis and enhance the credibility of the research results. Interestingly, two additional clinical trials (NCT04387409 and EUCTR2020-001376-15-DE) were conducted to evaluate the effectiveness and safety of the VPM1002 vaccine in reducing the absenteeism of HCWs in the SARS-CoV-2 pandemic. The clinical trial EUCTR2020-001376-15-DE was conducted to evaluate the effectiveness and safety of VPM1002 in reducing the absence of HCWs. Regrettably, due to the premature termination of the study, only 59 subjects were recruited, of which 29 received VPM1002 and 30 received placebo. The primary assessment objective, a reduction in HCWs’ absenteeism, was not statistically analyzed due to the early termination of the trial. Nonetheless, preliminary data suggest that the serious adverse events (0.00% vs 3.33%), vaccine compatibility (10.34% vs 23.33%), and COVID-19 infection rate (6.90% vs 10.00%) in the VPM1002 vaccine group were lower than these in the placebo control group. These data preliminarily show that the VPM1002 vaccine may have a specific protective effect on the prevention and control of COVID-19. However, this clinical trial was terminated prematurely due to poor recruitment, and only the result data of 59 participants were analyzed. We expect that the results of the other three clinical trials will provide more substantial evidence of whether the VPM1002 vaccine can prevent COVID-19 infection.

As a low TB incidence and mortality, BCG is not generally recommended for use in the United States. Currently, the Centers for Disease Control (CDC) of the United States recommends BCG vaccination only for HCWs and children who have a negative tuberculin skin test and who are continually exposed and cannot be separated from adults with a high risk of TB. However, as of March 25, 2022, the United States has confirmed 79,139,385 cases of COVID-19, with cumulative death of 967,905, ranking first in the world. Therefore, in the face of the world’s worst COVID-19 pandemic and widespread vaccination of specific COVID-19 vaccines, the United States has sought other anti-epidemic strategies, including BCG.

Researchers in the United States carried out two Phase III clinical trials (NCT04632537 and NCT04534803) to confirm the prevention and improvement of the BCG vaccine against COVID-19. However, both clinical trials were withdrawn due to capital and other reasons. Furthermore, a randomized and controlled Phase III clinical trial (NCT 04348370) was conducted in 1800 HCWs to assess the BCG vaccine-induced trained immunity to protect HCWs from SARS-CoV-2 infection. The HCWs were divided into the BCG vaccine group with 0.1 ml BCG (2 $\times$ 10${}^5$ CFUs via intradermal injection) and the placebo control group with 0.1 ml sodium chloride. This clinical trial will measure BCG vaccination’s impact on the incidence of COVID-19 infection in HCWs and the severity of the COVID-19. The study is expected to be completed in May 2022. In addition, one Phase II clinical trial (NCT02081326) was also conducted to evaluate the effect of BCG in COVID-19 patients with Type 1 diabetes. The participants in the BCG group will receive two doses of BCG vaccine in the first year (with an interval of 4 weeks) and one dose of BCG vaccine each year from the second to the fifth year. Participants in the placebo group will receive the same dose and frequency of 0.9% sodium chloride as the BCG group. This clinical trial was designed to observe whether revaccination with BCG has beneficial immune and metabolic effects in patients with type 1 diabetes and its impact on the incidence and severity of COVID-19 in patients with type 1 diabetes, which makes this clinical trial very distinctive and meaningful.

Like other high-income European countries, Greece has historically implemented universal BCG vaccination, but it is recommended only for high-risk groups, such as infants and children who have contact with an adult infected with TB, immigrants, and refugees (http://www.bcgatlas.org/). In Greece, as of March 25, 2022, as many as 2,884,100 confirmed cases of COVID-19 and 27,125 deaths have been reported to WHO, and a total of 20,181,739 vaccine doses have been administered. Currently, two clinical trials are being conducted in Greece to evaluate the efficacy of BCG in preventing COVID-19. As early as September 21, 2017, a randomized and controlled Phase IV clinical trial was conducted to explore the effect of BCG vaccination against influenza in the elderly. Interestingly, with the outbreak of the COVID-19 pandemic, the primary purpose of this clinical trial shifted from influenza to COVID-19. An interim analysis of this clinical trial was reported on April 29, 2020. The results showed that the first infection time (median 16 weeks vs 11 weeks) and the incidence of new infections (25.0% vs 42.3%) in the BCG group were significantly longer or lower than those in the placebo group [98]. These preliminary data suggest that BCG vaccination is safe, prolongs the first infection time, reduces the incidence of new infections, and has a protective effect on viral respiratory tract infection. This interim analysis provides evidence that “trained immunity induced by BCG vaccination may play a role in combating the COVID-19 epidemic”. Furthermore, another double-blind, placebo-controlled Phase IV clinical trial (NCT04414267 and EUCTR2020-002448-21-GR) was also performed to verify whether BCG vaccination can prevent COVID-19 and whether BCG vaccination can regulate the susceptibility of vaccinators to COVID-19 through clinical and immunological standards. The clinical trial is currently recruiting volunteers, and the trial results have not yet been announced. Overall, the two clinical trials in Greece were conducted by the same institution (Hellenic Institute for the Study of Sepsis), but the type of BCG strain, the number of volunteers, and the primary outcome were different between the two clinical trials (Table 4).