BCG as a stopgap vaccine for COVID-19

Studies make a case for BCG to be a bridge vaccine until a COVID-specific vaccine is developed, but larger clinical trials are needed to study the impact of such a strategy on the morbidity and mortality caused by the virus.

Published : Nov 04, 2020 06:00 IST

Clinical nurse  Wendy Dickinson receives a BCG injection at a trial clinic in Perth, Australia, on April 20. Health-care workers in Australia participated in a new trial to test whether the existing tuberculosis vaccine can help reduce their chances of contracting COVID-19.

Clinical nurse Wendy Dickinson receives a BCG injection at a trial clinic in Perth, Australia, on April 20. Health-care workers in Australia participated in a new trial to test whether the existing tuberculosis vaccine can help reduce their chances of contracting COVID-19.

Early into the COVID-19 pandemic, beginning March, and even up to August, several studies appeared claiming an empirical inverse correlation between mandatory national BCG vaccination policies and differential lower COVID-19 infection growth and related mortality rates at the gross population level. Nearly all of them were posted on different online pre-print repositories such as medRxiv and bioRxiv and hence were not peer-reviewed.

Studies by Aaron Miller and associates from the New York Institute of Technology, which first appeared in March, and by Mathuram Santosham and others from the Johns Hopkins Bloomberg School of Public Health in April drew particular attention, especially of the media.

Such studies were, of course, careful in covering their suggestions by concluding that the observed definitive ecological association warranted deeper epidemiological study and evaluation through randomised clinical trials. Nevertheless, they did draw a fair amount of criticism because, for one, the suggestion of a long-lasting protection from childhood immunisation at an individual level based on weak aggregate-level evidence was fundamentally flawed and, two, generalised conclusions were being drawn from what could be a spurious correlation, given the various other confounding factors that determine the impact of COVID-19 across the world. A notable critique by researchers from Canada soon appeared in Nature Microbiology  which, quoting the well-known dictum in research, said “correlation is not causation”. It also pointed to the biological implausibility of such a long-term protection at an advanced age based on the known facts that the BCG vaccine conferred protection against tuberculosis (TB) only in young children and that the protection waned by about 12 years of age. Despite such criticisms, such non-peer-reviewed studies continue to appear. Miller’s group has, in fact, posted an updated version of their March study in September. 

Repurposed BCG vaccine as a possible candidate for conferring protection against COVID-19 by vaccinating cohort groups, such as vulnerable health-care worker and elderly populations, is, of course, an entirely different proposition. As the Nature Microbiology  commentary observed, “If, in a well-performed trial, we do see a benefit in repurposing this vaccine, the evidence would be much more convincing.” Indeed, recent trials seem to suggest that a repurposed targeted BCG vaccination may be beneficial to populations at risk for COVID-19. Also read:The quest for a Covid vaccine

The Bacillus-Calmette-Guerin (BCG) vaccine is based on a live-attenuated strain derived from isolates of Mycobacterium bovis  that gives protection against mycobacterial infections, most notably against M. tuberculosis. It is the only known vaccine that provides effective protection against childhood TB and has been in use since 1921 the world over in national immunisation programmes to prevent TB.

BCG vaccine side benefit

An unexpected side benefit of BCG vaccination that has been seen for quite some time is that in vaccinated individuals not only TB but also other infections were less frequent. This has been termed “off-target or non-specific” (heterologous) effects. An oft-quoted study is the one in Guinea-Bissau in west Africa where the mortality of vaccinated newborns was nearly 40 per cent lower than that of unvaccinated babies.

Randomised controlled trials have shown that this is due to the protective effect of BCG against neonatal sepsis and respiratory infections. In South Africa, the vaccine reduced respiratory tract infections by 73 per cent in adolescents. In studies in Indonesia and Japan, BCG has also been found to protect the elderly against respiratory tract infections; the reduction found was 70-80 per cent.

The hypothesis of such non-specific protection was also tested successfully in healthy volunteers who were given the BCG vaccine or placebo and who received a trivalent Influenza A vaccine. Volunteers who received the BCG were found to have greater antibody titres against the influenza A virus. The vaccine has also been used to treat bladder cancer.

Although such non-specific effects on the immune system have been recognised for long, the causative mechanisms have not been fully delineated yet. One of the mechanisms proposed is what has been termed as “trained immunity”, wherein live-attenuated vaccines like the BCG can induce metabolic changes and ‘epigenetic’ changes (external modifications that do not alter the DNA but affect how it is read by the body cells) that enhance the immune response to subsequent infections; the capacity of the innate immune response became more efficient independent of the type of re-infection. Also, such non-specific protection by BCG vaccination in adults, particularly in the elderly, has not been investigated adequately.


In the October 15 issue of the journal Cell , the interim analysis of the results of phase III of a double-blind randomised clinical trial called ACTIVATE, designed to evaluate BCG vaccination-induced protection in elderly patients, was published. Interestingly, the trial predates COVID-19; it was conducted between September 2017 and August 2020. In view of the possible relevance of the findings to the COVID-19 pandemic, its interim results have been published before the trial analyses were fully completed.

The study, which is a collaboration between Radboud Medical Centre at Nijmegen in the Netherlands and National and Kapodistrian University of Athens, revealed that BCG vaccination was safe, increased the time to first infection and showed protection against viral respiratory infections. The study included 198 hospitalised patients over 65 years of age who were randomised to receive the BCG vaccine or a placebo at the time of their discharge from the hospital. The recruits were then followed up over 12 months to see if BCG protected them from a range of infections. The interim analysis included the data of 72 patients in the BCG group and 78 patients in the placebo group. The published results do not include the data of 48 patients who had not completed one year yet, and the final follow-up on them was scheduled for August.

The study found a noticeable difference between the placebo group and the BCG group. As regards the primary end-point of the study, BCG vaccination delayed the onset of first infection. In the BCG group, participants caught their first infection an average of 16 weeks after vaccination, compared to 11 weeks in the placebo group. While in the placebo group, 42.3 per cent of the elderly had an incidence at least one infection, in the BCG group, only 25 per cent contracted any infection. Also, in terms of patient-infections per year—the secondary end-point of the study—in the placebo group it was 57.7 per 100 patients while in the BCG group it was only 33.3 per 100 patients. No particular difference in the side effects was observed between the two groups.

In addition to the significant effect on infections in general, the other most important observation was that BCG vaccination mainly protected the participants against respiratory infections. BCG-vaccinated elderly people had 75 per cent fewer respiratory infections than those who received placebo. Also read:The vaccine race

Since the study was not specifically designed to evaluate the vaccine’s impact on COVID-19, and also given the low prevalence of COVID-19 among the participants of the study, the paper noted that while most observed protection had been against respiratory infections (probably) of viral origin, it was unclear whether BCG vaccination worked against coronaviruses in general and SARS-CoV-2 in particular.

However, The Lancet , in its May issue, reported that such randomised controlled trials were already under way in the Netherlands and Australia to assess whether the BCG vaccine based on the Danish sub-strain reduces incidence and severity of COVID-19 in health-care workers.

The group of researchers who are involved in these trials said: “If the BCG vaccine or another inducer of trained immunity provides non-specific protection to bridge the gap before a disease-specific vaccine is developed, this would be an important tool in response to COVID-19 and future pandemics.”

Other trials

The trials were part of a larger collaborative study on “trained immunity” among researchers from Radboud Medical Centre and the Universities of Nijmegen in the Netherlands, the University of Bonn, the University of Melbourne, and other Australian, American and Danish institutions.

In a paper published in June in the journal Cell Host & Microbe , this group reported its early findings. The BCG vaccine was administered to 15 volunteers and a placebo to five more. Three months later the researchers analysed their peripheral blood and bone marrow samples, which are the sources of (hematopoietic) stem cells, the ‘mother’ of all immune cells, such as T- and B-cells and NK (natural killer) cells. Some striking differences were found in the two groups.

The immune cells in the peripheral blood of vaccinated individuals released more cytokines, the inflammatory messengers, which strengthen the effectiveness of immune response; for instance, they trigger other immune cells to also mount a defence against the infection. According to Mihai Netea of Radboud University, in the immune cells of vaccinated individuals completely different genes than in the placebo group were found to have been activated, especially those required for cytokine production.

The vaccination also seemed to result in some long-term functional changes in the genetic programme of the individual in the sense that certain genes, especially those needed for increased cytokine production, become more accessible to cells, which means that they are read more frequently than prior to vaccination.

The research also observed a related interesting aspect: the increased access to these genes comes with a molecular regulatory mechanism. The molecule, called HNF (hepatic nuclear factor), ensures that cytokines are released only when there is a pathogen that needs to be attacked. According to Andreas Schlitzer of LIMES Institute of the University of Bonn, this find may be useful therapeutically, to specifically manipulate the trained immunity in a controlled manner.

These observations have given researchers hope that although BCG vaccination cannot prevent infection by SARS-CoV-2, it may have a positive effect by reducing the severity of the impact of the COVID-19 disease. This trial is expected to be expanded to include more participants.

The ACTIVATE trial, too, had included an assessment of production of innate immune responses at two time points, before BCG vaccination and three months after vaccination, in a subset of 57 (of the 198) participants (31 placebo and 26 BCG-vaccinated). The ACTIVATE study had found similar increased cytokine production in the immune cells of peripheral blood among the BCG-vaccinated individuals.

Like in the Australia-Netherlands study, the ACTIVATE trial too found reprogrammed cytokine producing genes in monocytes (kind of white blood cells produced by the myeloid progenitor arm of bone marrow derived stem cells; T- and B-cells are produced by the lymphoid progenitor arm; Fig. 1), which suggests that BCG vaccination induced epigenetic reprogramming.

To further validate the above findings that BCG vaccination induced trained immunity responses in the elderly, the authors carried out an independent study called 300BCG (using the same strain vaccine as in the ACTIVATE trial), in which they assessed the immune response in 14 healthy volunteers aged above 55 at three time points—before BCG vaccination, two weeks after and three months after. Here, too, similar trained immunity responses, such as significantly increased cytokine production and epigenetic reprogramming, were found.

The authors said: “Together, these findings indicate sustained trained immunity responses… and… non-specific beneficial effects against unrelated infections in the elderly upon BCG vaccination.”

Further, the study also found that trained immunity response (of increased inflammatory cytokine production) was not followed by excessive system inflammation. This would be of concern in the context of COVID-19, where exaggerated inflammatory reaction has been seen to contribute to severity and mortality in patients.

The authors also observed that the mechanism of protection induced by BCG vaccination could be due to non-specific T-cell responses or through trained immunity. They said: “Our data point toward induction of trained immunity, although it is likely that a combination of innate and heterologous T-cell immunity is responsible for the entire clinical effect [seen].” T-cells constitute a major component of the adaptive immune system. In particular, they trigger humoral immune response by signalling the B-cells to produce antibodies in response to exposure to viral and other infections.

Study by ICMR

A recent study by the Indian Council of Medical Research (ICMR), which has been posted on October 26 on the online preprint repository medRxiv, has found increased T-cell, B-cell and dendritic cell (DC) responses following BCG vaccination, thus confirming that a combination of the two routes (involving both myeloid and lymphoid derived immune cells) to enhanced immune response was indeed at work. This multi-centric clinical trial was carried out by scientists of the National Institute of Research in Tuberculosis (NIRT), Chennai.

(Dendritic immune cells are mainly derived from the myeloid precursor arm and are similar to monocytes but a very small fraction of DCs are also derived from the lymphoid arm (Fig. 1). While the former present the antigen surface to T-cells to mount an immune attack, the latter set produces large amounts of interferons in response to viral infections.)

This clinical trial investigated the impact of BCG vaccination on the levels of T-cell, B-cell, monocyte and DC response at one month after vaccination in a group of 86 (49 males and 37 females) healthy but elderly individuals of 60-80 years of age (with 54 in the vaccinated and 32 in the unvaccinated groups). The study found that BCG vaccination significantly enhanced the responses of T-cells (in particular what are called helper T-cells CD4 and CD8) and B-cells.

Alongside, the study also found that responses of naive T-cells (which are immature in the sense of not being exposed to external antigens) and regulatory T-cells (which tend to suppress the response of other immune cells) were significantly diminished. Similarly, an enhanced response of mature B-cells and suppression of naive B-cells were seen. While the researchers did not find any change in the response of monocytes, unlike the ACTIVATE study, which saw an epigenetic reprogramming of cytokine producing genes, they observed an enhanced response of both myeloid and lymphoid derived dendritic cells.

The study also found enhanced production of all kinds of antibodies in BCG vaccinated individuals. As the authors said, delayed activation of DCs could result in lagging adaptive response to pathogens. From that perspective, increased frequency of both types of DCs illustrates the important effect of BCG vaccination in enhancing the innate immune response (in terms of antigen presentation and interferon production) to both specific and non-specific pathogens in elderly individuals, they added. Also read:COVID-19: Race for immunity

Citing the ACTIVATE study results, the ICMR paper said: “These data fit very well with increased time to first infection and protection against viral respiratory pathogens. Moreover, BCG vaccination also diminishes the frequency of regulatory T-cells and could, therefore, potentially blunt or ameliorate the regulatory effects of these cells in down modulating protective immune system.” While both ACTIVATE and ICMR studies indicate an important role for BCG vaccination in boosting all-round immune response in the elderly, and make a case for BCG vaccination to be a bridge vaccine until a SARS-CoV-2 specific vaccine is developed, larger clinical trials are needed to study the impact of such a strategy on the morbidity and mortality caused by the virus.

There are many BCG clinical trials currently planned or ongoing across the world from this perspective. The ICMR has scheduled another multi-centric study, involving institutions in Delhi, Lucknow and Kolkata, and coordinated by the AIIMS, New Delhi, which is likely to take off soon. As mentioned in an earlier article in Frontline, (“COVID-19: Race for immunity”, October 23), the Serum Institute of India’s clinical trial for repurposing the BCG vaccine has now reportedly entered Phase III. However, trial details have not been made public.

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