Bacillus Calmette-Guérin vaccination as defense against SARS-CoV-2 (BADAS): a randomized controlled trial to protect healthcare workers in the USA by enhanced trained immune responses

Beyond Tuberculosis: The Surprising Immunological Benefits of the Bacillus Calmette-Guérin (BCG) Vaccine in Infectious, Auto-Immune, and Inflammatory Diseases

The Bacillus Calmette-Guérin (BCG) vaccine, best known for its role in preventing tuberculosis, has recently garnered attention for its broader immunomodulatory effects. By inducing trained immunity, BCG reprograms innate immune cells, enhancing their responses to various pathogens. This process, driven by epigenetic and metabolic reprogramming, suggests that BCG may have therapeutic potential far beyond tuberculosis. Emerging evidence points to its potential benefits in conditions such as autoimmune diseases, cancer, and viral infections. Furthermore, by modulating immune activity, BCG has been proposed to reduce chronic inflammation and promote immune tolerance. This review delves into the multifaceted role of BCG, highlighting its potential as a versatile therapeutic tool for managing a wide range of diseases.

BCG-Induced DNA Methylation Changes Improve Coronavirus Disease 2019 Vaccine Immunity Without Decreasing the Risk for Severe Acute Respiratory Syndrome Coronavirus 2 Infection

Background

The BCG vaccine induces trained immunity, an epigenetic-mediated increase in innate immune responsiveness. Therefore, this clinical trial evaluated if BCG-induced trained immunity could decrease coronavirus disease 2019 (COVID-19)-related frequency or severity.

Methods

A double-blind, placebo-controlled clinical trial of healthcare workers randomized participants to vaccination with BCG TICE or placebo (saline). Enrollment included 529 healthcare workers randomized to receive BCG or placebo. Primary analysis evaluated COVID-19 disease frequency, while secondary analysis evaluated coronavirus immunity in a subset of participants. Study enrollment ceased early in December 2020 following introduction of COVID-19-specific vaccines.

Results

Study enrollment was halted early, prior to reaching the targeted recruitment, and was not powered to detect a decrease in COVID-19 frequency. Symptomatic COVID-19 occurred in 21 of 263 and 10 of 266 participants in the BCG and placebo arms, respectively (P = .50, Fisher exact test). Participants vaccinated with BCG, but uninfected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), demonstrated increased coronavirus vaccine immunity (increase spike-inducible levels of tumor necrosis factor, interleukin 6, and interleukin 1β) 12 months after BCG vaccination compared to participants receiving placebo. Immune responsiveness to SARS-CoV-2 antigens correlated with BCG-induced DNA methylation changes.

Conclusions

Due to early study closure, the study was not powered to evaluate COVID-19 frequency. Secondary analysis demonstrated that 12 months following vaccination, BCG increased coronavirus vaccine immunity compared to those who did not receive BCG. This increase in COVID-19 vaccine immunity correlated with BCG-induced DNA methylation changes.

Host Genetic Background Influences BCG-Induced Antibodies Cross-Reactive to SARS-CoV-2 Spike Protein

Mycobacterium bovis Bacillus Calmette-Guérin (BCG) protects against childhood tuberculosis; and unlike most vaccines, BCG broadly impacts immunity to other pathogens and even some cancers. Early in the COVID-19 pandemic, epidemiological studies identified a protective association between BCG vaccination and outcomes of SARS-CoV-2, but the associations in later studies were inconsistent. We sought possible reasons and noticed the study populations often lived in the same country. Since individuals from the same regions can share common ancestors, we hypothesized that genetic background could influence associations between BCG and SARS-CoV-2. To explore this hypothesis in a controlled environment, we performed a pilot study using Diversity Outbred mice. First, we identified amino acid sequences shared by BCG and SARS-CoV-2 spike protein. Next, we tested for IgG reactive to spike protein from BCG-vaccinated mice. Sera from some, but not all, BCG-vaccinated Diversity Outbred mice contained higher levels of IgG cross-reactive to SARS-CoV-2 spike protein than sera from BCG-vaccinated C57BL/6J inbred mice and unvaccinated mice. Although larger experimental studies are needed to obtain mechanistic insight, these findings suggest that genetic background may be an important variable contributing to different associations observed in human randomized clinical trials evaluating BCG vaccination on SARS-CoV-2 and COVID-19.