BCG vaccination: a long-lasting protection against tuberculosis?--Authors' reply

Countries with high deaths due to flu and tuberculosis demonstrate lower COVID-19 mortality: roles of vaccinations

Deaths due to the ongoing COVID-19 pandemic vary (3-1681 deaths/million and mortality rates 0.71-14.54%) and are far greater in some countries compared to others. This observation led us to perform epidemiological analysis, using data in the public domain, to study the correlation of COVID-19 with the prevalence and vaccination strategies for two respiratory pathogens: flu and tuberculosis (TB). Countries showing more than 1000 COVID-19 deaths were selected at three time points during the ongoing pandemic: 17 May, 1 October and 31 December 2020. The major findings of this study that are broadly consistent at all three time points are: First, countries with high flu deaths negatively correlate with COVID-19 deaths/million. Second, TB incidences and deaths negatively correlate with COVID-19 deaths/million. Countries displaying high TB and flu deaths (Nigeria, Ethiopia, Myanmar, Indonesia, India) display lower COVID-19 deaths/million compared to countries with low TB and flu deaths (Italy, Spain, USA, France). Third, countries with greater flu vaccination display lower flu incidences but higher COVID-19 deaths/million and mortality rates. On the other hand, Bacillus Calmette Guerin (BCG) vaccination negatively correlates with Covid-19 deaths/million. Fourth, countries with only BCG, but no flu, vaccination show delayed and lower number of COVID-19 deaths/million compared to countries with flu, but no BCG, vaccination. Fifth, countries with high BCG vaccination coverage as well as high TB deaths display the lowest COVID-19 deaths/million. The implications of this global study are discussed with respect to the roles of respiratory infections and vaccinations in lowering COVID-19 deaths.

PE_PGRS33, an Important Virulence Factor of Mycobacterium tuberculosis and Potential Target of Host Humoral Immune Response

PE_PGRS proteins are surface antigens of Mycobacterium tuberculosis (Mtb) and a few other pathogenic mycobacteria. The PE_PGRS33 protein is among the most studied PE_PGRSs. It is known that the PE domain of PE_PGRS33 is required for the protein translocation through the mycobacterial cell wall, where the PGRS domain remains available for interaction with host receptors. Interaction with Toll like receptor 2 (TLR2) promotes secretion of inflammatory chemokines and cytokines, which are key in the immunopathogenesis of tuberculosis (TB). In this review, we briefly address some key challenges in the development of a TB vaccine and attempt to provide a rationale for the development of new vaccines aimed at fostering a humoral response against Mtb. Using PE_PGRS33 as a model for a surface-exposed antigen, we exploit the availability of current structural data using homology modeling to gather insights on the PGRS domain features. Our study suggests that the PGRS domain of PE_PGRS33 exposes four PGII sandwiches on the outer surface, which, we propose, are directly involved through their loops in the interactions with the host receptors and, as such, are promising targets for a vaccination strategy aimed at inducing a humoral response.

Current challenges and opportunities for bacillus Calmette-Guérin replacement vaccine candidates

Bacillus Calmette-Guérin (BCG) remains the only licensed vaccine against human tuberculosis (TB). BCG is a live-attenuated strain of Mycobacterium bovis, with limitations in efficacy against respiratory TB, the most common form of the disease responsible for transmission. However, continues to be used in the immunization programmes of different countries in the absence of another alternative. In order to improve BCG efficacy against pulmonary TB, in the current clinical TB vaccine pipeline, there are live-attenuated TB vaccines to replace BCG. This review discusses the current status of the development of live vaccine candidates designed to replace BCG from the rational strategies and immunological challenges to its clinical trial and identify key areas in the next years considered essential to confer improved safety and efficacy over BCG.

Development of tuberculosis vaccines in clinical trials: Current status

Tuberculosis (TB) is an important infectious disease worldwide. Currently, Bacillus Calmette-Guérin (BCG) remains the only TB vaccine licensed for human use. This TB vaccine is effective in protecting children against severe military TB but offers variable protective efficacy in adults. Therefore, new vaccines against TB are needed to overcome this serious disease. At present, around 14 TB vaccine candidates are in different phases of clinical trials. These TB vaccines in clinical evaluation can be classified into two groups including preventive pre- and post-exposure vaccines: subunit vaccines (attenuated viral vectors or adjuvanted fusion proteins), and whole-cell vaccines (genetically attenuated Mycobacterium tuberculosis (M. tb), recombinant BCG, killed M. tb or M. vaccae). Although, over the last two decades a great progress in the search for a more effective TB vaccine has been demonstrated there is still no replacement for the licensed BCG vaccine. This article summarizes the current status of TB vaccine development and identifies crucial gaps of research for the development of an effective TB vaccine in all age groups.

Progress and challenges in TB vaccine development

The Bacille Calmette Guerin (BCG) vaccine can provide decades of protection against tuberculosis (TB) disease, and although imperfect, BCG is proof that vaccine mediated protection against TB is a possibility. A new TB vaccine is, therefore, an inevitability; the question is how long will it take us to get there? We have made substantial progress in the development of vaccine platforms, in the identification of antigens and of immune correlates of risk of TB disease. We have also standardized animal models to enable head-to-head comparison and selection of candidate TB vaccines for further development.  To extend our understanding of the safety and immunogenicity of TB vaccines we have performed experimental medicine studies to explore route of administration and have begun to develop controlled human infection models. Driven by a desire to reduce the length and cost of human efficacy trials we have applied novel approaches to later stage clinical development, exploring alternative clinical endpoints to prevention of disease outcomes. Here, global leaders in TB vaccine development discuss the progress made and the challenges that remain. What emerges is that, despite scientific progress, few vaccine candidates have entered clinical trials in the last 5 years and few vaccines in clinical trials have progressed to efficacy trials. Crucially, we have undervalued the knowledge gained from our "failed" trials and fostered a culture of risk aversion that has limited new funding for clinical TB vaccine development. The unintended consequence of this abundance of caution is lack of diversity of new TB vaccine candidates and stagnation of the clinical pipeline. We have a variety of new vaccine platform technologies, mycobacterial antigens and animal and human models.  However, we will not encourage progression of vaccine candidates into clinical trials unless we evaluate and embrace risk in pursuit of vaccine development.