Non-specific effects of BCG vaccine on viral infections

Background: Some strains of Bacillus Calmette-Guerin (BCG) vaccine not only confer protection against disseminated forms of tuberculosis, but also reduce all-cause mortality by the induction of protection against infections with non-related pathogens. Objectives: We review evidence for non-specific protection induced by BCG vaccination against viral infections, discuss possible mechanisms of action, and summarize implications for vaccination policies and vaccine discovery. Sources: Relevant studies retrieved from PubMed and clinicaltrials.gov. Content: Numerous epidemiological, clinical and immunological studies demonstrate that BCG vaccination impacts the immune response to subsequent infections, resulting in reduced morbidity and mortality. Important lines of evidence indicating that BCG protects against viral pathogens comes from experimental studies in mice showing that BCG offers protection against various DNA and RNA viruses, including herpes and influenza viruses. Recently, the effect of BCG on an experimental viral infection in humans has been demonstrated. These effects are thought to be mediated via the induction of innate immune memory and heterologous lymphocyte activation, resulting in enhanced cytokine production, macrophage activity, T-cell responses and antibody titres. (C) 2019 European Society of Clinical Microbiology and Infectious Diseases. Published by Elsevier Ltd. All rights reserved.