• Kanak Parmar, Afzal Siddiqui, and Kenneth Nugent.
• Department of Internal Medicine, Texas Tech University Health Sciences Center, Lubbock, TX, USA.
• Am. J. Med. Sci. 2021 Jun 1; 361 (6): 683689683-689.

AbstractBacillus Calmette-Guerin (BCG) vaccine is one of the most widely used vaccines in the world. It protects against many non-mycobacterial infections secondary to its nonspecific immune effects. The mechanism for these effects includes modification of innate and adaptive immunity. The alteration in innate immunity is through histone modifications and epigenetic reprogramming of monocytes to develop an inflammatory phenotype, a process called "trained immunity." The memory T cells of adaptive immunity are also responsible for resistance against secondary infections after administration of BCG vaccine, a process called "heterologous immunity." Bacillus Calmette-Guerin vaccine is known to not only boosts immune responses to many vaccines when they are co-administered but also decrease severity of these infections when used alone. The BCG vaccine by itself induces a TH1 type response, and its use as a vector has also shown promising results. This review article summarizes the studies showing effects of BCG vaccines on various viral infections, its role in enhancing vaccine responses, the mechanisms for this protective effect, and information on its effect on COVID-19.Copyright © 2021 Southern Society for Clinical Investigation. Published by Elsevier Inc. All rights reserved.

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