Increase in the incidence of invasive bacterial infections following the COVID-19 pandemic: potential links with decreased herd trained immunity - a novel concept in medicine.

Abstract

A global increase in the incidence of various infectious diseases has been observed since the end of the COVID‑19 pandemic. This may be due to 2 independent phenomena. One of them is impaired immunity of long‑COVID patients. The second (major) one is associated with long‑term isolation of many people during the global pandemic‑related lockdown, resulting in an extreme reduction of exposure to natural environmental human microbiota. This, in turn, led to a silencing state of the body's defense systems, including a decline of the prepandemic trained immunity (innate memory), which only persists for weeks to months after exposure to a pathogen. This decrease in the performance of trained immunity may be especially important for morbidity of infectious diseases without currently available vaccines, such as invasive group A Streptococcus pyogenes (GAS) infections, primarily streptococcal toxic shock syndrome. This review discusses data that support an important role of trained macrophages in host defense, and demonstrates the potential clinical implications of β‑glucan, the major inducer of trained macrophages, for prophylactic and therapeutic use in individuals with impaired personal innate immunity. Altogether, it might be speculated that trained innate immunity within an entire population can lead to the development of herd trained immunity (HTI), a newly‑coined medical term. HTI can supplement classic, antigen‑specific herd immunity (memory B and T cells), and it plays a key role in preventing the spread of various infectious diseases, including invasive GAS infections. Unfortunately, the global HTI has been overthrown during the COVID‑19 pandemic; however, it should be restored shortly.