Abstract
Viral respiratory tract infection (RTI) is the most frequent cause of infectious illnesses including the common cold. Pharmacological solutions for treating or preventing viral RTIs are so far limited and thus several self-care products are available in the market. Some dietary supplements such as probiotics have been shown to modulate immune system function and their role in reducing the risk and the course of RTIs has been investigated extensively within the past decade. However, the mechanism of action and the efficacy of probiotics against viral RTIs remains unclear. We searched PubMed, Google Scholar, and Web of Knowledge for pre-clinical and clinical studies investigating the effect of probiotics on respiratory virus infections, immune response, and the course of upper and lower respiratory tract illness. The literature summarized in this narrative review points out that specific probiotic strains seem effective in pre-clinical models, through stimulating the immune system and inhibiting viral replication. Clinical studies indicate variable efficacy on upper respiratory illnesses and lack proof of diagnosed viral infections. However, meta-analyses of clinical studies indicate that probiotics could be beneficial in upper respiratory illnesses without specific etiology. Further studies aiming at discovering the mechanisms of action of probiotics and clinical efficacy are warranted.
Highlights
Respiratory viruses cause the most common infectious illnesses in humans—acute respiratory tract infection (RTI) that can be divided into upper RTIs (URTI), e.g., the common cold, and lower RTIs (LRTI), e.g., bronchitis and pneumonia
We focus on evaluating the current knowledge in the scientific literature on the immunomodulatory effects of probiotics in the context of viral RTIs by reviewing key pre-clinical and clinical evidence
B and T cell responses initiate within four–six days post-infection and peak later at days 7–14 depending on the respiratory virus [28,29,30]. Common respiratory viruses, such as rhinovirus and influenza virus, are cleared before adaptive immune responses are activated [22,30] indicating that memory responses and innate immunity are essential in viral eradication
Summary
Respiratory viruses cause the most common infectious illnesses in humans—acute RTIs that can be divided into upper RTIs (URTI), e.g., the common cold, and lower RTIs (LRTI), e.g., bronchitis and pneumonia. These illnesses affect all age groups annually and cause a high burden on health care systems and global economics due to absenteeism from daycares, school, and work. “respiratory tract infections”, “viral respiratory tract infections”, and “respiratory virus” in common databases such as PubMed, Google Scholar, and Web of Knowledge up to March 2020 In this narrative review, we focus on evaluating the current knowledge in the scientific literature on the immunomodulatory effects of probiotics in the context of viral RTIs by reviewing key pre-clinical and clinical evidence
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