Effects of a Nasal Spray Based on an Antigen Complex from Opportunistic Bacteria in Experimental Models of SARS-CoV-2 and Influenza A Infection

Background/Objectives: Acute respiratory infections represent a global health burden due to their high incidence, morbidity and mortality. Despite advances in prevention and treatment, strategies providing broad protection against respiratory pathogens remain limited. This study evaluated the antiviral activity of nasal spray forms based on an antigen complex from opportunistic bacteria, with or without a mucoadhesive copolymer, and their influence on cellular and humoral components of the immune response. Methods: Antiviral activity was assessed in experimental models of SARS-CoV-2 infection in Syrian hamsters and influenza A pneumonia in mice under prophylactic and therapeutic–prophylactic regimens. Parameters of cellular and humoral immune response were assessed using delayed-type hypersensitivity, antibody-forming cell assays, and leukocyte phagocytic activity. Results: Both forms showed comparable influence on cellular and humoral components of the immune response. In a mouse model of lethal influenza pneumonia induced by A/California/04/2009 (H1N1)pdm09 virus, intranasal administration at 100 µg/kg increased mean survival time and survival rate, and reduced body weight loss. In SARS-CoV-2-infected hamsters, both forms reduced clinical signs, body weight loss, weight lung index, lung pathology, and decreased viral titers. The copolymer-containing form showed the most pronounced effect under the therapeutic–prophylactic regimen, with greater protection against body weight loss and stronger suppression of viral replication. Conclusions: The findings support further investigation of these nasal spray forms as candidates for prophylaxis and adjunctive therapy of respiratory infections, particularly when pathogen variability may limit the effectiveness of pathogen-directed treatments. Further mechanistic studies are needed to clarify the pathways underlying the observed antiviral effect.