Efficacy and Safety of Live Attenuated COVID-19 Vaccine Candidate Based on Omicron-like Temperature-Sensitive Mutant in Preclinical Study

Background/Objectives: Although the COVID-19 pandemic has ended, emerging SARS-CoV-2 variants remain a future threat. This study evaluated the safety and efficacy of an intranasal live attenuated vaccine (LAV) prototype based on a temperature-sensitive (ts) Omicron-like SARS-CoV-2 mutant in a Syrian hamster COVID-19 model. Methods: SARS-CoV-2 Dubrovka (Wuhan-like), Otradnoe (BA.1.1) and FEB2 (BA.5.2) strains and the ts F-F3 mutant derived from FEB2 were characterized by electron microscopy, nanopore sequencing, cytopathic effect titration, and RT-PCR. In vitro studies used Vero CCL-81 cells; in vivo transmissibility and efficacy studies were assessed in female Syrian hamsters. Serum antibody titers were measured by ELISA and neutralization assays. Immunization efficacy was assessed by weight change; viral loads in the lungs, nasal passages, and brain; and histological examination of the lungs at day 4 post-challenge. Safety criteria included evaluation of pathogenicity and transmissibility in hamsters and ts phenotype stability during prolonged in vitro cultivation. Results: The F-F3 strain displayed a stable ts phenotype in vitro, did not infect hamster lungs or brain, and showed no transmission from immunized to naïve co-housed animals. A single intranasal immunization by F-F3 induced seroconversion, reduced pneumonia severity by 76.2% and 79.5% in two independent experiments (p < 0.01), protected the lungs against productive infection while reducing viral load in the nasal passages, and resulted in no detectable viral transmission after homologous BA.5.2 challenge. Following heterologous Wuhan-like challenge, immunization prevented weight loss and significantly reduced viral loads in the lungs, nasal passages, and brain; however, protection against pneumonia was only modest (18.6%; p < 0.05). Conclusions: Preclinical evaluation of the F-F3-based LAV prototype demonstrated a favorable balance between safety and efficacy. These characteristics support its further consideration as a candidate strain for COVID-19 LAV development.