Molecular Surveillance Reveals F-Gene Mutations and Constrained G-Gene Evolution in Human Respiratory Syncytial Virus: Implications for Vaccine Efficacy in Saudi Arabia

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Background/Objectives: Human Respiratory Syncytial Virus (HRSV) is a major global cause of acute lower respiratory infections in children. With recent approval of pre-fusion F protein-based vaccines and monoclonal antibodies, ongoing molecular surveillance is critical. This study examined HRSV molecular epidemiology and evolution in Riyadh, focusing on mutations in the attachment (G) and fusion (F) glycoproteins and their potential impact on vaccine efficacy. Methods: Nasopharyngeal aspirates (NPAs) (200 samples) were collected from pediatric patients. HRSV-positive samples were typed, and the G gene hypervariable region and F gene were sequenced. Sequence and phylogenetic analyses were performed to identify circulating genotypes and amino acid substitutions. Results: HRSV was detected in 15% of samples, with HRSV-B slightly predominant over HRSV-A. Infants aged 2–5 months had the highest incidence rate of infection. The ON1 subgenotype remained dominant. The duplicated region of the G gene showed constrained evolution, with 18 variable and 6 conserved residues over 13 years. In the F protein, HRSV-A isolates exhibited high conservation, with only three amino acid substitutions in antigenic sites (Ø and II). Sites III, IV, and V remained fully conserved. In contrast, HRSV-B isolates displayed eight substitutions in antigenic sites, including six in site II (palivizumab-binding epitope). Conclusions: Given the highly effective HRSV prophylactics, including the approved vaccines and monoclonal antibodies, these mutations raise critical concerns regarding vaccine efficacy against HRSV-B. These findings underscore the necessity of sustained, seasonal molecular surveillance to monitor the emergence of variants and provide a molecular basis for further clinical studies.

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