Genetic characteristics and epidemiology of influenza and respiratory syncytial viruses associated with acute respiratory infections in Thailand.

backgroundInfluenza and respiratory syncytial virus (RSV) are significant contributors to acute respiratory infections and play a substantial role in seasonal morbidity. This study sought to elucidate the clinical characteristics, seasonal trends, and genetic evolution of circulating influenza and RSV strains in Thailand during the period from 2022 to 2023.

methodsNasopharyngeal swabs were obtained from 113 patients presenting with respiratory symptoms. Laboratory confirmation identified 57 cases of influenza and 25 cases of RSV infections. Clinical and demographic data were subjected to analysis. The hemagglutinin (HA) and attachment glycoprotein (G) genes from representative strains of influenza and RSV viruses were sequenced to facilitate phylogenetic and amino acid analyses.

resultsInfluenza cases were predominantly observed in younger adults aged 18–35 years, whereas RSV infections were distributed evenly across various age groups and were more prevalent among females. The clinical manifestations of influenza included cough, nasal congestion, sore throat, and myalgia. In contrast, RSV cases were associated with vomiting, pneumonia, bronchitis, and elevated rates of respiratory distress. The most prevalent influenza subtype was A/H3N2, accounting for 63.2% of cases, followed by A/H1N1 and B/Victoria. RSV-B was more prevalent than RSV-A, with a prevalence rate of 64.0%. The A/H3N2 strains exhibited divergence from the vaccine strain A/Darwin/9/2021, characterised by notable amino acid substitutions. Conversely, A/H1N1 strains remained closely aligned with the vaccine strains. The B/Victoria strains were categorised under the subclade V1A.3a.2 and consistently demonstrated a deletion of three amino acids in the HA gene. RSV-A isolates were identified as belonging to genotype ON1, while RSV-B isolates were classified under genotype BA9, both displaying characteristic duplications and substitutions.

conclusionThis study elucidates significant distinctions in the epidemiology and clinical outcomes of influenza and RSV in Thailand during 2022–2023. Influenza, particularly the A/H3N2 subtype, was more prevalent among younger adults and exhibited genetic divergence from the recommended vaccine strain, highlighting the necessity for continuous surveillance to inform vaccine updates. Conversely, RSV affected individuals across various age groups and was associated with more severe respiratory complications, with RSV-B being predominant. The genetic profiles of influenza and RSV strains, including amino acid substitutions and lineage-specific deletions or duplications, offer valuable insights into viral evolution and immune evasion. Collectively, these findings underscore the importance of integrated clinical and molecular surveillance to inform vaccine strain selection, enhance diagnostic accuracy, and bolster public health preparedness against seasonal respiratory pathogens.