Epidemiology, Distinct Seasonal Dynamics, and Age-Specific Characteristics of Eight Respiratory Viruses in Thailand,...

Acute respiratory infections (ARIs) impose a substantial annual public health burden, largely driven by a diverse range of respiratory pathogens. Given the dynamic and continuously evolving circulation of these pathogens, as well as their markedly different prevalence across age groups, ongoing surveillance is essential. To characterize the current prevalence, seasonal patterns, and age-related distribution of eight key respiratory pathogens among ARI patients in Thailand, from January to December 2025, a retrospective study of 4697 respiratory samples from ARI patients aged 1 month to 97 years was conducted. Using real-time RT-PCR to screen for eight respiratory viruses, 61.1% (2871) of samples tested positive, with coinfections identified in 267 of the positive cases. Influenza virus (IFV) was the most prevalent pathogen at 22.2%, demonstrating a clear biannual pattern with peaks in January-March and July-December. Human rhinovirus (HRV) was the second most frequently detected virus (16.6%) and circulated year-round, whereas SARS-CoV-2 accounted for 9.9% of cases, with a pronounced peak in May. Significant age-specific differences in viral detection were observed. HRV predominated in children < 5 years, while IFV exhibited a broader distribution, with higher prevalence among individuals aged 6-18 years and ≥ 19 years. In contrast, SARS-CoV-2 was more frequently detected in adults and older populations. Coinfections were most common in children aged 3-5 years and were frequently associated with HRV. Notably, IFV, HRV, and RSV showed a significant increase in 2025 compared with 2024. These findings provide updated insights into the circulation of respiratory viruses in 2025, revealing distinct seasonal patterns, increasing influenza activity significantly with prolonged circulation in the latter half of the year, and clear age-related differences in pathogen distribution. This provides effective support for continuous surveillance to inform effective public health strategies for ARI control.