Emergence and Molecular Epidemiology of a New Norovirus GII.17[P17] Variant in Thailand, 2024

Watchaporn Chuchaona, Chulalongkorn University

Watchaporn Chuchaona, Sarawut Khongwichit, and Yong Poovorawan Affiliation of all Authors: Center of Excellence in Clinical Virology, Department of Pediatrics, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand

Molecular Epidemiology & Evolution

Norovirus is a leading cause of epidemic acute gastroenteritis worldwide, resulting in significant morbidity and mortality, particularly among young children, the elderly, and immunocompromised individuals. In Thailand, a newly emerging norovirus GII.17[P17] variant began replacing the previously predominant genotypes starting in July 2024. This variant has been detected in multiple regions, including Bangkok, Nakhon Si Thammarat, and notably Chaiyaphum Province. This study investigated the molecular epidemiological trends and evolutionary dynamics of GII.17[P17] strains using representative samples collected from patients with acute gastroenteritis in Thailand between 2016 and 2024. Phylogenetic analysis of the complete VP1 gene revealed that the newly emerging GII.17[P17] variant is genetically distinct from previously circulating strains and closely related to those reported in the United States and several European countries. The evolutionary rate of the complete VP1 gene among representative strains analyzed in this study, compared with reference strains found in Thailand, was estimated to be 3.18 × 10⁻³ nucleotide substitutions per site per year (95% highest posterior density [HPD]: 2.54 × 10⁻³ to 3.81 × 10⁻³). In comparison, the overall evolutionary rate of the GII.17 capsid sequences was estimated at 4.77 × 10⁻³ substitutions per site per year (95% HPD: 2.86 × 10⁻³ to 6.74 × 10⁻³). Interestingly, genetic variations of recombinant strains—such as GII.17[P25], identified in 2018, and GII.17[PNA6], detected in 2024—were also observed. Continued molecular surveillance of norovirus and emerging variants is essential and may inform future vaccine development targeting GII.17 strains.