Historical drivers of HCV Subtypes 1b and 3a in Thailand and 6f in Phetchabun, an HCV endemic area of the country.

The World Health Organization has set a target to eliminate viral hepatitis as a public threat by 2030. In pursuit of this goal, Thailand initiated a hepatitis C virus (HCV) microelimination project targeting Phetchabun province, a well-recognized high-burden HCV endemic area. However, the historical transmission dynamics of HCV in Phetchabun, and in Thailand in general, remain unclear. This study investigates the epidemic histories of HCV in Phetchabun, focusing on Subtypes 1b, 3a, and 6f, and their relationship with HCV in other regions of Thailand, using molecular phylogenetic analyses. Our results reveal nationwide the presence of Subtypes 1b and 3a, while Subtype 6f is mainly confined to Phetchabun. The initial spread of Subtype 1b was inferred to coincide with World War II and the period of suboptimal medical and hygienic standards in Thai blood transfusion services, suggesting a correlation between the two. The early expansion of Subtype 3a was, on the other hand, found to correlate with the epidemic of intravenous drug use in Thailand during the time of Vietnam War. The early expansion of Subtype 6f, in contrast, appears to coincide with the period of severe regional political conflict and social and economic instability. All these findings suggest the complex interplay between social determinants of health and HCV transmission. Post the mid-1990s/early 2000s, all subtypes showed significantly reduced population growth rates, aligning with improvements in blood transfusion safety standards, the nationwide "War on Drugs" policy, and enhanced accessibility to public healthcare and HCV treatments. These combined efforts likely have contributed to curbing the spread of HCV in Thailand. Nevertheless, our analyses reveal that the prevalence of HCV in Thailand remains high overall, emphasizing the need for further research and a nationwide approach to more effectively reduce the HCV burden in Thailand.