Transition from Quadrivalent to Trivalent Influenza Vaccination Compared with Continued Quadrivalent Vaccination for Preventing Severe Influenza Outcomes in Populations in Indonesia: Implications from the 2025-2026 Epidemiological Landscape.

Seasonal influenza remains a global public health challenge, with an estimated 1 billion infections and 290,000-650,000 respiratory deaths each year. The 2024-2025 season recorded a high disease burden in the United States, with tens of millions of symptomatic cases and hundreds of thousands of hospitalizations, as well as the highest hospitalization rates in the FluSurv-NET surveillance era. Entering the 2025-2026 season, a shift in dominance toward influenza A(H3N2) has been observed in various regions, including the emergence of subclade K (J.2.4.1), which has spread globally and become the dominant variant in several regions. In Indonesia, surveillance data indicate an increase in influenza activity in late 2025, followed by a decline in early 2026, with the positivity rate peaking during this period before returning to low activity levels. Subclade K has been detected across multiple provinces with wide geographic distribution, and most cases present with mild to moderate clinical manifestations. Serological studies have shown reduced antigenic reactivity between the vaccine and circulating viruses; however, real-world vaccine effectiveness data indicate that protection against severe disease remains preserved, with higher effectiveness observed in children compared to adults. The World Health Organization has recommended transitioning from quadrivalent to trivalent vaccines since 2023, following the absence of the B/Yamagata lineage in global circulation since 2020. This study presents an updated overview of global and national influenza epidemiology and its implications for vaccination policy, providing a scientific basis to support consideration of transitioning to trivalent vaccines in Indonesia.