Influenza Vaccine effectiveness is lower for A(H3N2) 2024-2025

Influenza A (H3N2) viruses have entered a new phase of accelerated evolution during the 2024–2025 season after several years of relative genetic stability. Analysis of more than 24,000 viral sequences showed continued diversification within clade J.2 and the emergence of a new subclade, K (formerly J.2.4.1

Subclade K carries multiple mutations in key hemagglutinin regions associated with immune escape and antigenic cluster transitions. Laboratory data indicate reduced antibody neutralization of emerging variants, with the greatest reduction observed for subclade K, suggesting potentially lower—but not absent—vaccine effectiveness against this lineage

Although the WHO updated the vaccine strain to a J.2 virus for the 2025 southern hemisphere and 2025–2026 northern hemisphere seasons, J.2 viruses continued to diversify. Subclade K was first detected in the southern hemisphere in 2025 and may become dominant in the 2025–2026 northern hemisphere season

These findings underscore the need for strengthened influenza surveillance integrating genomic sequencing, antigenic testing, serology, and epidemiologic data to support timely vaccine strain selection and public health risk assessment

References:

World Health Organization (WHO). Recommended composition of influenza virus vaccines for use in the 2025–2026 northern hemisphere influenza season. WHO, 2025

World Health Organization (WHO). Influenza virus characterization reports and vaccine strain selection documents, 2024–2025

GISAID Initiative. Global Initiative on Sharing All Influenza Data (H3N2 sequence data, 2024–2025