Hemagglutinin (influenza B virus, Yamagata lineage) (HA)

Hemagglutinin (HA) is the primary surface glycoprotein of the influenza B virus, specifically the Yamagata lineage, which was one of the two main lineages circulating globally until its apparent extinction in 2020 (Wikipedia, 2024; NIH, 2024). HA is a type I transmembrane protein that functions as a lectin, binding to sialic acid receptors on host respiratory cells to facilitate viral attachment and entry via membrane fusion (CDC, 2024; PNAS, 2024). As the most prominent antigen on the viral surface, HA is the central component of seasonal influenza vaccines, where it induces the production of neutralizing antibodies that prevent infection (NIH, 2024; Biomedicus, 2026). Historically, quadrivalent vaccines included a Yamagata lineage HA, such as the B/Phuket/3073/2013-like strain, to provide broad protection against both B lineages (WHO, 2024; TGA, 2024). However, due to the lack of detected Yamagata cases since the COVID-19 pandemic, the World Health Organization (WHO) has recently recommended its removal from future vaccine formulations to prevent potential reintroduction of the virus into the population (Pharmacy Times, 2024; Eurosurveillance, 2022). This transition back to trivalent vaccines highlights the dynamic nature of influenza surveillance and the importance of matching vaccine antigens to circulating strains (NIH, 2024). Understanding the structure and evolution of this protein remains essential for monitoring vaccine efficacy and responding to potential viral re-emergence in the future (ResearchGate, 2014; NIH, 2025). Therapeutic challenges include antigenic drift and egg-adaptation mutations, which can reduce the effectiveness of the immune response induced by the vaccine (CDC, 2024; NIH, 2024).

Other names

Hemagglutinin proteinHA1HA2Influenza B Yamagata HAB/Yamagata HAB/Phuket/3073/2013-like HAB/Yamagata hemagglutinin

Active immunization inducing neutralizing antibodies that block viral attachment to sialic acid receptors and inhibit membrane fusion.