For more than a decade, the annual flu shot has protected against four types of the influenza virus — but this year, that has changed.
In the United States, vaccines for the 2024-2025 flu season provide protection against only three subtypes of influenza: two influenza A viruses, called H1N1 and H3N2; and one influenza B virus, known as the "Victoria lineage." A fourth kind of flu virus — the "Yamagata lineage" — has been dropped from the formulation. This will be the first time since 2012 that U.S. flu vaccines have been trivalent, meaning three-component, as opposed to quadrivalent, or four-component.
