Flu shots have changed this year — here's why

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.

But why the change?

Based on all available evidence, the Yamagata lineage is now extinct. So, in March 2024, the U.S. Food and Drug Administration (FDA) announced that this year's flu vaccines would not include that subtype of virus, because it hasn't sickened anyone in years. We know this because there is a global consortium of scientists that regularly isolates flu viruses from sick patients and then analyzes the viruses' genetic material.

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The FDA made this decision following recommendations from its Vaccines and Related Biological Products Advisory Committee, and shortly after, the World Health Organization issued the same recommendation on a global scale. However, it may take time for global manufacturers to switch to the new formula, STAT News reported.

Flu vaccines are designed to protect people against the most common subtypes of iNFLuenza that sicken people each season. Within those subtypes, scientists predict which strains are most likely to circulate in the upcoming flu season by tracking infections in the opposite hemisphere, since the Northern and Southern hemispheres' flu seasons alternate.