This year, the flu shot in the US will only protect against three subtypes of the influenza virus: H1N1, H3N2, and the “Victoria lineage” of the influenza B virus. The “Yamagata lineage” has been dropped from the formulation because it hasn’t been detected since 2020. This decision was made by the FDA in March 2024 based on recommendations from experts.
Flu vaccines are designed to protect against the most common strains of the flu each season. Scientists predict which strains will circulate by tracking infections in the opposite hemisphere. The flu shot works by priming the immune system to produce antibodies that can fight off the selected flu strains.
The Yamagata lineage likely disappeared due to public health measures put in place during the COVID-19 pandemic. Many people were wearing masks, social distancing, and staying home when sick, which helped reduce the spread of other respiratory infections like the flu. Since no one has been infected with the Yamagata virus since 2020, it was removed from the flu vaccine formulation.
Removing the Yamagata lineage from the flu shot won’t affect its effectiveness against other strains. Manufacturers may be able to produce more doses of the vaccine without Yamagata, and the production process may be safer. If the Yamagata virus were to reappear, scientists could easily make the vaccine quadrivalent again.
Overall, the change in the flu vaccine formulation for the 2024-2025 season is based on scientific evidence and recommendations from health experts. This adjustment ensures that the vaccine remains effective in protecting against the most common flu strains.