6 October 2025

Key points

  • The World Health Organization now advises moving from the quadrivalent flu vaccine to the trivalent one.
  • The quadrivalent vaccine contains two influenza A strains and two influenza B strains, and so protects against four strains.
  • The trivalent vaccine carries the same two A strains, but only one B strain.
  • The strain dropped is influenza B/Yamagata (山形株): it is no longer being found anywhere in the world, which is why WHO advised removing it.
  • From 2025, therefore, the quadrivalent flu vaccine becomes a trivalent one.