Ferrets With Earlier Flu Immunity Survived H5N1 Bird Flu, and Mostly Survived H5N5

CDC researchers report that ferrets that had recovered from a seasonal H1N1 flu infection three months earlier survived a later challenge with an H5N1 bird flu virus, and that most of the ferrets challenged with H5N5 survived too. H5N5 is the subtype behind a fatal human case reported in Washington state in 2025.

The team writes that H5N5's appearance raised the question of whether immunity left by seasonal flu also covers H5 viruses carrying a different version of their second surface protein, the neuraminidase. Both challenge viruses belong to clade 2.3.4.4b, the H5 lineage that has spread through wild birds and poultry.

Twelve ferrets with no previous exposure to seasonal flu were infected with the H1N1 strain A/Nebraska/14/2019 and confirmed three months later to have raised antibodies, according to the research letter published Nov. 10, 2026, in the CDC journal Emerging Infectious Diseases. Half were challenged with H5N1 and half with H5N5, alongside control ferrets that had no previous flu immunity.

Xiangjie Sun and colleagues at CDC report that the previously infected animals challenged with H5N1 showed minimal disease, a mean maximum weight loss of 2.4% and 100% survival. In the H5N5 group the mean maximum weight loss was 7.6%, and one animal was euthanized on day 7 after reaching a predefined humane endpoint. The control ferrets developed severe disease and were all euthanized at those endpoints.

The letter states that antibodies raised by the earlier infection bound the N1 protein of both the seasonal virus and H5N1, but not the N5 protein of H5N5, and the authors say the absence of those cross-reactive antibodies potentially accounts for the weaker protection there. In both groups, the earlier infection did not substantially cut virus levels in the nose, although it largely kept the virus out of the lungs and other organs.

The letter is an early release ahead of the journal's December issue. The journal notes that early-release articles are not considered final versions, and that changes appear in the version posted when an article is officially released.