New animal research suggests that the H1N1 influenza pandemic of 2009 may have provided the public some protection against illness during the ongoing bird flu outbreak, which has affected wild birds, poultry, dairy cattle, and other mammals.
Between March 2024 and July 2025, there have been 70 confirmed bird flu cases among people in the US and 1 death. But most of these bird flu infections with H5N1 avian influenza have been mild compared with historical cases, despite dairy workers being exposed to high levels of the virus from infected cows.
The new study, published in Science Translational Medicine, assessed H5N1 susceptibility based on preexisting immunity to seasonal influenza viruses and the novel H1N1 virus that caused the 2009 flu pandemic. The study was conducted in ferrets, a commonly used animal model for human influenza.
The researchers exposed ferrets with varying influenza immunity to ferrets infected with H5N1 virus from dairy cattle. Animals without any influenza immunity quickly developed severe and lethal disease. All the ferrets with immunity to H3N2 seasonal influenza lost weight and developed mild clinical illness, and half required euthanasia. However, only half of the ferrets with H1N1 immunity became infected, none developed clinical signs, and all survived.
“Every person has been exposed to H1N1 as the virus caused a pandemic in 2009 and is now the predominant circulating influenza strain in 1 out of every 3-4 years,” Troy Sutton, PhD, a study author and an associate professor of veterinary and biomedical sciences at Penn State, said in a press release. “Our findings suggest that this immunity is protective against the more recent H5N1 strain and may explain why we’re seeing fewer cases and less severe disease than we would expect.”