Prior seasonal flu infection may reduce severity of H5N1 bird flu: study

A new study suggests that prior infection with seasonal flu (H1N1) may provide some protection against severe illness from H5N1 bird flu.

The findings, published in the journal Emerging Infectious Diseases, may help explain why most human cases of H5N1 in the US have not resulted in fatal outcomes.

Researchers from the University of Pittsburgh and Emory University conducted the study to better understand how viruses spread and how the immune system responds to infections. Using a ferret model, they discovered that pre-existing immunity from prior exposure to H1N1 flu significantly reduced the severity of H5N1 infection.

Ferrets that had previously been infected with H1N1 flu showed a greater ability to fight off severe disease and survived H5N1 infection, whereas those without prior immunity suffered more severe symptoms and higher fatality rates.

"All human flu pandemics emerge in the context of pre-existing immunity," said Valerie Le Sage, lead author and research assistant professor of microbiology and molecular genetics at the University of Pittsburgh. She emphasised that while ferrets are not a perfect model for humans, their immune responses closely resemble those of people, making them a valuable tool for studying flu infections.

The research revealed several important differences between ferrets with prior immunity and those without. Ferrets previously exposed to H1N1 cleared the virus faster from their nasal passages, reducing the risk of severe illness. Despite similar lung tissue damage, ferrets with pre-existing immunity showed milder symptoms such as lower fever, less weight loss, and more activity. Those without immunity experienced widespread infection, with the virus spreading to multiple organs, including the heart, liver, and spleen.

These findings indicate that prior flu infections may help the body mount a stronger and more localised immune response, preventing the virus from spreading throughout the body.