Previous infections with seasonal H1N1 flu may boost immunity and reduce the severity of H5N1 bird flu, according to a study.

The study, published in the journal Emerging Infectious Diseases, may help explain why most reported human cases of H5N1 bird flu in the US have not resulted in lethal outcomes.

Researchers from the universities of Pittsburgh and Emory conducted a study to decode the potential of viruses to spread between people.

Using a ferret model, they found that pre-existing immunity influences infection severity. It protected them against severe disease and death by a strain of H5N1 bird flu — currently circulating in wild birds, poultry, and cows. On the other hand, ferrets without prior immunity had more severe illness and fatal outcomes.

“All human flu pandemics emerge in the context of pre-existing immunity,” said lead author Valerie Le Sage, research assistant professor of microbiology and molecular genetics at the University of Pittsburgh.

“Our model isn’t perfect because the human immune response is complicated. But if we are using ferrets as surrogates for humans, it is very important to do so in the context of prior immunity,” Le Sage said.

Ferrets develop clinical symptoms of flu infection that closely resemble humans. They also develop a fever, sneeze, and have a runny nose.

The team showed that among the small number of ferrets intranasally infected with H5N1, only those with previous exposure to H1N1 survived the infection. Despite a similar degree of lung tissue damage, ferrets without prior immunity exhibited higher fever, greater weight loss, and a decrease in playfulness compared to those with H1N1 prior immunity.

Immunity from previous infections also seemingly helped the animals to clear out the virus from their nasal passages faster and limited the infection to the respiratory tract.

In contrast, immunologically naive ferrets displayed signs of systemic infection with virus particles spreading throughout the body, including the heart, liver, and spleen.

The study highlighted the need for a concerted effort to consider pre-existing immunity in the context of pandemic risk assessment.

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