Researchers believe they now understand how severe cases of influenza damage the heart, providing an explanation for the annual uptick in heart attacks during flu season.

“We have known for years that the frequency of heart attacks increases during flu season, yet outside of clinical intuition, scant evidence exists of the underlying mechanisms of that phenomenon,” study leader Filip Swirski of the Icahn School of Medicine at Mount Sinai said in a statement.

Studying tissue samples from hospitalized patients who died of influenza, the researchers learned that a type of immune cell known as a pro-dendritic cell 3 becomes infected in the lungs and travels to the heart.

There, instead of performing an immune cell's usual job of clearing the virus, the pro-dendritic cell 3 produces large amounts of an inflammatory protein called type 1 interferon (IFN-1) that triggers the death of heart muscle cells, impairing cardiac output.

“The pro-dendritic cell 3 acts as the ‘Trojan horse’ of the immune system during influenza infection, becoming infected in the lung, trafficking the virus to the heart, and disseminating it to cardiomyocytes,” study co-author Jeffrey Downey, also from Mount Sinai, said in a statement.

Vaccination against the flu offers some protection against this type of heart damage, the researchers also reported in Immunity.

Downey noted that in lab experiments, an mRNA drug that controls IFN-1 activity reduced the influenza-related heart muscle damage in test tubes and in mice and improved the muscles’ pumping ability.

The new findings “offer great promise for the development of new therapies, which are desperately needed since there are currently no viable clinical options to prevent cardiac damage” from the flu, Swirski said.