The researchers followed 926 healthcare workers during the 2023–2024 influenza season, using regular PCR screening to detect even mild infections. Individuals with higher mucosal IgA levels were significantly less likely to contract influenza A (H1N1), while no protective effect was seen for IgG antibodies in the blood.

“This is, to our knowledge, the first large prospective study in a real-world setting to show that pre-existing mucosal IgA antibodies are linked to protection against influenza infection,” says Oscar Bladh, infectious disease physician at Danderyd Hospital and doctoral student at Karolinska Institutet, and first author of the study.

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The study also found that natural infection boosted mucosal IgA levels, whereas current injected influenza vaccines did not.

“Today’s influenza vaccines provide good protection against severe disease. However, our results suggest that protection against infection itself likely requires high levels of antibodies at the site where the virus first enters the body,” says Charlotte Thålin, adjunct professor at Karolinska Institutet and principal investigator of the Community study.

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The findings point to the need for new strategies, such as nasal vaccines, aimed at strengthening mucosal immunity against respiratory viruses.