Swine influenza virus antibodies in humans, Western Europe, 2009

Serologic studies for swine infl uenza viruses (SIVs) in humans with occupational exposure to swine have been reported from Americas not from Europe. We compared levels of neutralizing antibodies against 3 infl uenza viruses – pandemic (H1N1) 2009, an avian-like enzootic subtype H1N1 SIV, and a 2007-08 seasonal subtype H1N1 – in 211 persons with swine contact and 224 matched controls in Luxembourg. Persons whose profession involved contact with swine had more neutralizing antibodies against SIV and pandemic (H1N1) 2009 virus than did the controls. Controls also had antibodies against these viruses although exposure to them was unlikely. Antibodies against SIV and pandemic (H1N1) 2009 virus correlated with each other but not with seasonal subtype H1N1 virus. Sequential exposure to variants of seasonal infl uenza (H1N1) viruses may have increased chances for serologic cross-reactivity with antigenically distinct viruses. Further studies are needed to determine the extent to which serologic responses correlate with infection.