A case report of a patient in whom antibodies against the 2009 pandemic influenza A/H1N1 virus have been present since June 1999.

A 69-y-old man with a history of hepatitis C since May 1985 and his 6 healthy immediate relatives were examined for hemagglutination inhibition antibodies against 2009 pandemic influenza A/H1N1 virus. This patient had a hemagglutination inhibition antibody titer of 640 against 2009 pandemic influenza A/H1N1 virus in a serum sample collected on July 4, 1999, and the antibody titers fluctuated between 40 and 320 in serum samples collected after 1999. The fluctuations in hemagglutination inhibition antibody titers against pandemic 2009 influenza A/H1N1 virus were not consistent with his history of seasonal influenza, and our results suggest a relationship to his vaccination with seasonal trivalent inactivated influenza vaccine. This patient as well as three relatives showed cross-reactive antibody titers of 10 or more against 2009 pandemic A/H1N1 influenza virus in serum samples taken after June 1999. From these results we conclude that the cross-reactivity to pandemic 2009 A/H1N1 influenza virus emerged after June 1999.