Inactivated seasonal influenza vaccines increase serum antibodies to the neuraminidase of pandemic influenza A(H1N1) 2009 virus in an age-dependent manner.

Levels of preexisting antibodies to the hemagglutinin of pandemic influenza A(H1N1) 2009 (hereafter pandemic H1N1) virus positively correlate with age. The impact of contemporary seasonal influenza vaccines on establishing immunity to other pandemic H1N1 proteins is unknown. We measured serum antibodies to the neuraminidase (NA) of pandemic H1N1 in adults prior to and after vaccination with seasonal trivalent inactivated influenza vaccines. Serum antibodies to pandemic H1N1 NA were observed in all age groups; however, vaccination elevated levels of pandemic H1N1 NA antibodies predominately in elderly individuals (age, ⩾60 years). Therefore, contemporary seasonal vaccines likely contribute to reduction of pandemic H1N1-associated disease in older individuals.