Partial protection induced by 2011–2012 influenza vaccine against serologically evidenced A(H3N2) influenza virus infections in elderly institutionalized people

Abstract

Ninety-two institutionalized elderly subjects were vaccinated with trivalent influenza inactivated vaccine available for the 2011–2012 season, characterized by a prevalent circulation of A(H3N2) influenza viruses (A/Victoria/208-clade) presenting antigenic and genetic patterns different from the A(H3N2) vaccine component (A/Perth/16/2009-clade). Haemagglutination inhibiting (HI) antibody titers were determined in sera collected before, 1 and 6 months after vaccination and patients were considered positive for serological evidence of recent infection if they had a seroconversion on comparing HI titers found in sera collected 1 and 6months after vaccination. No seroconversions were found against A(H1N1) and B vaccine components. Instead 17 volunteers seroconverted against all or at least some of the different A(H3N2) antigens examined, i.e. the 2011–2012 (A/Perth/16/2009) and the 2012–2013 (A/Victoria/361/2011) vaccine strains and four drifted viruses belonging to the A/Victoria/208-clade circulating in the area were the elderly people were living. The results obtained suggest that influenza infections in the vaccinated volunteers might be due both to a poor match between vaccine and circulating A(H3N2) viruses, since 1 month after vaccination 15 of the 17 volunteers had post-vaccination HI titers considered protective (≥40) against the A(H3N2) vaccine antigen, but not always against the epidemic strains, and to a waning of vaccine induced immune response, since 6 months after vaccination HI titers of non-infected volunteers were found to be decreased as compared with those found 1 month after vaccination.