Serological evidence of partial protection against infection with A/H3N2 circulating drifted strains after immunization with enhanced 2011/2012 trivalent influenza vaccines

A(H3N2) influenza viruses presenting antigenic and genetic patterns (A/Victoria/208-clade) different from the A(H3N2) vaccine component (A/Perth/16/2009-clade) were isolated during the 2011-2012 season in Europe. Results obtained using a test-negative case-control study design demonstrated a suboptimal protective effect of the vaccine especially in older people and it was postulated that this could be due to a combination of a poor match between vaccine and circulating viruses and a waning protection. Our results obtained using a different method support this hypothesis. Ninety-two institutionalized elderly subjects were vaccinated with an influenza enhanced vaccine (Fluad® or Intanza® 15mcg) for the 2011-2012 season. Haemagglutination inhibiting (HI) antibody titers were determined in blood samples collected before, 1 and 6 months after vaccination against the three vaccine antigens and against four drifted A(H3N2) viruses circulating in the area where the elderly people were living. A serological diagnosis of influenza infection was made on comparing HI titers found in sera collected 1 and 6 months after vaccination. Patients were considered positive if they had a seroconversion. No seroconversions were found against the A(H1N1) and B vaccine components and 6 months after vaccination HI titers decreased as compared with those found at one month. Instead, 17 (18.5%) volunteers seroconverted against A(H3N2) antigens. In most instances the infections were evidenced using both the vaccine and the epidemic A(H3N2) viruses as antigens. One month after vaccination, 15 of the 17 volunteers (88.2%) had post-vaccination seroprotective HI titers (≥40) against vaccine antigen, but not always against the epidemic strains.