Duration of Serum Antibody Response to Seasonal Influenza Vaccines: Summary

1. Duration of Serum Antibody Response to Seasonal Influenza Vaccines: Summary • The level of antibody response made to seasonal influenza vaccines depends on the vaccine preparation, dose, prior antigenic experience, and age or underlying disease conditions of an individual • Antibody responses are typically greatest among primed healthy older children, adolescents and young adults and are lower among the elderly and young children • Following vaccination, anti-HA antibody titers (measured by the hemagglutination-inhibition assay) peak 2- 4 weeks post-vaccination in primed individuals but may peak 4 weeks or later in unprimed individuals or older adults • Serum antibody titers may fall by 50% or more by 6 months after vaccination, with the degree of reduction being proportional to the peak titers achieved • Vaccine-induced serum antibody titers then remain stable for two to three years

2. Duration of Serum Antibody Response to Inactivated A/USSR/77 (H1N1) Vaccines(Cate et al., Rev Infect Dis 1983; 5:737) • Evaluation of persistence of vaccine-induced antibody to H1N1 virus in the absence of circulating virus in the community • Mean HI titers at 6 months, were generally 2-fold lower than peak titers achieved shortly after vaccination • There was no difference in antibody persistence among whole virus vaccine versus split virus vaccine recipients • Mean decreases in titers were greater (~4-fold) in younger adults (20-25 yrs) compared with older adults (>44 yrs) • Two thirds of younger adults lacked detectable pre-vaccination HI titers to USSR/77 • These results suggest that prior experience with related influenza viruses influence both the titer achievement and persistence of the anti-HA antibody response

3. Duration of Protection Following Seasonal Influenza Vaccination • Protection without revaccination persists for at least 3 years (children and young adults) • TIV: • Foy et al., JAMA 1973; 226:758 • School aged children were vaccinated with a single dose of either A/Hong Kong/68 (H3N2) or an influenza B vaccine (control) in 1968 • Vaccine efficacy against H3N2 viruses was estimated to be 76-83% in the first two epidemics • ~ 60% effective in preventing serologically-confirmed H3N2 influenza in third year (1972) of the study • Couch et al., In Options for the Control of Influenza, 1996 • Young healthy adults were vaccinated with trivalent influenza vaccine in 1986 • Vaccination was not repeated in year 2 or 3 • Vaccine efficacy against infection was 85% in the 1986-87 H1N1 season • Vaccine efficacy against infection was 57% in the 1988-89 H1N1 season • Circulating H1N1 viruses in 1986-87 and 1988-89 were antigenically similar • LAIV • Belshe et al., Clin Infect Dis 2004;39:920 • Subgroup analyses of clinical trials used for licensure • Among children 60-83 months, efficacy against culture-confirmed influenza in year two was 86.9% (95% CI 78.8%-94.1%) • In year 2, 93% of infections were due to drifted H3N2 strain • Galgani et al., Arch Ped Adoles Med 2004; 158:65 • Young healthy children aged 1.5-18 years were vaccinated with live attenuated vaccine over 3 seasons (1999-2000, 2000-01, 2001-02) in community-based, non randomized study (total pop vaccinated during study ~5000) • For some (self selected, not randomized) vaccination was not repeated in year 2 (2000-01) and/or 3 (2001-02) • Vaccine effectiveness against medically attended acute respiratory illness (MAARI, not lab confirmed) was ~20% in the 2000-01 season in overall group (NB: VE using MAARI outcome expected to be lower than lab confirmed; 20% is "good") • Vaccine effectiveness against MAARI was 22% (95% CI=11%-32%) in the 2001-02 season among group only vaccinated in 2000-01 season - approximately same as for children vaccinated all 3 years • Circulating viruses in 2000-01 and 2001-02 were H1N1 and B . The H1N1 viruses in 2001-02 were drifted compared to those circulating in 2000-01 • Not enough in dataset to evaluate children only vaccinated in year 1