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

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

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 H1N1 season Vaccine efficacy against infection was 57% in the H1N1 season Circulating H1N1 viruses in and were antigenically similar LAIV –Belshe et al., Clin Infect Dis 2004;39:920 Subgroup analyses of clinical trials used for licensure Among children 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 years were vaccinated with live attenuated vaccine over 3 seasons ( , , ) 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 ( ) and/or 3 ( ) Vaccine effectiveness against medically attended acute respiratory illness (MAARI, not lab confirmed) was ~20% in the 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 season among group only vaccinated in season - approximately same as for children vaccinated all 3 years Circulating viruses in and were H1N1 and B. The H1N1 viruses in were drifted compared to those circulating in Not enough in dataset to evaluate children only vaccinated in year 1