Influenza illness and its complications follow infection with influenza viruses. Global surveillance of influenza identifies yearly antigenic variants. For example, since 1977, antigenic variants of influenza A (H1N1 and H3N2) viruses and influenza B viruses have been in global circulation. Specific levels of hemagglutination-inhibition (HI) antibody titer post-vaccination with inactivated influenza virus vaccines have not been correlated with protection from influenza illness but the HI antibody titers have been used as a measure of vaccine activity. In some human challenge studies, HI antibody titers of ≥ 1:40 have been associated with protection from influenza illness in up to 50% of subjects.^1,2 Antibody against one influenza virus type or subtype confers little or no protection against another virus. Furthermore, antibody to one antigenic variant of influenza virus might not protect against a new antigenic variant of the same type or subtype. Frequent development of antigenic variants through antigenic drift is the virological basis for seasonal epidemics and the reason for the usual incorporation of one or more new strains in each year's influenza vaccine.³ Therefore, inactivated influenza vaccines are standardized to contain the hemagglutinins of strains (i.e., typically 2 type A and 1 type B), representing the influenza viruses likely to circulate in the United States in the upcoming winter.

Immune Response to FLUARIX: In a randomized, double-blind, placebo-controlled study conducted in healthy subjects 18 to 64 years of age in the United States (Study FLUARIX-US-001), the immune responses to each of the antigens contained in FLUARIX were evaluated in sera obtained 21 days after administration of FLUARIX (n = 745) and were compared to those following administration of a placebo vaccine (n = 190). For each of the influenza antigens, the percentage of subjects who achieved seroconversion, defined as a 4-fold increase in HI titer over baseline following vaccination, and the percentage of subjects who achieved HI titers of ≥ 1:40 are shown in Table 1. The lower limit of the 2-sided 95% CI for the percentage of subjects who achieved seroconversion or an HI titer of ≥ 1:40 exceeded the pre- defined lower limits of 40% and 70%, respectively.

No controlled trials demonstrating a decrease in influenza disease after vaccination with FLUARIX have been performed.

Table 1. Rates With HI Titers ≥ 1:40 and Rates of Seroconversion to Each Antigen Following FLUARIX or Placebo (21 Days After Administration of a Dose) in Study FLUARIX-US-001 (ATP cohort)

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