Mechanism of Action

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 titers post-vaccination with inactivated influenza virus vaccine have not been correlated with protection from influenza illness. In some human studies, antibody titer of ≥ 1:40 have been associated with protection from influenza illness in up to 50% of subjects [see REFERENCES].

Antibody against one influenza virus type or subtype confers limited or no protection against another. 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 virologic basis for seasonal epidemics and the reason for the usual change of one or more new strains in each year's influenza vaccine. Therefore, inactivated influenza vaccines are standardized to contain the hemagglutinin of strains (i.e., typically two type A and one type B), representing the influenza viruses likely to be circulating in the United States in the upcoming winter.

Annual revaccination with the current vaccine is recommended because immunity declines during the year after vaccination, and because circulating strains of influenza virus change from year to year [see REFERENCES].

Clinical Studies

Between 1982 and 1991, twelve clinical studies were conducted in healthy adult and geriatric subjects and one in children between 4 and 12 years of age who were considered to be 'at risk'. Since 1991 an annual clinical study has been conducted in the UK in healthy adults aged 18 years or older. FLUVIRIN® was also used as a control in a US clinical trial in adults (18-49 years of age). In all the trials, blood samples were taken prior to vaccination and approximately three weeks after vaccination to assess the immunogenic response to vaccination by measurement of anti-HA antibodies.

Three clinical studies were carried out between 1995 and 2004 in a total of 520 pediatric subjects (age range 6-47 months). Of these, 285 healthy subjects plus 41 'at risk' pediatric subjects received FLUVIRIN®.

FLUVIRIN® should only be used for the immunization of persons aged 4 years and over.

Immunogenicity in Adults (18 to 64 years of age)

Tables 4 and 5 show the immunogenicity data for the adult age group. The seven clinical studies presented enrolled a total of 774 adult subjects. In the adult group, for all antigens (A/H1N1, A/H3N2 and B) at least one of the following point estimate criteria was met: the proportion of subjects with seroconversion (post-vaccination titer ≥ 1:40 from a pre-vaccination titer < 1:10) or significant increase (at least a four-fold increase from pre-vaccination titer ≥ 1:10) in antibody titer was greater than 40%; the geometric mean titer (GMT) increase was > 2.5; the proportion of subjects with a post-vaccination hemagglutination inhibition (HI) antibody titer ≥ 1:40 was greater than 70%.

TABLE 4 : Summary of the Seroconversion and Proportion of Subjects Achieving an HI titer ≥ 1:40 for Adult Subjects

TABLE 5 : Summary of the Geometric Mean Hemagglutination Inhibition Antibody Titers, Pre- and Post-Immunization, for Adult Subjects

Immunogenicity in Geriatric Subjects (65 years of age and older)

Tables 6 and 7 show the immunogenicity of FLUVIRIN® in the geriatric age group. The six clinical studies presented enrolled a total of 296 geriatric subjects. For each of the influenza antigens, the percentage of subjects who achieved seroconversion and the percentage of subjects who achieved HI titers of ≥ 1:40 are shown, as well as the fold increase in GMT.

For all antigens (A/H1N1, A/H3N2 and B) at least one of the following point estimate criteria was met: the proportion of subjects with seroconversion (post-vaccination titer ≥ 1:40 from a pre-vaccination titer < 1:10) or significant increase (at least a four-fold increase from pre-vaccination titer ≥ 1:10) in antibody titer was greater than 30%; the geometric mean titer (GMT) increase was > 2.0; the proportion of subjects with a post-vaccination hemagglutination inhibition (HI) antibody titer ≥ 1:40 was greater than 60%. The pre-specified efficacy criteria were met in each study, although a relatively lower immunogenicity of A/H1N1 strain was seen in the last four studies (the same strain was in each of the formulations).

TABLE 6 : Summary of the Seroconversion and Proportion of Subjects Achieving an HI titer ≥ 1:40 for Geriatric Subjects

TABLE 7 : Summary of the Geometric Mean Hemagglutination Inhibition Antibody Titers, Pre- and Post-Immunization, for Geriatric Subjects

Immunogenicity in Pediatric Subjects

A small-scale study, was conducted in 1987 to evaluate safety and immunogenicity of FLUVIRIN® in 38 'at risk' children, with diabetes and/or asthma, or lymphoid leukemia. Thirty-eight participants aged between 4 and 12 years of age were assessed. Ten subjects had diabetes, 21 had asthma, two had both diabetes and asthma, and one had lymphoid leukemia. There were four healthy control subjects. All participants received a single 0.5-mL dose of FLUVIRIN®.

Immunogenicity results were obtained for 19 of the 38 subjects enrolled in the study. The point estimate of the percentage of subjects achieving a titer of ≥ 1:40 was 84% for the A/H1N1 strain 79% for the B strain, and 53% for the A/H3N2 strain. The GMT fold increases were 5.8 for the A/H1N1 strain, 40 for the B strain and 17.7 for the A/H3N2 strain.

Three clinical studies were carried out between 1995 and 2004 in a total of 520 pediatric subjects (age range 6-47 months). Of these, 285 healthy subjects plus 41 'at risk' pediatric subjects, received FLUVIRIN®.

In a 1995/1996 clinical study, 41 subjects (aged 6-36 months) at increased risk for influenza-related complications received two 0.25-mL doses of FLUVIRIN®. At least 49% of subjects showed a ≥ 4-fold increase in HI antibody titer to all three strains. HI antibody titers of 1:40 or greater were seen in at least 71% of the subjects for all three influenza strains, with increases in geometric mean titer of 6.0-fold or greater to all three strains.

Two clinical studies (1999-2000 and 2004) indicated a lower immunogenicity profile for FLUVIRIN® compared with two commercial split vaccines; in a study in the age group 6-47 months the comparator was a US licensed vaccine, Fluzone®, and in another study in the age group 6-36 months the comparator was a non-US licensed inactivated influenza vaccine. Despite the small sample size (a total of 285 healthy subjects received FLUVIRIN® in these two clinical studies) the lower immunogenicity profile of FLUVIRIN® was greatest versus the comparator vaccines in children < 36months but was also evident in those 36-47 months of age, though the differences were less.

FLUVIRIN® should only be used for the immunization of persons aged 4 years and over.

REFERENCES

1 Hannoun C, Megas F, Piercy J. Immunogenicity and protective efficacy of influenza vaccination. Virus Res 2004; 103:133-138.

2 Hobson D, Curry RL, Beare A, et. al. The role of serum hemagglutinin-inhibiting antibody in protection against challenge infection with influenza A2 and B viruses. J Hyg Camb 1972; 767-777.

3 Centers for Disease Control and Prevention. Prevention and Control of Influenza with Vaccines. Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2010; 59(RR-8):1-62.

Last reviewed on RxList: 1/10/2012
This monograph has been modified to include the generic and brand name in many instances.