Immunogenicity and Safety of a Trivalent Inactivated Influenza Vaccine in Children 6 Months to 17 Years of Age, Previously Vaccinated with an AS03-Adjuvanted A(H1N1)Pdm09 Vaccine: Two Open-label, Randomized Trials.

Background: During the influenza pandemic 2009-2010, an AS03-adjuvanted A(H1N1)pdm09 vaccine was used extensively in children 6 months of age and older, and during the 2010-2011 influenza season, the A(H1N1)pdm09 strain was included in the seasonal trivalent inactivated influenza vaccine (TIV) without adjuvant. We evaluated the immunogenicity and safety of TIV in children previously vaccinated with the AS03-adjuvanted A(H1N1)pdm09 vaccine.

Methods: Healthy children were randomized (1:1) to receive TIV or a control vaccine. Children were aged 6 months to 9 years (n = 154) and adolescents 10-17 years (n = 77) when they received AS03-adjuvanted A(H1N1)pdm09 vaccine at least 6 months before study enrolment. Hemagglutination inhibition (HI) and neutralizing antibody responses against the A(H1N1)pdm09 strain were evaluated before (day 0) and at day 28 and month 6 after study vaccination. Reactogenicity was assessed during the 7 day postvaccination period, and safety was assessed for 6 months.

Results: At day 0, >93.9% of all children had HI titers ≥1:40 for the A(H1N1)pdm09 strain, which increased to 100% at both day 28 and month 6 in the TIV group. Between days 0 and 28, HI antibody geometric mean titers against A(H1N1)pdm09 increased by 9-fold and 4-fold in children 6 months to 9 years of age and 10-17 years of age, respectively.

Conclusion: AS03-adjuvanted A(H1N1)pdm09 vaccine-induced robust immune responses in children that persisted into the next season, yet were still boosted by TIV containing A(H1N1)pdm09. The reactogenicity and safety profile of TIV did not appear compromised by prior receipt of AS03-adjuvanted A(H1N1)pdm09 vaccine.