Does vaccine dose predict response to the monovalent pandemic H1N1 influenza a vaccine in children with acute lymphoblastic leukemia? A single-centre study.

Background: Vaccination against influenza is an important strategy in preventing severe infection among children with acute lymphoblastic leukemia (ALL). Successful vaccination depends on both vaccine and host-related factors. We conducted a study on factors predicting the immunogenicity of the monovalent pandemic H1N1 (pH1N1) influenza A vaccine in children with ALL.

Methods: Children with ALL in our hospital were recruited and received two doses of the inactivated split-virion AS03-adjuvanted vaccine. The serological response was measured before each vaccine dose (Day 0 and 28) and 3 months after the second dose. Antibody titres were measured using a hemagglutination-inhibition assay. Seroconversion was defined as a ≥fourfold increase in antibody titre and a post-vaccination titre ≥1:40.

Results: Pre and post-vaccination titres were available from 45 children with ALL after one dose of the vaccine and 39 children after two doses. The seroconversion rate was 11.1% after one dose and 25.6% after the second dose. Univariate analysis demonstrated a significantly higher (P = 0.01) seroconversion rate among children who received the adult dose (0.5 ml) of the vaccine and a trend towards increased seroconversion (P = 0.07) by multivariate analysis. Factors including age, gender, lymphocyte count, treatment phase and regimen did not significantly affect the seroconversion rate. Children who received the adult dose demonstrated a significantly greater magnitude of serological response after both one dose (P = 0.04) and two doses (P = 0.001).

Conclusions: These data suggest that the immunogenicity of the pH1N1 vaccine among children with ALL is improved by repeated and adult doses of the vaccine.