Immunogenicity and safety of two doses of a non-adjuvanted influenza A H1N1/2009 vaccine in young autoimmune rheumatic diseases patients

Objectives The aim of this study was to evaluate the immunogenicity and safety of the influenza A H1N1/2009 vaccine in children under 9 years old with autoimmune rheumatic diseases (ARD). Methods Thirty-eight ARD patients and 11 healthy children received two doses of non-adjuvanted influenza A/California/7/2009 (H1N1) virus-like vaccine. Subjects were evaluated before and 21 days after vaccination. Seroprotection (SP) and seroconversion (SC) rates, geometric mean titers (GMT) and factor increases (FI) in GMT were calculated. Results Mean ages were comparable between patients and controls. Pre-vaccination SP and GMT were similar in patients and controls (p > 0.05). Three weeks after immunization, SP (81.6% vs. 81.8%, p = 1.0), SC (81.6% vs. 90.9%, p = 0.66), GMT (151.5 vs. 282.1, p = 0.26) and the FI in GMT (16.7 vs. 36.3, p = 0.23) were similar in patients and controls, with both groups achieving an adequate response, according to the European Medicines Agency and Food and Drug Administration standards. Analysis of the possible factors influencing SC showed no difference in demographic data, leukocyte/lymphocyte counts or immunosuppressant use between seroconverted and non-seroconverted patients (p > 0.05). The vaccine demonstrated a satisfactory safety profile in this population. Conclusions Two doses of influenza A H1N1/2009 vaccination induced an effective antibody response and caused adverse events in rare instances, suggesting this vaccine is appropriate and can be recommended for this age group.