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OBJECTIVE to determine the immunogenicity of the monovalent vaccine against 2009 pandemic influenza A/H1N1 in HIV-1-infected individuals. DESIGN a total of 192 participants, including 44 HIV-1-positive individuals and 148 HIV-1-negative healthy controls were enrolled to receive a single dose of MF59-adjuvanted 2009 A/H1N1v vaccine formulated to contain 7.5 microg of haemagglutin antigen. METHODS standard haemagglutination inhibition (HAI) assay was performed to evaluate seroconversion and seroprotecsion rates against the pandemic virus in serum samples collected at baseline (T0) and 3-5-week postvaccination (T28). Seroconversion to vaccination was defined by either prevaccination HAI titer less than 1: 10 with a postvaccination titer higher than 1: 40, or a prevaccination titer higher than 1: 10 and increase of at least four-fold or more after vaccination. Seroprotection was defined by HAI titers higher than 1: 40. RESULTS the vaccine induced specific antibody titers in HIV-1-positive individuals similar to those of HIV-1-negative controls [215.3, 95% confidence interval (CI) 150.4-308.1 vs. 275.9, 95% CI 232.6-327.3] with postvaccination seroprotection rates higher than 97%. In contrast, the seroconversion rate was lower in the HIV-1-positive individuals as compared with the HIV-1-negative controls (36.4 vs. 79.0%, P < 0.0001), likely as a consequence of their high HAI baseline titers. Multivariable logistic regression analysis showed that seroconversion was less likely in HIV-1-positive individuals [odds ratio (OR) = 0.237, 95% CI 0.104-0.539, P = 0.0006) and with increasing age (OR = 0.805, 95% CI 0.684-0.947, P = 0.009). CONCLUSIONS a single dose of MF59-adjuvanted 2009 influenza H1N1 vaccine induced an immune response against pandemic H1N1 virus in HIV-1-positive individuals reaching titers similar to those of HIV-1-negative individuals. The seroconversion rate was negatively associated with HIV infection and increasing age.