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To determine rates of seroconversion after single vaccination with a novel split virion, inactivated, adjuvanted pandemic H1N1 influenza vaccine (A/California/7/2009) in HIV-1-infected patients (ClinicalTrials.gov Identifier: NCT01017172).Single center diagnostic study.Institutional HIV outpatient department of an urban university clinic.Adult HIV-1-infected individuals.Serum samples were taken before and 21 days after vaccination.Antibody titers determined by hemagglutination inhibition assay. Seroconversion to vaccination was defined by either an antibody titer of 1: 10 or less before and of at least 1: 40 after or at least 1: 10 before and at least four-fold increase in antibody titer 21 days after single vaccination.One hundred and sixty patients (125 men/35 women) were analyzed. Before vaccination, 23 patients (14.4%) had a hemagglutination inhibition assay titer of at least 1: 40. A median of 22 ± 3 days after vaccination, 110 (69%) patients seroconverted. Seroconverters were younger (45.1 ± 10.0 vs. 48.8 ± 11.3 years; P = 0.04), had a higher CD4 cell count (532 ± 227 vs. 475 ± 281 cells/μl; P = 0.03) and were more likely to have received a previous H5N1 vaccination in 2009 (25 vs. 8%; P = 0.02) when compared to nonresponders. No other significant differences were found comparing the two groups (prevaccination hemagglutination inhibition assay titer of ≥1: 40, AIDS, HAART, HIV RNA PCR <50 copies/ml or CD4 nadir, CD4 and CD8 percentage, sex, BMI, chronic hepatitis B or C).Seroconversion after one dose of a split virion, inactivated, adjuvanted pandemic H1N1 influenza vaccine of HIV-infected patients was 69%. Studies to investigate whether a second dose of the vaccine will increase seroconversion rate are needed.