Background:  The objective of this study was to determine seroconversion after a single dose of the novel split virion, inactivated, adjuvanted pandemic H1N1 influenza vaccine (A/California/7/2009) in HIV-1 infected patients (ClinicalTrials.gov Identifier: NCT01017172). Response rates in the general population are reported to be 79% to 100% across all ages.

Methods: Adult HIV-1 infected individuals scheduled for H1N1 vaccination where included. Serum samples where taken before and 21 days after vaccination. Antibody titers were determined by H1N1 hemagglutination inhibition assay (HAI), clinical and HIV related data were extracted from the charts. Seroconversion was defined by an HAI titer ≥1:40 and a minimum 4-fold increase in antibody titer post vaccination.

Results:  To present, 160 patients (125 males/35 females) have received a single vaccination. Before vaccination 23 patients (14.4%) had a HAI titer ≥1:40. At a median of 22 days post vaccination, 110 (69%) patients had seroconverted. Seroconverters were younger (45.1 vs 48.8 years; P =0.04), had a higher CD4 cell count (532 vs 475 cells/µL; P =0.03) and had more often received a previous H5N1 vaccination in 2009 (25 vs 8 %; P =0.009) when compared to non-responders. No other significant differences were found comparing the 2 groups (prevaccination HAI titer ≥1:40, AIDS, HAART, HIV RNA PCR <50 copies/mL, CD4 nadir, CD4 and CD8 percentage, sex, BMI, chronic hepatitis B or C).

Conclusions:  Response rates to a split virion, inactivated, adjuvanted pandemic H1N1 influenza vaccine of HIV infected patients are lower as reported for the general population. Further studies are needed to investigate whether a second dose will increase response rates.