Poor immunogenicity of the H1N1 2009 vaccine in well controlled HIV-infected individuals.

Objective: To evaluate the safety and immunogenicity of the H1N1 2009 vaccine in HIV-positive individuals.

Design: A single-arm study.

Setting: Clinic at the Hospital of the University of Pennsylvania, Philadelphia, Pennsylvania, USA.

Participants: HIV-infected adults with an indication for H1N1 vaccination.

Intervention: Single intramuscular 15 microg dose of the monovalent, unadjuvanted, inactivated, split virus H1N1 vaccine.

Main Outcomes: Immunogenicity, safety and tolerability.

Results: A total of 120 participants were enrolled, 71% men, 68% African-American, with median age of 46 years. All of them but one were on antiretroviral treatment, with a median current CD4 cell counts of 502 cells/microl, and a nadir CD4 cell counts of 132 cells/microl. The HIV RNA level was below 400 copies/ml in 92% of participants. All participants completed the 3 weeks of follow-up. Thirty of the 120 (25%) participants had antibody hemagglutination-inhibition assay titers equal or greater than 1: 40 at baseline. Among participants without evidence of previous exposure, only 61% develop protective titers by week 3 of the study. Nonresponders had lower current and nadir CD4 cell counts than responders. Only four of nine participants with detectable HIV viral load at baseline developed protective antibody titers. Age and race were not predictors of the response to the vaccine. The vaccine was well tolerated.

Conclusion: These results suggest that only 60% of well controlled HIV-infected individuals without preexisting immunity to H1N1 develop protective antibody titers after immunization. Alternative vaccines, dosing, adjuvants or schedule strategies are needed to achieve effective immunization of this vulnerable population.