Improved serological response to H1N1 monovalent vaccine associated with viral suppression among HIV-1-infected patients during the 2009 influenza (H1N1) pandemic in the Southern Hemisphere.

OBJECTIVES

Patients infected with HIV-1 were targeted for vaccination against H1N1 influenza because of their anticipated increased risk of mortality associated with H1N1 infection. Reports regarding the efficacy of vaccination in HIV-1-infected patients have suggested a reduced immunogenic response compared with the general population. Hence, the study aimed to determine the serological response to pandemic H1N1 influenza vaccine in HIV-1-infected patients in a clinical setting.

METHODS

A retrospective review of all HIV-1-infected patients who attended mass H1N1 vaccination between October 2009 and March 2010 at an Australian HIV clinic was carried out. Pre- and post-vaccination H1N1 antibody titres were measured. The main outcome measure was response to the vaccination, which was defined as an H1N1 antibody titre of ≥ 1:40 using a haemagglutination inhibition (HI) assay.

RESULTS

Baseline blood samples were collected from 199 patients, of whom 154 agreed to receive vaccination; of these, 126 had pre- and post-vaccination HI titres measured. Seventy-seven of 199 patients (38.7%) showed a baseline antibody titre of ≥ 1:40. Eighty-five (67.4%) showed a fourfold or greater increase in titre and 109 of 126 (86.5%) achieved an antibody titre of ≥ 1:40 after vaccination. The serum HI H1N1 antibody geometric mean titre (GMT) for the 126 paired samples was 39.32 ± 3.46 pre-vaccination and increased to 237.36 ± 3.94 [standard deviation (SD)] post-vaccination (P<0.001). In a binary logistic regression analysis, HIV viral load and baseline HI antibody titre were significantly associated with post-vaccination increase in HI H1N1 antibody titre.

CONCLUSIONS

A high prevalence of HI H1N1 antibodies was found before vaccination in the cohort, consistent with previous exposure to H1N1 influenza virus. The response to vaccination was considered adequate, as more than two-thirds of patients achieved a fourfold or more increase in antibody titre after vaccination. The response to vaccination was significantly greater in those patients who were aviraemic for HIV, suggesting that antiretroviral therapy improves the humoral response, which is important in optimizing vaccine effectiveness.