Durability of antibody responses after receipt of the monovalent 2009 pandemic influenza A (H1N1) vaccine among HIV-infected and HIV-uninfected adults

Background: Human immunodeficiency virus (HIV)-infected persons are at risk for severe influenza infections. Although vaccination against the H1N1 pandemic influenza strain is recommended, currently there are no data on the durability of post-vaccination antibody responses in this population.

Methods: HIV-infected and HIV-uninfected adults (18-50 years old) received a single dose of monovalent 2009 influenza A (H1N1) vaccine (strain A/California/7/2009H1N1). Antibody levels to the 2009 H1N1 pandemic strain were determined at day 0, day 28, and 6 months by hemagglutination-inhibition assay. A seroprotective response was a post-vaccination titer of ≥1:40 among those with a pre-vaccination level of ≤1:10. Geometric mean titers (GMT) and factors associated with higher levels were also evaluated.

Results: We studied 127 participants with a median age of 35 (interquartile range (IQR) 28, 42) years. Among the HIV-infected arm (n=63), the median CD4 count was 595 (IQR 476, 819)cells/mm(3) and 83% were receiving HAART. Thirty-five percent of all participants had a pre-vaccination level of >1:10. HIV-infected compared to HIV-uninfected adults were less likely to generate a seroprotective response at day 28 (54% vs. 75%, adjusted OR 0.23, p=0.021) or have a durable response at 6 months post-vaccination (28% vs. 56%, adjusted OR 0.19, p=0.005). Additionally, although pre-vaccination GMT were similar in both arms (median 7 vs. 8, p=0.11), the GMT at 6 months was significantly lower among HIV-infected versus HIV-uninfected adults (median 20 vs. 113, p=0.003). Among HIV-infected persons, younger age (p=0.035) and receipt of HAART (p=0.028) were associated with higher GMTs at 6 months.

Conclusions: Despite vaccination, most HIV-infected adults do not generate durable seroprotective antibody responses to the 2009 influenza A (H1N1) virus, and hence may remain vulnerable to infection. In addition to HAART use, more immunogenic vaccines are likely needed for improving protection against influenza in this population.