Improved neutralizing antibody response in the second season after a single dose of pandemic (H1N1) 2009 influenza vaccine in HIV-1-positive adults.

Neutralizing antibody titers were determined before and after a single dose of pandemic (H1N1) 2009 influenza vaccine in HIV-1-positive Japanese adults in the first season of the pandemic and in those in the second season who had already received the vaccine in the first season. The antibody response rate at 2-month post-vaccination increased significantly from 49.0% (50/102, 95%CI: 39.0–59.1%) in the 2009/2010 season to 66.7% (42/63, 95%CI: 53.7–78.1%) in the 2010/2011 season. Geometric mean antibody titers (fold dilution) at baseline, at 2 months, and at 4 months also increased significantly from 4.4 (95%CI: 3.3–5.7), 19.0 (95%CI: 13.4–26.8) and 13.7 (95%CI: 9.3–20.2), respectively, in the 2009/2010 season to 8.3 (95%CI: 5.8–11.7), 47.0 (95%CI: 32.2–68.6) and 38.2 (95%CI: 23.8–61.4), respectively, in the 2010/2011 season. Although the vaccine response was low in the first season, it was improved in the second season.