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2009 H1N1 Vaccine: Will One Dose Be Enough?

A single 15-µg dose of 2009 H1N1 vaccine, without an adjuvant, produced strong immune responses in 97% of healthy adults.

A safe, effective vaccine against the 2009 pandemic influenza A (H1N1) virus is badly needed. Given that most people do not have cross-reactive antibodies against this new virus (JW Infect Dis Sep 16 2009), experts have speculated that vaccine-induced immunity would require higher doses, a two-dose series, or an adjuvanted vaccine. Now, in an industry-sponsored trial involving 240 healthy adults in Australia (half aged 18–49, and half 50–64), researchers have compared the safety and immunogenicity of two different doses of 2009 H1N1 hemagglutinin antigen.

Participants were randomized to receive either 15 or 30 µg of hemagglutinin antigen intramuscularly; antibody titers were determined at baseline and 21 days postvaccination. Antibody titers 1:40 on the hemagglutinin-inhibition assay were considered seroprotective.

At baseline, 31.7% of all participants had antibody titers 1:40. At 3 weeks, such titers were observed in 96.7% (95% confidence interval, 91.7–98.7) and 93.3% (95% CI, 87.4–96.6) of 15-µg and 30-µg dose recipients, respectively. Among participants with no detectable antibodies at baseline, a single 15-µg dose elicited 21-day titers 1:40 in 92.5% (95% CI, 80.1–97.4). Responses among persons aged 50 were slightly decreased but still strong: 93.5% (vs. 100% of individuals aged 18–49) had titers 1:40 after the 15-µg dose. Side effects were comparable to those seen with seasonal influenza vaccines, with no deaths or serious adverse events reported.

Comment: These promising preliminary results suggest that a standard dose of 2009 H1N1 vaccine, without an adjuvant, is safe and immunogenic in healthy adults. The need for only a single 15-µg dose will allow more people to be vaccinated more easily during the early stages of the vaccine campaign, when supplies are likely to be limited. As suggested by an editorialist, these findings may not apply to young children or to adults with immune suppression or high-risk medical conditions, so additional data for these groups are needed.

— Daniel J. Diekema, MD, MS

Published in Journal Watch Infectious Diseases September 16, 2009

Citation(s):

Greenberg ME et al. Response after one dose of a monovalent influenza A (H1N1) 2009 vaccine — Preliminary report. N Engl J Med 2009 Sep 10; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0907413)

Neuzil KM. Pandemic influenza vaccine policy — Considering the early evidence. N Engl J Med 2009 Sep 10; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMe0908224)

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