Longevity of the protective immune response induced after vaccination with one or two doses of AS03A-adjuvanted split H5N1 vaccine in ferrets.

It is crucial that a safe and effective pandemic vaccine be rapidly available to combat a new pandemic threat. In this study we investigated the magnitude and persistence of the protective efficacy induced by one or two doses (3.75 μg HA/dose) of AS03(A)-adjuvanted H5N1 A/Indonesia/5/05 split vaccine in a lethal ferret challenge model. All ferrets that received at least one dose of adjuvanted vaccine 4 weeks before homologous challenge survived and showed reduced or undetectable virus replication in the lungs and the upper airways. Ferrets receiving two doses of adjuvanted vaccine 19 and 16 weeks before the challenge also showed high level of protection from replication in the lungs and the upper airways, albeit with only 83% survival. Animals in the control groups (non-adjuvanted vaccine or saline) and animals immunized with one dose of adjuvanted vaccine administered 10 or 16 weeks before challenge showed only 17-33% survival rate after challenge. In conclusion, our observations support the possibility that a single dose of AS03(A)-adjuvanted H5N1 split vaccine can offer a rapid and short term but partial protection against disease. A second dose of the adjuvanted vaccine, which can be given with a flexible injection schedule, was shown to be essential to induce appreciable levels of antibodies and long-term protection.