Antibody persistence after 2-dose priming and booster response to a third dose of an inactivated, adjuvanted, whole-virion H5N1 vaccine.

An inactivated, alum-adjuvanted, whole-virion H5N1 vaccine had been evaluated previously. Hemagglutination inhibition (HI) assays showed that the antibody levels declined significantly, with 4.8%-20.8% and 0%-18.8% of participants retaining seroprotection (HI titer >or=1:40) 6 and 12 months after the second dose, respectively. A third dose of the same vaccine given 12 months after the second dose significantly boosted immune responses. Thirty days after the third dose in the 1.25-, 2.5-, 5-, and 10-microg dose groups, 29.4%, 31.3%, 78.6%, and 90.0% of participants had HI titers >or=1:40, and 52.9%, 81.2%, 92.9%, and 100% of participants had microneutralization titers >or=1:40, respectively. Both the 5-microg and 10-microg doses met European Union criteria.