An Australian group has reported results of a clinical trial to determine the safety and immunogenicity of a 2009 pandemic H1N1 influenza virus vaccine. Twenty-one days after the first of two scheduled doses, the vaccine proved to be immunogenic in adults. This is good news, as many had believed that two doses of the vaccine would be needed to achieve adequate protective immune responses.
A total of 240 individuals were enrolled in the trial and given 15 or 30 microgram doses of an inactivated, H1N1 vaccine propagated in eggs by CSL Biotherapies, Parkville, Australia. Anti-influenza antibody titers were measured at enrollment and 21 days after each vaccination, by hemaggultination-inhibition (HI) and microneutralization assays.
A single 15-µg or 30-µg dose of the H1N1 vaccine produced a robust immune response in most subjects. Day 21 titers of 1:40 or more on HI assay were observed in 96.7% of those who received the 15 µg dose and in 93.3% of those who received the 30 µg dose. In total, 74.2% of subjects showed a significant seroconversion or increase in HI titer. Protection of humans against seasonal influenza is generally believed to require a HI titer of 1:40 or more.
Some of the side effects of immunization included injection-site tenderness, pain, swelling, and headache.
Whether similar results will be obtained in children, and using vaccine manufactured by other companies remains to be determined.
The ability of one dose of adjuvant-free H1N1 vaccine to induce a strong immune response has two important benefits. One is that twice as many individuals can be immunized than if two doses were required. And a return visit for a second dose of pandemic H1N1 vaccine won’t be needed.
Michael E. Greenberg, M.D., M.P.H., Michael H. Lai, B.Med.Sc., M.B., B.S., M.Med.Sc., Gunter F. Hartel, M.S., Ph.D., Christine H. Wichems, Ph.D., Charmaine Gittleson, B.Sc., M.B., B.Ch., Jillian Bennet, M.Sc., M.P.H., Gail Dawson, B.Pharm., Wilson Hu, M.D (2009). Response after One Dose of a Monovalent Influenza A (H1N1) 2009 Vaccine New England Journal of Medicine : 10.1056/NEJMoa0907413