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H5N1 vaccines produced to date have been poorly immunogenic at low antigen doses, raising concerns about production capacity if widespread infection occurs. Could adding an adjuvant spare antigen and also produce cross-protection against drifted H5N1 viruses? In a manufacturer-sponsored study, investigators tested the safety and immunogenicity of a split-virion H5N1 vaccine in 400 adults. Participants were randomized to receive two doses of vaccine at one of four antigen doses (3.8 μg, 7.5 μg, 15 μg, or 30 μg hemagglutinin), given with or without an oil-in-water–based emulsion adjuvant.
After the second vaccine dose, given 21 days after the first, 84% of participants receiving the lowest antigen dose plus adjuvant but only 4% of those receiv…