Immune responses of healthy subjects to a single dose of intramuscular inactivated influenza A/Vietnam/1203/2004 (H5N1) vaccine after priming with an antigenic variant

Background. We administered a single dose of influenza A/Vietnam/1203/2004 (H5N1, clade 1) vaccine to subjects who had received 2 doses of influenza A/Hong Kong/156/1997 (H5N1, clade 0) vaccine in 1998. Methods. Thirty-seven subjects previously vaccinated with a baculovirus-expressed recombinant hemagglutinin A/Hong Kong/156/1997 vaccine in 1998 received a single intramuscular dose of 90 mu g of inactivated subvirion A/Vietnam/1203/2004 vaccine in 2006. Serum antibody was measured before vaccination and 28 and 56 days after vaccination. Antibody responses were compared with those measured after one or two 90-mu g doses in H5-naive subjects. Results. On day 28 after a single dose, the geometric mean titer ( GMT) of hemagglutination-inhibition antibody in primed subjects was 64.0 (95% confidence interval [Cl], 37.8-108.5), with 68% responding (4-fold increase in antibody level to a titer of >= 1: 40). In contrast, H5-naive subjects who received two 90-mu g doses had a day 56 ( 28 days after the second dose) GMT of 27.7 ( 95% CI, 20.3-38.0), with only 43% responding. Conclusions. This study suggests that priming can result in immune responses to a single dose of an antigenically variant strain of H5N1 influenza virus and could be a useful strategy for pandemic control. Trial registration. ClinicalTrials. gov identifier: NCT00240903.