We conducted a multi-center, randomized and laboratory-blinded clinical trial with subgroup analyses, involving adults aged greater than 60 years old (range 61-86 years old), to investigate the immunogenicity and the potential factors affecting the immune response of a monovalent, unadjuvanted, inactivated, split-virus vaccine. A total of 107 subjects were randomized to receive 15 and 30 microg of hemagglutinin antigen in a 1:1 ratio. The immunogenicity was detected through hemagglutination inhibition (HAI) test of serum obtained before and 3 weeks after vaccination. By 3 weeks after vaccination, HAI titer >or=1:40 was observed in 75.5% and 81.1% of participants receiving 15 and 30 microg of hemagglutinin antigen, respectively. Positive seroconversion was observed in 71.7% and 81.1% of recipients of the 15 and the 30 microg, respectively. The GMTs increased by a factor of 10.7 and 17.4 in the groups of 15 and 30 microg, respectively. This study indicated that one dose of 15 microg hemagglutinin antigen without adjuvant induced protective immune response in the majority of elderly. Multivariate logistic regression analyses showed that gender, age and diabetes were statistically significant factors affecting the seroprotection rate (p=0.04, 0.01 and 0.01, respectively) and seroconversion rate (p=0.01, 0.01 and 0.01, respectively).
Trial registration: ClinicalTrials.gov NCT01003145.
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