Comparison of Live, Attenuated H1N1 and H3N2 Cold-Adapted and Avian-Human Influenza A Reassortant Viruses and Inactivated Virus Vaccine in Adults

Abstract

The infectivity, immunogenicity, and efficacy of live,attenuated influenza A/Texas/1/85 (H1N1) and A/Bethesda/1/85 (H3N2) avian-human (ah) and cold-adapted (ca) reassortant vaccines were compared in 252 seronegative adult volunteers. The immunogenicity and efficacy of the H1N1 reassortant vaccine were also compared with those of the trivalent inactivated virus vaccine. Each reassortant vaccine was satisfactorily attenuated. The 50% human infectious dose was 10^4.9 for ca H1N1, 10^5.4 for ah H1N1, 10^6.4 for ca H3N2, and 10^6.5 TCID⁵⁰ for ah H3N2 reassortant virus. Within a subtype, the immunogenicities of ah and ca vaccines were comparable. Five to seven weeks after vaccination, volunteers were challenged with homologous wild-type influenza A virus. The magnitude of shedding of virus after challenge was >100-fold less in H1N1 vaccinees and >10-fold less in H3N2 vaccinees compared with unimmunized controls. The vaccines were equally efficacious, as indicated by an 86%-100% reduction in illness. Thus, the ah A/Mallard/New York/6750/78 and the ca A/Ann Arbor/6/60 reassortant viruses are comparable.