The A/Mallard/6750/78 Avian-Human, but Not the A/Ann Arbor/6/60 Cold-Adapted, Influenza A/Kawasaki/86 (H1N1) Reassortant Virus Vaccine Retains Partial Virulence for Infants and Children

Characteristics of avian-human (ah) and cold-adapted (ca) influenza A/Kawasaki/9/86 (H1N1) reassortant vaccine viruses were compared in 37 seronegative adults and 122 seronegative infants and children. The 50% human infectious dose ($HID_{50}$) in infants and children was $10^{2.9}$ and $10^{2.6}$ $TCID_{50}$) for the ah and ca vaccine, respectively. The ah influenza A/Kawasaki/9/86 reassortant was reactogenic: 24% of infants and children infected with ≥100 $HID_{50}$ had fever ≥39.4°C. Since H3N2 ah vaccines were previously shown to be adequately attenuated, it is reasonable to suggest that the genes that code for hemagglutinin and neuraminidase of the H1N1 virus apparently influence the reactogenicity of reassortant viruses derived from the avian influenza A/Mallard/New York/6750/78 donor virus. Because this avian virus does not reproducibly confer a satisfactory level of attenuation to each subtype of influenza A virus, it is not a suitable donor virus for attenuation of wild-type influenza viruses. In contrast, the ca A/Ann Arbor/6/60 donor virus reliably confers attenuation characteristics to a variety of H1N1 and H3N2 influenza A viruses.