Immunogenicity of influenza A/USSR (H1N1) subunit vaccine in unprimed young adults.

Purified subunit vaccines (HANAflu) containing 20 microgram hemagglutinin of influenza A/USSR/90/77 (H1N1) alone or with 1-5% whole virus were compared to commercially available vaccines for reactogenicity and immunogenicity in unprimed young adults. Reactions to all vaccines were minimal. Sera from volunteers who received two intramuscular doses of vaccine or placebo were tested by hemagglutination-inhibition and neutralization tests. HANAflu with 1 microgram (5%) whole virus added was not different in immunogenicity from commercial vaccine. Both commercial vaccine and HANAflu with 1 microgram whole virus added gave higher seroconversion rates and more neutralization titers greater than or equal to 4 log2 than HANAflu alone and HANAflu + 0.2 microgram (1%) whole virus. Thus, the HANAflu subunit vaccine alone was less immunogenic than commercial vaccine in unprimed persons. However, addition of 1 microgram (5%) whole virus, but not 0.2 microgram (1%), eliminated this difference, There may be a role for addition of small amounts of whole virus to subunit influenza vaccines to overcome lower immunogenicity in unprimed populations.