Correlated studies of a recombinant influenza-virus vaccine. 3. Protection against experimental influenza in man.

A recombinant (X-31) influenza A2 vaccine (Hong Kong variant) and a standard A2 vaccine (Hong Kong variant) were given to 51 normal adult volunteers. One month later, 29 vaccinated and 14 control volunteers were challenged with an infectious strain of the Hong Kong variant. The other 22 volunteers were given a second vaccination. All vaccinated volunteers developed a rise in neutralizing antibody in serum, and a minimal increase was noted from the second vaccination. Twenty-five of the 29 volunteers who were challenged with live virus showed a significant rise in neuraminidase-inhibiting (NI) antibody from vaccination. A rise in neutralizing antibody in nasal secretions was seen in 14 of 29, but only five subjects developed a rise in NI antibody. For each type of antibody in each location, the recombinant vaccine was slightly more antigenic, a finding compatible with the suggestion that it contained slightly more antigen (556 versus 503 chickcell-agglutinating units). After challenge with live virus, a reduction in frequency of infection and illness was noted in both vaccinated groups when compared to controls. Quantification of virus in specimens of nasal wash revealed an association between quantity of virus shed and clinical illness. Titers of NI antibody in serum correlated best with reduced shedding of virus, but an effect of neutralizing antibody could not be excluded. From the serologic responses to vaccination and the protection afforded against live-virus challenge, it was concluded that the recombinant vaccine was as effective as a standard A2 influenza vaccine.