Trials of aqueous killed influenza vaccine in Canada, 1968-69.

The appearance of the pandemic A/Hong Kong/1/68 (H3N2) influenzavirus strain provided an opportunity for a clinical field trial of influenza vaccines in Canada during the winter of 1968-69. As by November 1968 there were reports of influenza B activity and as supplies of A2/HK/68 vaccines were limited, it was decided to make a series of strictly randomized double-blind trials comparing A2/HK/68 vaccines not only with B/Mass/66 vaccines but also with a bivalent vaccine that was already in production and contained B/Mass/66 and A2/Mtl/68, the latter a strain isolated in Canada during January 1968. In 4 trials, a total of 13 729 military personnel and 4 795 primary schoolchildren were vaccinated. Reported vaccine reactions were less than 0.1% with zonally-purified vaccines and 2.6% with the "standard" aqueous killed bivalent vaccine. Three children had serious reactions. Surveillance detected an outbreak of influenza in the first two trials on the military. The 3 vaccines containing A2 strains gave similar clinical protection conservatively estimated at 42-55% but probably about 80%. The effectiveness of the A2/Mtl/68 vaccine, which was in production before the Hong Kong variant had been isolated, was unexpected. In the absence of a vaccine specific to a new pandemic strain, it should not be assumed that a vaccine made from another recent strain could not be useful.