Efficacy of “Hong Kong” Vaccine in Preventing “England” Variant Influenza a in 1972

The antigenic drift that occurs among influenza A viruses may impair the effectiveness of available vaccines containing older antigens. We therefore studied the efficacy of such a vaccine containing Hong Kong virus antigen (A2/Aichi/2/68) during an outbreak of A2/England/72 influenza in a population of 979 vaccinated and 2955 unvaccinated men. Attack rates for laboratory-confirmed febrile influenza were 18.4 and 46.0 per 1000, respectively. One dose of the vaccine thus reduced the frequency of influenza in those vaccinated by 60 per cent (p<0.01). Of 171 men studied before the outbreak, 84 per cent had hemagglutination-inhibition titers of ≤1:8 against the epidemic strain, but only 22 per cent had such low titers two weeks after vaccination. A significantly greater attack rate was found in men with acute-phase hemagglutination-inhibition titers of ≤1:8 than among those with titers of ≥1:16. There was a direct relation between the clinical effectiveness of the vaccine and its ability to produce ...