Studies with inactivated influenza vaccines purified by zonal centrifugation. 2. Efficacy

High (3000 CCA units) and standard low (300 CCA units) doses of the monovalent purified vaccines described in an earlier paper were evaluated in a double-blind manner in 2 adult populations for protective effectiveness against illness caused by the Hong Kong influenza virus. Epidemics in both populations occurred 4-6 weeks after single injections of vaccine were given.The attack rates among recipients of low or high doses of A2/Japan vaccines in a prison were virtually identical to those among the control groups receiving B/Massachusetts vaccines. The attack rate among those receiving 3000 CCA units of Aichi vaccine was nearly 70% less than the rate among recipients of A2/Japan or B/Mass. vaccines. Similarly, in a retirement community the attack rate among the recipients of 3000 CCA units of Aichi vaccine was 50%-70% lower than the attack rate among the recipients of the A2/Japan vaccine. In both populations, recipients of high doses of Aichi vaccine who later became ill tended to have less morbidity, fewer and lower fevers, and shorter stays in bed. Attack rates among the groups receiving low doses of Aichi vaccine were somewhat lower than rates among those receiving A2/Japan or B/Mass. vaccines, but the effect was not statistically significant. Divalent and polyvalent commercial vaccines were also without protective effect.The results indicate that optimally constituted influenza vaccines at standard dosage levels have little, if any, effectiveness and that even very large doses of vaccine do not approach the high degrees of effectiveness that have been achieved with other virus vaccines.