The attack rate [AR = (number of cases)/(popultion at risk) × 100] was first determined for both populations, and from those data the relative risk (RR = the ratio of the proportion of the treated cases to the proportion of the nontreated cases) was then calculated as the ratio of those two attack rates. Finally, the percent vaccine efficacy was computed by the following equation:(1-[ARvaccinated/ARunvaccinated])×100=(1-RR)×100