Work out vaccine efficacy instantly with clear inputs, formula shown and shareable results.
Compares attack rates between vaccinated and unvaccinated groups to give efficacy, and converts the absolute difference into a number needed to vaccinate.
Vaccine efficacy
VE (%) = (attack rate unvaccinated - attack rate vaccinated) / attack rate unvaccinated x 100 = (1 - relative risk) x 100
Efficacy comes from trial conditions; effectiveness is measured in routine use where storage, timing and uptake vary.
High efficacy against a rare outcome can still mean many people must be vaccinated to prevent one case.