The efficacy of a vaccine to protect against a target disease can be calculated by comparing attack rates for the disease between vaccinated and unvaccinated persons.
Parameters:
(1) number of people unvaccinated
(2) number of unvaccinated people who develop target disease
(3) number of people vaccinated
(4) number of vaccinated people who develop target disease
percent vaccination =
= (number of people vaccinated) / ((number of people vaccinated) + (number of people unvaccinated)) * 100
attack rate in unvaccinated persons =
= (number of unvaccinated people who develop target disease) / (number of people unvaccinated)
attack rate in vaccinated persons =
= (number of vaccinated people who develop target disease) / (number of people vaccinated)
vaccine efficacy =
= ((attack rate in unvaccinated persons) – (attack rate in vaccinated persons)) / (attack rate in unvaccinated persons) * 100
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