How is Vaccine Efficacy Calculated?

Vaccine Efficacy (VE) was designed and calculated by Greenwood and Yule in 1915 for cholera and typhoid vaccines. VE is a percentage decrease in cases in a vaccinated group compared to an unvaccinated group. Studies on VE helps measure possible outcomes such as disease attack rates, hospitalizations, medical visits, and cost. Vaccine efficacy can be measured using double-blind, randomized, clinical controlled trials. The basic formula for VE is: OR VE = (1 − risk ratio) x 100 The risk ratio is a relative risk of developing the disease in the vaccinated group compared to the unvaccinated group. Let’s calculate the VE from the given data, What would be the VE for this COVID-19 vaccine?

A. 75% B. 85% C. 93% D. 99% Source: CDC, NewYork Times