Vaccine efficacy (VE) refers to the percentage reduction of disease in a vaccinated group compared to an unvaccinated group under optimal conditions. It is a measure of how well a vaccine works in controlled clinical trials. The formula to calculate VE is:
\[ VE = \left(1 - \frac{AR_V}{AR_U}\right) \times 100 \]
where ARV is the attack rate in the vaccinated group and ARU is the attack rate in the unvaccinated group.
