Efficacy is typically measured during Phase III clinical trials. These trials involve thousands of participants who are randomly assigned to receive either the vaccine or a placebo. The incidence of the disease in both groups is then monitored over time. The formula used to calculate efficacy is:
\[ \text{Efficacy} = \left( \frac{\text{Attack Rate in Unvaccinated Group} - \text{Attack Rate in Vaccinated Group}}{\text{Attack Rate in Unvaccinated Group}} \right) \times 100 \]
