Amid the COVID-19 pandemic and the development of new vaccines, we hear the term “vaccine efficacy” more than ever. But, this raises some questions: what do these efficacy numbers exactly mean? How do the researchers calculate these numbers? And, how should these numbers be interpreted?

Vaccine candidates undergo clinical trials to determine vaccine efficacy, and researchers can calculate the efficacy by considering the difference in new cases of the disease between the group receiving a placebo and the other receiving the vaccine candidate. This video explains what happens through clinical trials and how vaccine efficacy is calculated from this journey.