The pandemic has brought many tricky terms and ideas from epidemiology into everyone’s lives. Two particularly complicated concepts are vaccine efficacy and effectiveness. These are not the same thing. And as time goes on and new variants like omicron emerge, they are changing, too. Melissa Hawkins is an epidemiologist and public health researcher at American University. She explains the way researchers calculate how well a vaccine prevents disease, what influences these numbers and how omicron is changing things.