Herd immunity appears unlikely for COVID-19, but CDC says vaccinated people can ditch masks in most settings When COVID-19 first began spreading, public health and medical experts began talking about the need for the U.S. to reach herd immunity to stop the coronavirus from spreading. Author: Noam Galai/Getty Images When COVID-19 first began spreading, public health and medical experts began talking about the need for the U.S. to reach herd immunity to stop the coronavirus from spreading. When COVID-19 first began spreading, public health and medical experts began talking about the need for the U.S. to reach herd immunity to stop the coronavirus from spreading. Experts have estimated that between 60% and 90% of people in the U.S. would need to be vaccinated for that to happen. Only about 35% of the population has been fully vaccinated, and yet the CDC said on May 14, 2021 that fully vaccinated people can lose their masks in most indoor and outdoor settings.