With new CDC guideline, Coloradans in 38 counties should wear masks in public indoor spaces COLORADO (KRDO) -- The majority of Colorado is included in a new mask recommendation made by the US Centers for Disease Control and Prevention. Tuesday, the CDC recommended vaccinated people should wear masks while in indoor public places in counties with "High" or "Substantial" spread of the virus. The CDC defines substantial transmission as 50 or more cases per 100,000 people over a one-week period. According to CDC data, more than 60 percent of US counties have high or substantial transmission of COVID-19. In Colorado, 38 out of the 64 counties fall under high or substantial, that includes El Paso County, Pueblo County, and Teller County.