AP By Eric Levenson and Jessica Firger, CNN Health guidance issued Tuesday recommends that fully vaccinated people wear masks indoors if they’re in areas with “substantial” or “high” transmission of Covid-19. But what do “substantial” and “high” actually mean? The two terms are part of the US Centers for Disease Control and Prevention’s four-tiered system that measures the level of community transmission in each county: low, moderate, substantial and high. On the CDC’s website, you can run a search by state or county and click around the color-coded map to see how active the virus is where you live.