Why do vaccinated people need to mask? See Gupta's answer

CNN's Dr. Sanjay Gupta discusses the CDC's updated guidance that recommends fully vaccinated people wear masks indoors in areas with "substantial" and "high" transmission of Covid-19.

source https://www.cnn.com/videos/health/2021/07/27/gupta-cdc-guidance-masks-sot-vpx-lead.cnn