This week, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said new data could allow coronavirus delta variants to be transmitted from vaccinated individuals to others in rare breakthrough infections. He said it shows that he has sex. The CDC has revised its guidance, stating that everyone should wear a mask in areas with “substantial and high” levels of delta infection. With this revised guidance, how can people stay safe in social situations and fully vaccinated people need to worry? Dr. Lina Wen, an emergency physician and author of the new book Lifeline: A Doctor’s Journey in the Battle for Public Health, explains what Americans can do.