The Delta variant of the coronavirus is threatening to put the world in an entirely new stage of the pandemic.The variant is spreading fast, particularly in places with low vaccination rates — it is thought to be around 50 percent more transmissible than previous versions.What can be done to stop Delta, and how will the variant hamper global efforts to return to normalcy?Guest: Carl Zimmer, a science writer and author of the “Matter” column for The New York Times.Sign up here to get The Daily in your inbox each morning. And for an exclusive look at how the biggest stories on our show come together, subscribe to our newsletter. Background reading: Vaccines are driving down coronavirus case numbers in the U.S., but it’s unclear whether Delta will reverse that trend. Here’s what scientists know about it.Conflicting advice from the health authorities about masks has bewildered a worried public.For more information on today’s episode, visit nytimes.com/thedaily. Transcripts of each episode will be made available by the next workday.