(Encore) The Pfizer-BioNTech and Moderna COVID-19 vaccines are the first authorized vaccines in history to use mRNA technology. In light of the authorization for some children and teens now, we are encoring the episode in which Maddie Sofia chats with Dr. Margaret Liu, a physician and board chair of the International Society for Vaccines, about the history and science behind these groundbreaking vaccines. We'll also ask what we can expect from mRNA vaccines in the future. Have a question for us? Send a note to shortwave@npr.org — we'd love to hear it.