Scientists are growing concerned the U.S. may be headed for a fourth wave. COVID-19 cases are rising rapidly, mirroring an increase in many countries around the world. Harvard epidemiologist Bill Hanage tells NPR he's worried another surge in the U.S. will fuel the spread of the variant known as B.1.1.7. In the meantime, there's new evidence that vaccines from Pfizer and Moderna are effective at preventing viral spread — and that they produce "robust" antibody response in children ages 12-15. NPR's Joe Palca has more. In participating regions, you'll also hear from local journalists about what's happening in your community.Email us at considerthis@npr.org.