There's evidence of at least seven U.S. variants of the coronavirus, while another that emerged from the U.K. is poised to become the dominant strain here by the end of March. One adviser from the Food and Drug Administration tells NPR there's a tipping point to watch for: when a fully vaccinated person winds up hospitalized with a coronavirus variant.NPR science correspondent Richard Harris reports on concerns that COVID-19 vaccines themselves could cause the virus to mutate. NPR science reporter Michaeleen Doucleff explains why the story of one COVID-19 patient may hold clues to how variants develop in the first place. For a deeper dive on variants, listen to Michaeleen's recent episode of NPR's Short Wave on Spotify or Apple Podcasts. In participating regions, you'll also hear a local news segment that will help you make sense of what's going on in your community.Email us at considerthis@npr.org.