Worried about coronavirus variants? Here's what you need to know

Concern about new coronavirus variants has grown quickly in recent months.

First, scientists in the United Kingdom spotted a more contagious coronavirus strain that spread like wildfire through the London area. Then, researchers in South Africa spotted one that appears to evade the immune system. Next, another variant was flagged in Brazil because it looked like it could infect people who had already been infected once before.

And now there has been a flurry of reports about homegrown variants in the United States. What's going on? How worried do we really need to be about them?