Video by Xueying Chang, Kaz Fantone and Ben de la Cruz/NPR
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When will the pandemic end? How many more COVID-19 waves will the U.S. goes through? Will we go back to “normal” in the fall? All these questions depend largely a one key factor: how the variants behave.

As the virus spreads around the world, it mutates — which all viruses do. Along the way, variant strains emerge. Some of these variants are more effective at infecting humans and spread more quickly. Some variants may be better at evading the immune system and are more likely to cause a second bout of COVID-19.

Variants with these properties have caused massive waves of COVID in Brazil, South Africa, the United Kingdom — and now India. And in our interconnected world, they are already spreading to many other countries.

How does a mutated version of the virus improve its chances of being transmitted to humans? How does avoid recognition by the immune system? If you imagine viruses as puzzle pieces, as this video does, that can help explain what’s happening when a coronavirus variant comes into contact with human cells.