Here's why the CDC reversed course on masks indoors—and how it might affect you

The change in guidance comes as the highly transmissible Delta variant sweeps a nation that is still struggling to get enough shots in arms.

U.S. public health officials’ guidance on mask-wearing is evolving because the virus behind COVID-19 is evolving—and flexible tactics are needed to fight a pandemic that is clearly far from over.

Less than three months after the Centers for Disease Control and Prevention had said that vaccinated people need not mask indoors or outdoors, based on the high protection conferred by the three COVID-19 vaccines authorized in this country, the CDC updated its guidance, urging everyone regardless of vaccination status to wear masks in schools and in public indoor spaces, in parts of the country with high or substantial coronavirus transmission.

Behind the stark warning were even starker facts: As SARS-CoV-2 has spread around the world and replicated in hundreds of millions of

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