Hi and welcome to COVID Quickly, a scientific American podcast series.

This is your fast track update on the COVID pandemic.

We bring you up to speed on the science behind most urgent questions about the virus and

the disease.

We demystify the research and help you understand what it really means.

I'm Tanya Lewis.

I'm Josh Fishman.

We're scientific American senior health editors.

Today we'll explain why the CDC now wants vaccinated people to wear masks indoors again.

And we'll discuss one big reason why some people still refuse to get vaccinated.

Just a few months ago, the CDC said vaccinated people didn't have to wear masks in most settings,

but this week the agency reversed course saying vaccinated people should wear masks at times.

Back to the recommendations actually say, Tanya, and why the sudden shift in position.

So in its latest guidance, the CDC says that vaccinated people should once again wear masks

in public indoor settings under certain circumstances.

For example, if you live in an area with high levels of COVID transmission, which includes

people who live in places with more than 50 cases per 100,000 people in the last week.

The agency also recommended universal masking in all K through 12 schools, including all students,

teachers, and staff.

The CDC says the reason for the shift is new data about the Delta variant and breakthrough infections.

Delta is known to be much more transmissible than previous strains and appears to produce

about a thousand times as much virus in the body as the original strain.

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