New York City officials suggested the most vulnerable wear high-quality masks.
Covid has been spreading unabated in China since the government lifted its policy of strict quarantines and isolation.
The latest versions of the virus appear particularly adept at evading the body's immune response.
U.S. authorities said they want to monitor cases and hospitalizations driven by a new omicron variant.
People are increasingly relying on at-home rapid tests if they decide to test at all.
Studying those who have not yet caught Covid has become just as critical as studying those who have.
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