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Health care workers across the United States were among the first Americans to get the federally approved coronavirus vaccine, marking a critical moment in the fight against Covid-19.

Dr. Anthony Fauci, the nation's top disease expert, predicted that after vaccines are more widely deployed the country could achieve herd immunity against Covid-19 by "the end of the second quarter 2021."

The beginning of this new phase in fighting the virus came as the U.S. death toll passed 300,000, and recorded more than 200,000 infections in one day.

• Map of U.S. hot spots and worldwide Covid-19 cases.
• Tracking surges in states across the country this winter.
• Map of travel restrictions and which states have a mask mandate.
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