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New York City has announced a coronavirus vaccine mandate for all private employers as it aims to fight the spread of the omicron variant.

Mayor Bill de Blasio (D) on Monday described the measure as the first of its kind nationwide and said he hopes to prevent another wave of infections before the end-of-the-year holidays.

Scientists are racing to learn more about the omicron variant, which they worry could prove more transmissible or more vaccine-resistant than other forms of the virus. The variant has spread to many states since California reported its first case Wednesday, with some cases confirmed among New York City residents.

Here’s what to know

  • Tighter rules for entry into the United States for international travelers went into effect Monday. All incoming travelers will need to show proof of a negative coronavirus test taken within a day of their departure, regardless of vaccination status.
  • Americans’ blood pressure rose during the pandemic — especially among women, according to new research that underscores the ways the coronavirus has exacted an uneven toll.
  • Key updates from around the world: Thailand, Nepal and Russia detected their first cases of omicron, officials said Monday.