The fast-spreading Omicron variant of the coronavirus has become dominant in the United States with lightning speed, and claimed the life on Monday of an unvaccinated man in Texas, officials said.
The spike in Covid-19 cases is alarming public health officials, who fear an explosion of infections after the Christmas and New Year's holidays.
The Omicron variant now accounts for 73 percent of US coronavirus infections based on sequencing data for the week ended on Saturday, the US Centres for Disease Control and Prevention said on Monday.
With the new variant in circulation, Covid-19 cases are now doubling in 1-1/2 to three days in areas with community transmission, the World Health Organization said on Saturday.
Lines for Covid-19 tests wrapped around the block in New York, Washington and other US cities over the weekend as people clamoured to find out if they were infected before celebrating the holidays with family.
"I just want to make sure before seeing my wife's 70-year-old mom that I'm negative," said David Jochnowitz while waiting for a test in Washington.
With the rapid rise in infections, Washington Mayor Muriel Bowser on Monday reinstated an indoor mask mandate until the end of January and required government workers to get vaccinated, including a booster shot.
"I think we're all tired of it," Bowser told reporters. "I'm tired of it too, but we have to respond to what's happening in our city and what's happening in our nation."
In New York City, Covid-19 cases rose 60 percent in the week that ended on Sunday as Omicron spread rapidly around the US Northeast. New York has set records for the most new cases reported in a single day since the pandemic started for three consecutive days.
"It is a predictor of what the rest of the country will see soon, and the minimum - since NYC is highly vaccinated - of what other parts of the country will experience in under-vaccinated cities and states," said Georges Benjamin, executive director of the American Public Health Association.
Many Broadway productions cancelled performances as cast and crew have become infected. The popular Hamilton production on Monday extended cancellations until after Christmas due to breakthrough Covid-19 infections.