Another COVID-19 variant has been detected in two cases in the United Kingdom, Health Secretary Matt Hancock said.

"Both are contacts of cases who have traveled from South Africa over the past few weeks,” Hancock said.

Hancock said the “new variant is highly concerning because it is yet more transmissible and it appears to have mutated further than the new variant that has been discovered in the U.K."

The health secretary said both cases and close contacts of the cases have been quarantined.

U.K. travel to and from South Africa is suspended, Hancock said.

The coronavirus is constantly mutating, and there are many thousands of lineages of the virus, each with distinct mutations. There's no evidence this new variant is more deadly and there's no evidence it will affect the vaccine.

ABC News' Mike Trew contributed to this report.

Another 803,000 workers lost their jobs and filed for unemployment insurance last week as the pandemic rages, the U.S. Department of Labor said Wednesday. 

The latest figure is slightly less than last week’s figure, but still remains well above pre-pandemic levels. 

The DOL also said Wednesday that some 20.3 million people were still receiving some form of unemployment benefits through all government programs as of the week ending Dec. 5. That figure was 1.7 million for the comparable week in 2019. 

The latest economic data from the DOL also comes as the COVID-19 relief package, which includes extended unemployment benefits, faces a new hurdle as President Donald Trump has indicated he will not sign the bill yet. 

The unemployment rate in the U.S. was 6.7% last month, according to the DOL's most recent employment situation report. In February, prior to the pandemic, the unemployment rate was 3.5%.

-ABC News’ Catherine Thorbecke contributed to this report.

The U.S. government has ordered 100 million additional doses of the Pfizer-BioNTech COVID-19 vaccine, Business Wire announced Wednesday morning. 

This brings the total doses of the vaccine for the country to 200 million.

"With these 100 million additional doses, the United States will be able to protect more individuals and hopefully end this devastating pandemic more quickly," said Albert Bourla, the chairman and CEO of Pfizer. "We look forward to continuing our work with the U.S. government and healthcare providers around the country."

All 200 million doses are expected to be delivered by July 31, 2021, allowing for 100 million people in the U.S. to be vaccinated.

"Securing more doses from Pfizer and BioNTech for delivery in the second quarter of 2021 further expands our supply of doses across the Operation Warp Speed portfolio," said U.S. Department of Health and Human Services Secretary Alex Azar. "This new federal purchase can give Americans even more confidence that we will have enough supply to vaccinate every American who wants it by June 2021."

Tuesday became the second deadliest day on record since the start of the COVID-19 pandemic in the U.S., with 3,401 new deaths, according to new data compiled by Johns Hopkins University. 

The deadliest day was last week, Dec. 16, when 3,656 new deaths from the virus were reported. 

The U.S.' seven-day average of daily deaths is now 2,654, according to The COVID Tracking Project. 

December is set to surpass April as the deadliest month since the start of the pandemic.