Texas attorney general Ken Paxton waded into a battle between the city of Austin and Gov. Greg Abbott Friday over whether the city's four-day late-night indoor dining shutdown should be allowed.

Austin's ban, which was set to run between 10:30 p.m. and 6 a.m. from Dec. 31 until Jan. 3, was upheld by a judge on Thursday. Hours later, at 7:39 p.m. local time on New Year's Eve, the governor released a statement on Twitter telling restaurants to stay open.

Paxton backed the governor on Friday and called for the Texas Supreme Court to halt enforcement of the ban. "We cannot have local declarations conflicting with Gov. Abbott’s clear order," he said in a statement. "I will continue to fight for Texans, small businesses and for an open economy."

-ABC News' Jenna Harrison contributed to this report.

The United States surpassed 20 million coronavirus cases on New Year's Day, with 20,007,149 infections, according to data from Johns Hopkins University.

In different terms, 1 out of every 16 Americans has tested positive for COVID-19.

As the nation's third COVID wave continues, California has emerged as a major epicenter of the U.S. outbreak and at least three states have reported cases of the new COVID variant, which originated in the U.K. Health experts believe it to be more transmissible than the old variant, although it is not thought to be more deadly.

According to JHU, 346,408 Americans had died of the virus as of Friday.

Sen. Mitt Romney criticized the Trump administration's vaccine rollout in a statement he released Friday, writing: "when something isn’t working, you need to acknowledge reality and develop a plan—particularly when hundreds of thousands of lives are at stake."

Relying on states in lieu of developing a federal vaccination plan is "as incomprehensible as it is inexcusable," Romney wrote, and offered up his own suggestions as examples of the brainstorming he said ought to be happening in Washington.

The government should enlist every medical professional not currently delivering care, such as retired veterinarians, combat medics and corpsmen and medical students, to administer vaccines, Romney suggested, noting that they could be paid using the funding Congress has appropriated for states. Additionally, schools could serve as vaccine sites and vaccinations could be scheduled for specific days according to a person's priority category and birthdate.

While public health professionals will easily point out errors in his plan, he said, the nation needs new strategies based on "experience, modeling and trial," especially as the U.S. begins vaccinating more complex populations.

"We are already behind," Romney added. "Urgent action now can help us catch up."

-ABC News' Trish Turner and Chris Howell contributed to this report.

Construction on a 30-bed emergency field hospital is slated to start in western North Carolina Friday, as COVID-19 cases in the state continue to rise.

The facility, which is being built next to Caldwell Memorial Hospital, will treat COVID patients who aren't sick enough to need a ventilator and is meant to relieve pressure on five health systems in the region.

As of Thursday, 3,472 people were hospitalized because of the virus, according to the state health department.

Public Health England does not recommend mixing COVID-19 vaccines from different suppliers, according to the agency's head of immunizations, Dr. Mary Ramsay.

"We do not recommend mixing the COVID-19 vaccines -- if your first dose is the Pfizer vaccine you should not be given the AstraZeneca vaccine for your second dose and vice versa," Ramsay said in a statement Saturday. "There may be extremely rare occasions where the same vaccine is not available, or where it is not known what vaccine the patient received. Every effort should be made to give them the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all."

The clarification comes amid questions over the British government's updated guidance on COVID-19 vaccines, which now says that if individuals who received the first shot go to an immunization site where that same vaccine is not available for a second shot, or if the first vaccine received is unknown, "it is reasonable to offer one dose of the locally available product to complete the schedule."

"This option is preferred if the individual is likely to be at immediate high risk or is considered unlikely to attend again," the guidance adds.

The guidance, which was updated Thursday, also notes that "there is no evidence on the interchangeability of the COVID-19 vaccines although studies are underway," and thus "every effort should be made to determine which vaccine the individual received and to complete with the same vaccine."

Next week, the United Kingdom is set to begin distribution of a COVID-19 vaccine developed by England's University of Oxford and manufactured by British-Swedish pharmaceutical giant AstraZeneca, after the drug was approved Wednesday for emergency supply. Another COVID-19 vaccine developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech was approved in the U.K. on Dec. 2 and rollout began a week later.

ABC News' Zoe Magee contributed to this report.