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.

New Zealand will require all travelers from the United States and the United Kingdom to test negative for COVID-19 before departing for the country.

The new rule, which will come into effect Jan. 15, is in addition to a requirement that was implemented from Dec. 31 for passengers arriving from both nations to be tested within 24 hours of their arrival in New Zealand and to remain isolated until a test result is available. The U.S. and the U.K. are two of the hardest hit countries in the coronavirus pandemic.

The latest measures "are extra precautionary steps which provide another layer of protection for New Zealand from COVID-19," New Zealand's Ministry of Health said in a statement Sunday.

New Zealand, an island nation of 5 million people in the southwestern Pacific Ocean, has reported at least 2,181 confirmed cases of COVID-19 since the start of the pandemic, including 25 deaths, according to the latest data from the health ministry.

The U.S. death toll from the coronavirus pandemic has surpassed 350,000.

The number of people who have died from COVID-19 in the United States was 350,186 as of early Sunday morning, according to a real-time count kept by Johns Hopkins University.

The country has by far the highest death toll from the disease.

The U.S. set another record for most daily cases, despite some states not even reporting totals due to the New Year's holiday.

There were a record 275,897 cases reported Saturday, even though seven states did not relay any data, according to The COVID Tracking Project.

"Some states are reporting normally, some are reporting multiple days of data, and almost all are dealing with at least some holiday backlogs," the project said.

The seven-day average for hospitalizations stands at a record 123,689 despite incomplete data. There were 2,367 deaths reported on Saturday, too.

Officials expect numbers to spike after the holidays as people return home from gatherings with family and friends.