Coronavirus updates: US reports nearly 300,000 new cases in all-time high

The new, more contagious strain of the novel coronavirus, which is sweeping rapidly across London and other parts of southeast England, has been detected in California's San Bernardino County, officials said.

The latest variant of the virus that causes COVID-19 was found in two members of the same household in the Big Bear area who were tested on Dec. 20, according to a press release from the San Bernardino County Department of Public Health. One of them had contact with a traveler who returned from the United Kingdom on Dec. 11 and began showing symptoms three days later.

Four other cases of the fast-moving strain, known as B117, have been detected in San Diego County.

"Based on the information currently available, we know that the B117 variant strain seems to spread more easily and quickly," Dr. Michael Sequira, San Bernardino County public's health officer, said in a statement Friday. "Therefore, following all safe practices is more important than ever."

Experts say there's currently no evidence that the variant is deadlier or causes more severe illness, or that existing vaccines are less effective against it.

The new strain was announced in England in late December and then confirmed in the United States for the first time on Tuesday, after a case was detected in Colorado.

More than 4 million people in the United States have received their first dose of a COVID-19 vaccine, according to the Centers for Disease Control and Prevention.

As of 9 a.m. ET on Saturday, 13,071,925 vaccine doses had been distributed nationwide and 4,225,756 doses had been administered, according to the CDC COVID Data Tracker.

The doses include both the Pfizer/BioNTech and Moderna vaccines.

India's drugs regulator granted an emergency use authorization for two COVID-19 vaccines on Sunday.

One is the British-developed Oxford/AstraZeneca vaccine, which is being produced locally by the Serum Institute of India, and the other, Covaxin, was developed by Indian biotechnology company Bharat Biotech. Both vaccines, which will be administered in two doses, "are being approved for restricted use in emergency situations," Drugs Controller General of India, Dr. Venugopal G Somani, said during a press conference Sunday.

Somani said the decision to approve the vaccines was made after "careful examination," by the Central Drugs Standard Control Organization, the national regulatory body in India for pharmaceuticals and medical devices.

Indian Prime Minister Narendra Modi welcomed the approval, calling it a "defining moment."

"It is a matter of pride that the two vaccines that have been approved for emergency use are both made in India," Modi wrote on his official Twitter account Sunday. "This reflects the will of our scientific community to fulfill the dream of a self-reliant India."

The first phase of India's mass immunization plan aims to vaccinate 300 million people by August, including health care workers, police officers and anyone deemed vulnerable due to their age or health conditions.

The country of nearly 1.4 billion people has the second-highest tally of diagnosed COVID-19 cases in the world, behind only the United States. Since the start of the pandemic, India's Ministry of Health and Family Welfare has reported more than 10.3 million confirmed cases, including at least 149,435 deaths.

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.