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California’s first cases of infection caused by the South Africa coronavirus strain have been confirmed in the Bay Area, Gov. Gavin Newsom said Wednesday.

California’s first cases of the South African coronavirus variant have been confirmed in the Bay Area, Gov. Gavin Newsom said Wednesday.

Newsom said one of the cases was found in Alameda County and the other in Santa Clara County. He did not share any additional information on those who tested positive or how they may have contracted the variant, called B.1.351. Newsom said the cases of the South Africa variant had been reported “as of a few hours ago.”

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The announcement comes amid growing concern that mutations of the coronavirus, some of which are believed to be more contagious, may be increasingly spreading at a time when California is still digging out from the devastation of its winter surge.

While currently available vaccines appear to still be effective against most of the coronavirus variants that have been identified to this point, the strain fueling a resurgence of COVID-19 in South Africa was not slowed down by one therapeutic — an experimental vaccine developed by AstraZeneca and Oxford University — that officials had been counting on to protect its frontline healthcare workers, prompting the government to shelve plans for an inoculation campaign that would have begun this month.

A COVID-19 vaccine developed by the U.S. company Novavax also was found to be nearly 90% effective against all types of COVID-19 when tested in Britain, yet only 49% effective in South Africa.

Neither the Novavax nor the AstraZeneca vaccines are being used in the United States at this point.

California has also confirmed 159 identifiable cases of the coronavirus variant first identified in the United Kingdom, designated as B.1.1.7. That is of particular concern for health officials, as it is believed to be 50% more transmissible than the conventional variety.

Given how easily that variant can spread, officials predict it could become the dominant coronavirus strain nationwide by the end of March.

The U.K. variant has been identified in San Diego, Los Angeles, Orange, San Bernardino, Alameda, San Mateo and Yolo counties.

L.A. County officials Monday confirmed the fourth and fifth cases of the B.1.1.7 variant. Public Health Director Barbara Ferrer said it’s clear that there are a fair number of mutant strains circulating in L.A. County.

If we have a more infectious virus, Ferrer said, “it just makes it easier for people to get infected. … The variants are concerning, because if we let our guard down, the more infectious strains can become dominant. And that just makes it a lot easier for this virus to spread.”

Orange County on Monday confirmed its first case of the U.K. variant in a resident who reported no international travel history, “which means there are likely more cases in OC,” the county’s Health Care Agency tweeted.

Officials on Monday also confirmed the first case of the U.K. variant in the Sacramento region — in Yolo County, west of Sacramento — after an individual was determined to have the strain and “may have acquired the variant through travel outside the community,” according to a statement issued by UC Davis and the city of Davis.

At least 138 cases of the U.K. variant have been confirmed in San Diego County, with 50 probable cases there. A UC San Diego scientist has warned government officials that the U.K. strain is so transmissible that its spread — combined with the rejection of mask use and physical distancing guidelines, as occurred across California in the fall — could result in an even worse surge within two months than occurred in the winter.

Times staff writers Melissa Healy and Emily Baumgaertner contributed to this report.

This story originally appeared in Los Angeles Times.