There's growing evidence that the more contagious coronavirus strain - first discovered in the UK - is spreading in LA County.
At least five cases of the UK variant have now been identified here amid limited genomic sequencing.
San Diego County has reported many more cases of the UK variant.
"We just haven't seen that kind of a cluster yet here but we are on the look out for it," says Barbara Ferrer, LA County's public health director. "There's no way that we don't have a fair amount of variants circulating and a fair amount of mutated virus circulating. It's just a question of how much and does it become dominant."
The CDC says the UK variant may become the predominant one in the US by March.
"We haven't yet seen the South African variant in any of the sequencing we have done but that doesn't mean it's not here," she says.
The CDC says preliminary data suggests that variant may also be more contagious. And there's growing concern that current vaccines are less effective against the South African strain.
"Assume it's here because we have already found, certainly enough of the UK variant to know it's circulating and do our very best to not have it turn into the dominant strain," she says.
Ferrer says that means doubling down on masking and other measures to prevent the spread of the virus.