As a more contagious coronavirus variant from the United Kingdom takes root in the United States, researchers in California have identified a separate, home-grown strain that may be contributing to the spike in cases there.
The California variant first appeared in July in just one sample out of 1,230 in Los Angeles County. As of December, it was found in one-quarter of all samples tested in Southern California.
The dramatic increase took place at the same time as cases spiked in California.
The virus "appears to be more transmissible," said study co-author Eric Vail, director of molecular pathology at Cedars-Sinai health system. More tests are under way to find out if it is, or whether the case spike "just so happened to be a right-place-at-the-right-time, coincidental moment."
California has the second-highest infection rate in the country, behind Arizona, according to the Covid Tracking Project. The number of new infections increased tenfold between the beginning of November and mid-December. The numbers are trending downward, but the state still logs nearly 40,000 new infections per day.
This strain has appeared as several other variants have raised concerns around the world. The U.K. strain is roughly 30% to 50% more infectious than others. While fewer than 200 cases have been reported in the United States so far, the U.S. Centers for Disease Control and Prevention expects it to become the dominant strain in March.
Other variants have appeared in South Africa and Brazil that also spread more easily.
Experts caution that the virus is not the only reason for the increase in cases. Failures to wear masks and keep social distance are driving outbreaks, too.
The rising number of variants is a by-product of an out-of-control pandemic, Vail said. The more people the virus infects, the more opportunities it has to mutate.
"Every single person is like a little incubator, a little evolution chamber for the virus," he said.
None of these strains appear to cause more severe disease than normal. But more infectious strains will likely increase the caseload at a time when many health systems are severely strained.
Experts also are watching carefully to see if the vaccines are less effective against the new strains.
So far, there is no indication that they are. But the variants all have mutations in part of a key protein the virus uses to attach to cells in the airway, which also is the protein that the vaccines target.
One mutation found in the South African and Brazilian variants is especially worrying. Some lab tests found that certain antibodies did not stick nearly as well to the part of the protein with this mutation.
But the immune system makes antibodies to many different parts of the protein, experts note. The Pfizer vaccine still worked in lab tests. The other vaccines are similar, so experts are cautiously optimistic.
"If I was a gambling man, I would say that (the variants) would not affect" how well the vaccines work, Vail said.