SAN JOSE — A potentially more infectious COVID-19 variant that’s increasingly spreading throughout California is now “relatively common” in Santa Clara County, where it has contributed to the infamous Christmas Day outbreak in a Kaiser emergency room and multiple other outbreaks.

“The variant has been identified in cases from many of these settings, including cases associated with the Kaiser outbreak, skilled nursing facility outbreaks, cases in jails and shelters, and specimens from testing sites in the community,” the county said in a statement sent to this news organization Monday.

“This suggests that the variant is now relatively common in our community,” the statement added.

On Sunday evening, county Public Health Officer Dr. Sara Cody joined state and other local health officials in announcing the 452R variant was behind several of the South Bay’s outbreaks, including the one where at least 74 Kaiser employees and 15 patients were infected in the ER of the South San Jose hospital.

A hospital receptionist died from the Kaiser outbreak, which has been largely attributed to an employee making an unannounced visit to the ER to bring some holiday spirit. She wore an inflatable Christmas tree costume that may have spread the virus because it used power-circulated air.

Kaiser confirmed the presence of 452R in the outbreak in a statement Monday, adding that those who tested positive “are now past their contagious period and symptom free.”

At Sunday’s press conference, Santa Clara County officials did not formally link any other outbreaks to the variant, including a series of large ones at the county jails and one affecting the San Jose State University football team before it played and lost a bowl game at the end of a historically successful season.

Monday’s statement did link the strain to the other outbreaks, though not to the football team.

In addition to Santa Clara County, the 452R variant has been detected in Humboldt, Lake, Los Angeles, Mono, Monterey, Orange, Riverside, San Francisco, San Bernardino, San Diego and San Luis Obispo counties.

Cody said more and more of the 452R strain is being detected in the South Bay because Santa Clara County is conducting more genomic sequencing than most other California counties.

“We have been doing more looking and thus more finding,” she said.

News of the spreading variant comes as virus trends are showing some positive signs, with the rise of COVID-19 cases and deaths reported in California on Sunday having slowed from a week ago. At approximately 39,700 per day, California’s average daily case count has fallen about 11% in the past week. The state is still averaging about 513 fatalities per day — more than one every three minutes — but that is just 7% higher than a week ago, compared to a 43.5% increase the previous week.

University of Washington scientists who maintain one of the most widely used and respected computer models for tracking and projecting outbreaks also say new infections appear to have peaked over the past week in the U.S. and large states, including California.

But the positive assessment is predicated on the public adhering to social distancing, avoiding gatherings and continuing to wear masks, as well as the absence of a vaccine-resistant virus strain and vastly improved vaccine distribution.

Meanwhile, the inoculation effort, which already has been rocky, hit another snag after a cluster of people suffered severe allergic reactions to a batch of Moderna vaccines in Southern California, prompting Dr. Erica Pan, the state’s top epidemiologist, to recommend that clinics put hundreds of thousands of doses on hold.

More than 330,000 doses from that batch were distributed across the state, and Santa Clara County was among several jurisdictions to announce they were holding off. The county said in a news release it has no indication that any of the 21,800 doses from the scrutinized batch that ended up with South Bay health providers has been administered.

Cody said the extent to which the variant factored into the Kaiser outbreak is still being investigated with the help of the state Department of Public Health and the national Centers for Disease Control and Prevention.

“That was a very unusual outbreak with a lot of illnesses, and it seemed to spread quite fast,” she said. “We are trying to understand whether the features of that outbreak are because of this variant … or does it have to do with other factors that are present at this hospital.”

Both Kaiser and public health officials are stressing that the rise of the mutated strain does not warrant changing existing safety protocols and practices to avoid contracting COVID-19.

“We do not have any signals at this point that this variant is associated with anything else such as increased severity of illness, although we of course are certainly looking for any signals to see if that would be emerging,” Cody said.

The latest variant is different from the B.1.1.7 variant, a more contagious strain first detected in the United Kingdom. Dr. Charles Chiu, a virologist and professor of laboratory medicine at the UC San Francisco, noted the variant’s “very high attack rate” that in some outbreaks being studied led to “80 and 90 percent” infection scenarios.

Chiu said in an interview Sunday night that the first case of the variant in California was discovered in May in Alameda County. But from mid-November to the last two weeks of December, the number of COVID-19 cases tied to the strain rose from 3.8% to 25%.

“It is concerning that it may potentially be more infectious,” Chiu said at a later briefing, stressing the need for more investigation to determine how quickly it can be transmitted.

Staff writers Evan Webeck, Lisa M. Krieger, John Woolfolk, Elliott Almond and Julia Prodis Sulek contributed to this report.