A review of coronavirus specimens collected in Los Angeles County shows the mu and lambda variants were circulating earlier this summer, but the delta variant remains dominant, officials said Friday.

The highly contagious delta has “crowded out all of the other previously circulating strains,” now accounting for 100% of all strains sequenced in L.A. County, said Barbara Ferrer, the county’s public health director.

But when the lab that sequences the cases re-analyzed samples with an updated version of the genetic library used to identify variants, it reclassified many older specimens as mu or lambda variants.

The county is now reporting a total of 232 cases linked to the mu variant so far, while last week it had said 167 cases were identified to date. Another 28 cases were linked to the lambda variant.

“Most of these specimens were actually collected earlier in the summer, even though this report was just made available in the past week,” Ferrer explained.

While the U.S. Centers for Disease Control and Prevention doesn’t consider either the lambda or mu to be variants of concern or interest, the World Health Organization does because they’ve fueled spread in other parts of the world.

Mu was a dominant strain in Colombia earlier this year, and in lab studies appeared more vaccine resistant than previous strains, but it has been outstripped by the delta variant in the U.S.

The lambda strain, meanwhile, has not resurged since peaking globally in early July. But Ferrer said the county will continue to monitor sequences isolated from cases in the region.

Public health officials remain concerned that new, potentially more dangerous strains could develop if L.A. County’s vaccination rate doesn’t increase.

As of Sept. 5, 66% of residents 12 and older were fully vaccinated, an increase of one percentage point over the previous week.

“We are making very little progress increasing vaccination coverage among our residents,” Ferrer said. “In order to avoid the cycle of surges every few months, we do need to see a significant increase in the vaccination coverage.”

Public health officials say they’re still working to send mobile teams into Black and Latino communities with lower vaccine coverage, and they hope numbers will rise after this week’s vaccine mandate from the L.A. Unified School District and President Joe Biden’s order that all employers with more than 100 workers require them to be vaccinated or test for the virus weekly.

In L.A. County, unvaccinated adults 50 and older are more than 17 times more likely to be hospitalized than their vaccinated counterparts, while unvaccinated 18- to 49-year-olds are hospitalized at a rate 23 times higher.

And in July and August, unvaccinated people were 10 times more likely to die from COVID-19 than those who were vaccinated, Ferrer said.

The county reported more than 2,000 new infections and 50 additional deaths for Friday.

Virus transmission in the county remains at a high level, according to thresholds set by the CDC. But the seven-day case rate of 104 new cases per 100,000 residents is a 35% decrease from the previous week, and a nearly 50% decrease from Aug. 19.

“These recent declines may reflect the masking requirements implemented early in the surge and the small increases in our vaccination coverage,” Ferrer said, adding that parts of the country without mask mandates aren’t seeing such steep declines.

The health director warned that it’s possible reported infections will tick up again following Labor Day celebrations and with more testing and intermingling in schools and workplaces.