California’s coronavirus case rates are approaching an all-time low, but in one county on the North Coast, the numbers are moving in the wrong direction.Humboldt County, whose biggest city, Eureka, is 230 miles north of San Francisco, recorded 137 new cases last week. That was the most since early February. Over the weekend, the county health department reported 33 additional infections, bringing the total number of residents who have tested positive for the virus since the pandemic began to 3,885.

Humboldt’s seven-day average positivity rate of 3.1% is now more than three times the state average, according to data compiled by The Chronicle. County officials attribute the recent uptick to the spread of the transmissible coronavirus variant first discovered in the United Kingdom, known as B.1.1.7.

The mutation is also behind recent surges in Oregon and Washington.

“Our recent increase in cases shows just how contagious this variant is,” said Dr. Ian Hoffman, Humboldt County’s health officer. “Until now, older adults experienced the most severe outcomes of COVID, but now we’re seeing younger people getting sicker and younger people being hospitalized.”

Health officials in the county of 135,558 said people ages 20 to 29 represent 23.3% of the new cases, more than any other age group. Nine of 17 residents hospitalized with COVID-19 last week were under 30.

Mass gatherings in the region, including one at a Pentecostal church that officials labeled a “super-spreader” event, are believed to be contributing to the increase.

“Given how easily this variant spreads, we still need to wear masks in public, maintain distance and do everything we can to avoid large gatherings, especially indoors, until more of us are vaccinated,” Hoffman said.

Humboldt County officials have also identified a single case of the Brazilian variant, known as P.1, through genomic sequencing.

County data shows about 27% of residents are fully vaccinated, compared with more than 40% statewide. The health department turned down 1,000 doses of the Moderna vaccine last week because of low demand, Hoffman said.

“We’re at a point where certainly the people who were well-connected to information in society knew how to get the vaccine, and have now gotten it,” he said. “Now we’re having to get out to people who didn’t have as much access in the past. So we’re going to keep vaccinating as long as people are there to show up for clinics.”

Aidin Vaziri is a San Francisco Chronicle staff writer. Email: [email protected]