When federal health authorities authorized COVID-19 vaccine boosters in September, they said the extra shots should be given to older adults or people at high risk. But Santa Clara County health officials took that a step further Wednesday, saying they will give any adult vaccinated six or more months ago another dose.

It was a bold pronouncement that once again puts Santa Clara County at the forefront of battling the virus, with a more generous interpretation of the federal and state booster eligibility guidelines than is being used elsewhere.

“No one will be turned away who wants a booster,” said Dr. Marty Fenstersheib, the county’s COVID-19 vaccine officer, setting new rules for county vaccination clinics.

With the late summer decline in COVID-19 cases leveling off at an uncomfortably high level and showing signs of rising again, county health officials urged vaccinated adults to get a booster shot ahead of the winter holidays — fearing another eruption of outbreaks as families and friends gather.

“We want to safeguard the public and prevent a COVID-19 surge as the holiday season approaches,” said county Health Officer Dr. Sara Cody. “Get your booster now and make your Thanksgiving gathering safer.”

So far, Santa Clara County stands alone in the Bay Area and perhaps in the state, as well. Though other county health officials voiced similar concerns about another winter COVID-19 surge, they weren’t jumping aboard their neighbor’s bandwagon Wednesday. And it was not clear whether drug stores and private health providers in the region would follow Santa Clara County’s lead, either.

Contra Costa County “continues to follow the eligibility guidelines” from the U.S. Centers for Disease Control and Prevention and California Department of Public Health, said spokesman Will Harper, though “we are indeed concerned about what impact the holidays will have as families gather indoors to celebrate.”

Alameda County health spokeswoman Neetu Balram said “we strongly encourage everyone 5 and older to get fully vaccinated as soon as possible,” citing concerns that “forecasts predict another wave of increasing cases starting in late November.”

But Balram added that Alameda County supports the federal and state recommendations “for who should receive boosters, including any updates they may make.”

San Mateo County is following the CDC eligibility guidelines.

Those guidelines say adults vaccinated with Pfizer’s or Moderna’s shots six or more months ago should get a booster if they are 65 or older, 50 or older with other health problems, or at least 18 and living in a long-term care facility. People who are 18 or older with other health problems or who live or work in a setting that puts them at high risk of exposure to the virus may consider a booster shot as well. And all adults who received Johnson & Johnson’s one-shot vaccine at least two months ago also should get a booster.

The state defines high-risk jobs to include health care workers, firefighters, police, congregate care staff, teachers, school support staff, daycare workers, food and agriculture workers, manufacturing workers, correctional officers, public transit workers, postal workers and grocery workers.

Gov. Gavin Newsom, 54, who got his booster at a public event last month, qualified because he originally had the Johnson & Johnson shot. Oakland Mayor Libby Schaaf, a healthy 55-year-old who’d had the Pfizer shots, got her booster at the same event and explained her job includes being an “emergency responder.”

Despite the restrictions, it’s unclear whether people seeking a booster have been turned away for not meeting the recommended criteria. Many online registration forms do not ask how an individual qualifies for a booster.

But Cody said only 20% of vaccinated adults in Santa Clara County who had the Pfizer or Moderna vaccines at least six months ago or the Johnson & Johnson shot at least two months ago have gotten boosters and blamed the eligibility guidelines, which she insists were not intended to be restrictive.

“The message was quite complicated and difficult for the public to understand,” Cody said. “When you drill down and see all the different medical conditions that make you eligible, you’re kind of talking about the whole population.”

President Biden in August announced plans for all vaccinated Americans to get boosters starting in September once they are eight months past their initial doses. But expert advisors to the CDC and the Food and Drug Administration were skeptical. For the Pfizer and Moderna vaccines, they recommended the shots only for older, chronically ill or otherwise high-risk people, positions the agencies for the most part adopted. The advisors agreed that Johnson & Johnson vaccine recipients should get a second shot after two months because of evidence of waning immunity.

Pfizer this week asked the FDA to amend its booster authorization to include all adults, citing results of a new clinical trial that shows it’s safe and effective. Those results have yet to be peer reviewed or published.

CDC data show only about one in three Californians 65 or older have gotten a booster vaccine, similar to figures in Texas, Florida and many other states. Among all adults 18 and older, the figures drop to 14%.

The booster campaign has been controversial from the start, with many health experts across the country arguing that shifting focus to boosters would distract from the far more important goal of getting shots into the unvaccinated and fueling skepticism about the vaccines’ effectiveness.

Cody said Wednesday that “we’ve always followed the science” in combating the virus and that knowledge has changed over the course of the pandemic. The vaccines, she said, offered amazing levels of protection.

“Now we’re seeing,” Cody said, “that protection does fade.”