After a worrisome upward trend last month, Bay Area counties have seen COVID-19 transmission rates decline and the state overall is seeing hopeful signs of improvement heading into the holidays — even as outbreaks worsen across much of the country.

Gov. Gavin Newsom noted the progress as he visited a vaccine clinic Monday in San Francisco to urge more people to get the shots. But he also sounded a wary tone as infections continued a sharp upward trend nationally in the upper Midwest and Northeast as well as in Europe, suggesting a coming winter surge.

"We saw a few weeks ago some troubling signs with case rates going up, positivity rates going up, hospitalizations and ICUs going up," Newsom said. "That said, in the last 10 or 11 days, we've seen some stability. Some good signs. That is good news."

Newsom said at 1.9%, California now has the lowest rate of positive tests for the virus in the United States, and the pathogen's effective reproductive rate has fallen to 0.91. A figure lower than 1 indicates it is not spreading.

California exited the U.S. Centers for Disease Control and Prevention's high red transmission level for the virus Monday. It is now is the only state outside the Southeast at the orange substantial transmission level, with the rest of the country seeing a higher rate of spread.

Other Bay Area counties now have joined San Francisco in the CDC's yellow moderate level of virus transmission, after being stuck in orange earlier this month.

"We're seeing a decline," Newsom said. "That's encouraging. We're starting to see a decline again in hospitalizations and ICUs, and we're seeing some stability in our case rate as well."

Newsom credited the state's turnaround to a willingness of many residents to wear face coverings to reduce the virus' spread and to get vaccines and boosters.

"That's because of the good work that all of you have been doing for many many months to put a lid on this virus, to get us through this winter and the prospects of a surge," Newsom said. "So we're here to celebrate that success of sorts, but soberly and humbly and reminding people of the importance not to let their guard down, the importance of face coverings, importance of continuing to promote the efficacy and promote the safety of these lifesaving vaccines."

Newsom also noted that 27 states have seen case rates increase 10% or more in the past week. The CDC on Friday reported that the level of community transmission of the virus in the U.S. remains high and that the 7-day average of daily new cases had risen 16.1% from the week before.

Health experts believe the virus is more transmissible in winter both because weather conditions are more favorable for its survival and colder weather drives more people to gather indoors, where they are more likely to be in close contact with others in poorly ventilated areas and spread the virus.

The new rise in cases nationally comes at a complicated moment. Last Thanksgiving, before vaccines were available, federal and local officials had urged Americans to forgo holiday gatherings. Now public health officials, including Dr. Anthony Fauci, the nation's leading infectious-disease expert, have mostly suggested that vaccinated people can gather in relative safety.

Still, although the CDC says the unvaccinated are 5.8 times more likely to test positive for COVID-19 and 14 times more likely to die from it than the vaccinated, their protection has its limits, particularly with the highly transmissible delta variant now dominant in the U.S.

The vaccinated can also become infected and transmit the virus, and the shots' waning protection has prompted a campaign for additional booster shots for all American adults.

Over the summer, as COVID-19 cases spread like wildfire across the Southeast, health experts blamed those states' reluctance to impose requirements for face masks or vaccines. But now, surprisingly, those states have the country's lowest case rates, and the virus is spreading in the more vaccinated and masked-up North.

Minnesota's total population, for example, is about 62.1% vaccinated, about equal with California's 62.8%. But Minnesota's case rate per 100,000 people over the last seven days is 524.5, nearly seven times California's 75.8. New York state, with 68.1% of its total population vaccinated, has a case rate of 325.6.

The course of the virus in Europe, where Austria is entering a lockdown and some areas of Germany have shut down Christmas markets, has raised fears about just how high case numbers might rise in the United States.

Dr. George Rutherford, an infectious disease specialist at the University of California-San Francisco, said the natural immunity people gain after recovering from infection also plays a role in keeping case rates low in places like the Southeast, which is coming off of severe summer outbreaks. How durable that immunity will be is unclear, but infections in other parts of the U.S. are mostly among the unvaccinated.

Rutherford said California appears to be in much better shape heading into the Thanksgiving holiday than last year, when infections were "taking off like a rocket." It remains "an open question" how much the shots will tamp down infections and hospitalizations this year, but they will be concentrated among the unvaccinated, he said.

"We've got to get people boosted and got to get 5-11-year-olds vaccinated," Rutherford said. "We do that stuff, we're going to be in very good shape."

The New York Times News Service contributed to this report.