A month ago the coronavirus seemed headed for a long winter’s nap in masked and well-vaccinated California. Gov. Gavin Newsom boasted that the Golden State “continues to lead the nation” as the only state to reach the Centers for Disease Control and Prevention’s yellow “moderate” tier of community virus transmission.

But the Mercury News reports that COVID-19 cases aren’t falling in California anymore. They have climbed back up to the CDC’s blood-red “high” level of virus transmission as the highly contagious Delta variant continues to wreak havoc.

“There are early indications that the decline in the Delta surge at the national level in the U.S. has ended,” said Ali H. Mokdad, professor of health metrics sciences at the University of Washington, which runs a widely followed model projecting the course of the pandemic. Currently, 19 states have increasing transmission, including several like California “that had previously appeared to have been declining.”

And while much of the Golden State’s current coronavirus woes are driven by virus spread in the less-vaccinated and restricted inland counties, the Bay Area hasn’t been immune. Most Bay Area counties that hoped to reach the yellow moderate level by now remain stubbornly stuck in orange. Marin and Santa Cruz counties, which had reached the yellow level, are back up to orange. San Francisco is the only county in yellow.

So why aren’t Golden Staters reaping more reward for their adherence to health guidance?

“You’re paying for your success, which is weird,” Mokdad said. “You succeed in controlling the virus, and now you’re having infections.”

But he and other health experts say it’s not because the health guidance isn’t sound. Outbreaks burn out once the virus runs out of enough new people without immunity to infect. And people can gain immunity both from infection recovery and vaccines.

With higher vaccination levels than in the Southeast, California saw a smaller wave of cases over the summer as the Delta variant ripped through the country, mostly infecting those who hadn’t been vaccinated. Now that they’ve recovered, they have immunity too, cutting off avenues for the virus to spread.

“These regions are now being partly protected by high prior infection rates,” said Dr. Bob Wachter, chair of the medical department at the University of California-San Francisco. “But these people whose immunity comes from COVID are not very well protected, and their immunity will wane with time.”

States in the Southeast hammered with big summer case surges now are faring better simply because, with their combination of vaccinations and infections, they have fewer left who are susceptible to the virus than in California, Mokdad said. But “they got there at a heavy price.”

Other factors also are in play. The Southeast’s hot, humid summers drive people to the air-conditioned indoors where the virus spreads easily, while Californians enjoy moderate weather out in the surf and sand. But the autumn chill is now bringing Californians inside, too.