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 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.

Meanwhile, the virus has gone quiet in Deep South states that abandoned mask orders, opposed vaccine mandates, posted lower vaccination rates and saw larger outbreaks over the summer. California’s case rate is now well above Texas’ and double Florida’s, which along with the rest of the Gulf Coast are down to the CDC’s orange “substantial” transmission level.

“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.

For Bay Area residents, that has real consequence. Local health officers have reimposed indoor face-mask wearing regardless of vaccination status and say they will lift the order only after their counties have dropped below the orange level for three weeks, among other conditions.

So why aren’t Golden Staters reaping more reward for their adherence to health guidance while the virus gives freewheeling Dixie a break?

“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.”

While California’s vaccination rate compares well with many other states, it still isn’t enough to snuff outbreaks. Currently 62% of California’s total population is fully vaccinated, compared with 60% in Florida, 54% in Texas, 49% in Georgia, 48% in Louisiana, 46% in Mississippi and 45% in Alabama. That still leaves more than one in three Californians unvaccinated.

In California, indoor face mask orders imposed over the summer for schools statewide and for other public buildings in the Bay Area and Los Angeles also helped keep the virus in check. But the state’s unvaccinated — especially those who haven’t been infected — remain vulnerable.

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.

What’s more, immunity through vaccination or infection wanes over time. Californians who were quick to line up for vaccines early in the spring are now wondering how long their protection will last.

Booster shots have been authorized mostly for older adults, people with weakened immunity or those with high exposure risk, which will spur their protection from infection. But among those 65 and older, just 30% in California got the extra shot, similar to the 29% in Texas and 27% in Florida.

Mokdad said immunizing newly eligible kids and unvaccinated adults, giving booster shots and encouraging or requiring mask wearing can blunt a projected winter rise in infections. Will people heed the call?

“California has done very well over the past few months, but we still have too many unvaccinated people,” Wachter said. “People are spending more time inside and being more active, and masking is going down.”

Combined with waning immunity and low booster uptake, he said, “the end result is that we’ve plateaued in our improvements, both nationally and in California, and it’s likely we’ll soon see some significant upticks.”