S.F., L.A. took disparate paths to coveted tier

San Francisco and Los Angeles took wildly divergent paths to arrive at the same coveted place on Tuesday: the yellow tier in California’s reopening blueprint.

Their trajectories couldn’t be more different, and the scale of misery is almost impossible to compare between the two counties. While San Francisco has largely avoided the worst of the coronavirus pandemic’s deadly scourge, Los Angeles County has been crushed, never more so than during the winter surge.

That California’s two most prominent counties arrived in the state’s least restrictive tier on the same day signals a remarkable shift in the pandemic, public health experts said: In the north and in the south, California is on the mend.

“It’s a sunny day and California is looking good,” said Dr. Kirsten BibbinsDomingo, vice dean for population health and health equity at UCSF.

She attributed the counties’ successes mostly to their residents’ willingness to be vaccinated. “The things that have really helped all of California, and you see it in our two big urban centers, is that we have been vaccinating at a really high rate, and there is a high degree of vaccine uptake,” Bibbins-Domingo said.

As of Tuesday, seven of the state’s 58 counties were in the yellow tier, which is assigned based on case rates and the percentage of people testing positive. San Francisco and Los Angeles are the only two large counties in yellow — the other five are in rural Northern California and the Sierra.

San Francisco often has been an outlier in the pandemic in California, an island of low infection and death rates even during the darkest days. Its public health response was broadly viewed as among the most conservative in the United States. It was among the six Bay Area counties that were the first in the nation to issue shelterinplace orders more than a year ago, and it’s not shocking that it is among the first in the state this year to reach the yellow.

Los Angeles County, though, never made it out of the most restrictive purple tier until this spring. Its cases remained steadily high, even during the relative lulls between surges. The county’s move to yellow is remarkable for several reasons, but perhaps mostly for what it means for the rest of the state.

San Francisco reaching the yellow tier is a local victory and cause for celebration, public health experts said. But what happens in Los Angeles County, which makes up roughly a quarter of California’s total population, in many ways foretells the fate of the rest of the state. California was the epicenter of the national pandemic just a few months ago and now has among the lowest daily case rates in the United States — and Los Angeles has propelled that trajectory, for worse and now for better.

“They’ve had an incredible recovery. And you have to hand it to them. It’s not a fluke, it’s by dint of effort,” said Dr. George Rutherford, a UCSF infectious disease expert.

That its cases have dropped so far, so fast is startling, experts say. At the peak of the winter wave, Los Angeles County was reporting more than 12,000 cases every day — nearly 150 per 100,000 residents. San Francisco peaked at about 500 cases a day, or 40 per 100,000 residents.

On Tuesday, when the state’s weekly tier data came out, Los Angeles’ adjusted case rate, incorporating how much testing a county is doing, was actually lower than San Francisco’s, by a hair: 1.6 cases per 100,000, compared with 1.8.

Los Angeles also has endured a much deadlier attack than San Francisco. Nearly 24,000 people have died of COVID19 in the county over the course of the pandemic, compared with 537 in San Francisco. The per capita death rate in the south is nearly fourfold higher.

But Los Angeles County has gone two days this week with no COVID deaths at all. The last time it reported consecutive days with zero deaths was last summer. San Francisco went five days last week without reporting any deaths.

“I kind of wrote L.A. off as toast. I would have thought that when we got to yellow, L.A. is probably miles behind,” said Shannon Bennett, chief of science at the California Academy of Sciences in San Francisco. “It’s phenomenal to me that they’re here.”

There are many reasons why the pandemic hit Los Angeles County so much harder than San Francisco, and not all of the answers are clear yet. Los Angeles is so much bigger and more diverse, requiring a more complex public health response. It has a larger population of essential workers, especially people from Latino and Black neighborhoods who suffered disproportionately high rates of disease.

How the two counties converged in the yellow tier is also not entirely clear. It’s an example of the concept of equifinality, said Dr. Stephen Shortell, former dean of the School of Public Health at UC Berkeley: the idea that a goal can be reached by many possible routes.

Dr. George Lemp, a former epidemiologist with the University of California, summed up his analysis similarly: “San Francisco took the guarded and cautious approach to get to where they are, and L.A. took the long and tortured road to get where they are. And they both arrived in the same place.”

That tortured road was built on acquired immunity, public health experts said. Cases have plummeted in both counties largely because so many people are no longer susceptible to the coronavirus. San Francisco accomplished that almost entirely by vaccination — about 72% of residents 16 and older have received at least one dose.

About 54% of residents 16 and older in Los Angeles County have had at least one dose. But Los Angeles had a grim head start on San Francisco. Experts believe as many as 20% of county residents have been infected with the coronavirus and therefore have some naturally acquired immunity. Fewer than 10% of people in San Francisco have been infected.

That might partly explain why Los Angeles County’s cases fell much more sharply than San Francisco’s and it has landed in the same spot now.

“Look at the thumping that Los Angeles has taken. And cases have fallen like a rock,” Rutherford said. “Naturally acquired immunity is the real thing.”

Another explanation for why the counties hit the yellow tier at the same time may come down to a trope familiar to marathon runners and dieters alike: The last stretch to the finish, be it in miles or pounds or case counts, is the hardest.

San Francisco saw its case rates plummet in January and February, then it stumbled against an invisible roadblock. For almost all of March and April, its adjusted case rate lingered around 2 per 100,000 — never quite low enough to dip into the yellow tier.

But Los Angeles County, starting from a much higher peak, kept on plunging, until finally it caught up with San Francisco’s low case rates. Now it may also linger at this low but steady pace, public health experts said.

“The last mile is the toughest, and we’re in the last mile,” Shortell said. “We’ve made great strides, and to get to zero cases, or even near zero, to finish the marathon is going to take longer and a little extra effort.”