Charts show why S.F. is at opposite end of coronavirus tier system from L.A., San Diego – San Francisco Chronicle

Amid a recent coronavirus surge across the U.S., San Francisco occupies a unique place in California’s least restrictive reopening tier — in sharp contrast with the state’s two largest counties, Los Angeles and San Diego.

San Francisco is one of only six among California’s 58 counties in the yellow “minimal” tier and the only large metro area in the state with that distinction.

But in Southern California, Los Angeles County has been stuck in the strictest purple “widespread” level since the state launched its tier system in August. It was joined there this week by San Diego, which was bumped back from the red “substantial” tier as cases have spiked.

Health officials cite many reasons for the surge, including social gatherings, household and workplace exposure, and travel. Young adults also continue to drive the virus. And health officials everywhere say that many people are becoming weary of health protocols and restrictions, and some are returning to normal pre-pandemic behavior despite the risks.

“It’s fair to say that people still aren’t responding well enough to public health policy,” said John Swartzberg, a UC Berkeley infectious disease expert. “We still have no leadership from the federal government.”

A coronavirus testing site is open at Pier 30 in San Francisco, which has restrictions far more stringent than its yellow tier allows, a cautious approach the city has maintained throughout the pandemic.

Swartzberg said that with the weather getting colder, more people are inside. More people are also traveling, and he expects many more to do so for the winter holidays, which will greatly exacerbate the problem.

The state’s reopening blueprint specifies what business and public activities are allowed at each tier, and health officials use it to dial back the spread of coronavirus when metrics indicate troubling trends. While counties must not exceed their tier limits, they may choose to loosen restrictions more slowly than their tier allows.

San Francisco’s cautious reopening approach, following an aggressive early pandemic response and strict safety protocols, can be credited for its position at the opposite end of the tier system from Los Angeles and San Diego counties.

Some of San Francisco’s restrictions are far more stringent than its yellow tier allows. The city has paused its reopening timeline several times since the summer, and this week shut down indoor dining in an effort to keep a lid on spiking coronavirus cases.

On Tuesday, in the state’s weekly assignments, 11 counties regressed to more restrictive tiers. They included Contra Costa in the Bay Area, which went backward from orange to red. Napa, Solano and Santa Clara are among the counties in danger of moving to tighter tiers next week.

Even with growing cases, Swartzberg said most of the Bay Area is starting in a better place than Southern California and the rest of the country.

“We’re experiencing very significant growth in the virus in the Bay Area, but we started with such low numbers,” Swartzberg said. “So we’re not experiencing problems like hospitals filling up.”

He has some guesses as to why Southern California is struggling more with the coronavirus compared to the Bay Area.

“Culturally there is a big difference between Northern and Southern California,” he said. “There’s a much more politically homogeneous population in the Bay Area versus Los Angeles. And people tend to follow public health dicta more so here than Southern California.”

He also said San Francisco’s history with the AIDS crisis resulted in a strong relationship among public health agencies and the wider community.

Swartzberg also pointed to the higher number of migrant workers in Southern California, who have been hit particularly hard by the coronavirus.

Placement in California’s four-tier reopening system, launched in early September, is determined by three coronavirus metrics: rate of new cases (adjusted by the state to account for the amount of testing), rate of positive tests and a new equity metric. Under state rules, counties that fail to meet one or more of those thresholds for their tier for two consecutive weeks are moved back into the next most restrictive tier.

In early October, the state introduced a health equity metric that aims to ensure the positive test rates in the most disadvantaged neighborhoods don’t lag behind a county’s overall positive test rate.

San Francisco

A vendor’s table is filled with face coverings at Fourth and Market streets in San Francisco, which is the only large metro area in California in the yellow “minimal” coronavirus reopening tier.

When the state launched its tier system on Sept. 1, San Francisco was in the red “widespread” category. It moved to the orange tier on Sept. 29 and on Oct. 28 became the first urban California county to enter the yellow tier.

But officials said cases have surged 250% since Oct. 2. The rapid spike prompted city health officials on Tuesday to shutter all indoor dining, halt high school reopenings and lower capacity at gyms and movie theaters from 50% to 25%, effective after Friday.

“I think they’re very, very wise,” Swartzberg said. “The question is, do you want to wait until the artificial rules we made are achieved to do something? Or do we want to look at the curve, and try and anticipate what we’re going to see and blunt that curve right away? Yeah we want to do that. You have to have your head in the sand not to preemptively do things right now.”

Currently, San Francisco has an adjusted case rate of 2.5, which is the seven-day average of daily COVID-19 cases per population of 100,000, adjusted for the number of tests performed. The rate has risen since the past two tier assessments of 1.7, and before that, the case rate was 1.5. But this is still very low compared to 5.1, where San Francisco started in the new tier system.

While the adjusted case rate is slightly above the yellow-tier threshold, the county got a boost by meeting the state’s health equity metric. San Francisco also has some of the most widespread testing in the state.

One area where San Francisco has excelled is its positive test rate, which has not risen above 3% since the new monitoring system was put in place. For the past three weeks, it was below 1%, and while it has bumped up since then, it’s still at 1.1%.

And its health equity metric has also remained low: It began at 3% but dipped below 2% starting the week of Oct. 12, and stayed below the yellow tier threshold of 2.2% ever since.

Experts have pointed to a number of reasons that San Francisco has done particularly well compared to the rest of the state, including its early shelter-in-place order, stringent safety protocols including mask-wearing and social distancing, and slow, cautious approach to reopening.

Los Angeles County

A masked pedestrian walks along Broadway Avenue  in Los Angeles in April. Los Angeles County, the state’s largest, is stuck in the purple, most restrictive tier. Los Angeles Mayor Eric Garcetti has recommended that the city's 4 million people wear masks when going outside amid the spreading coronavirus.

Since the new monitoring system launched, Los Angeles County has remained firmly in the purple tier. Last week, it hit its highest daily case total since Aug. 15, and has reported an average of more than 2,000 new cases and 15 deaths per day, according to the L.A. Times Tracker.

In its first tier assessment, the county’s adjusted case rate was 9.6. Since then, it’s hovered between 7 and 8.1 — never low enough to get out of the purple tier. But the percent positive rate has stayed at orange “moderate” tier levels, with the latest at 3.8%. The health equity metric hit its peak at 6.8% on Nov. 2, and remains high at 6.5%.

Health officials have said for some time that social gatherings are driving the spread in Los Angeles County. One specific example is the many people who got together to watch the Dodgers and Lakers through their championship runs, both culminating in victories in October that led to fans spilling out in the streets to celebrate.

Like much of the state and country, health officials point to pandemic fatigue causing many to eschew safety protocols and restrictions.

San Diego County

Diners sit at outside tables at the Beach House Grill in San Diego in May. San Diego County is one of three of California’s largest counties among 10 statewide that regressed on reopening businesses and other activities amid a resurgence in coronavirus cases, state officials said, while San Francisco imposed new restrictions including a ban on indoor dining in a bid to slow the spread of the virus there.

San Diego County was the lone Southern California county in the red tier when the state rolled out its new system Sept. 1, with the rest of the south state solid purple. The case rate was rocky at first for San Diego County, but officials managed to get it under the threshold of 7.

Still, the rate continued to inch close to the purple-tier threshold for weeks, creeping above the line on Oct. 19 before going down briefly the following week. But on Nov. 2, the case rate was at 7.4, and shot up to 8.9 this week, prompting state officials to move the county into the purple tier. In San Diego County, weekly cases are up 70%.

Coronavirus spread at universities has been a lesser problem in California compared to other states, since many schools are operating virtually. But since San Diego State began allowing some in-person classes for the fall semester, more than 1,300 students have become infected.

The county reports that 58% of all cases are in individuals ages 20 to 49. The 20- to 29-year-old age group had the highest number of cases at 25.1%, with the next highest at 18.4% in 30- to 39-year-olds. A disproportionate number of cases are attributed to the Latino community, accounting for 58.8% of cases but only 30.1% of the population.

The most recent COVID-19 Watch report for San Diego County on Nov. 10 shows the highest possible origin of recent exposure at 34.4% is the workplace, with the next highest bars and restaurants at 10.1%. Household exposure was also high at 34.2%, and travel-related exposure at 21.2% with 8.6% of respondents having visited Mexico, which the county notes is mostly work-related.

Kellie Hwang is a San Francisco Chronicle staff writer. Email: Twitter: @KellieHwang