Coronavirus

San Francisco just hit its lowest COVID case rate since one year ago. Here’s how other Bay Area counties compare

Before the winter surge, much of the Bay Area was celebrating low coronavirus transmission rates that allowed for more relaxed restrictions on what people could do for fun, work and life in general.

Now, case rates have plummeted again on the other side of that surge, and San Francisco has reported its lowest case rate since very early in the pandemic — a year ago when the city was in lockdown mode. That was before a summer spike sent the numbers skyrocketing ahead of a drop to lows in early October.

However, the picture across the Bay Area is uneven, with some counties still a ways off from achieving the lower levels seen last fall. And, even as we approach the lows, experts are concerned about the possibility a fourth surge could start in the coming weeks.

The lowest level San Francisco hit, right before the winter surge, was a seven-day average of 2.8 infections per 100,000 population, recorded on Oct. 1. Last week, the city reached a 2.6 case rate Thursday, although since then, the rate has ticked up to 3. San Francisco last recorded a lower case rate, 2.4, on March 20, 2020.

San Mateo County last week reported a 3.95 case rate, below the 4.35 rate it reached in mid-October before the winter surge took off. The county has also seen a rise since last week, to 6.5 as of Sunday.

Sonoma County, which struggled with high transmission even before the winter surge, finally has seen its infections dip below a rate of 7, for the first time since June 30. On Sunday, the county reported a case rate of 6.5, well below its 8.2 pre-surge low on Sept. 15.

Overall, the Bay Area infection rate is dropping close to its bottom transmission level seen in the fall. As of Sunday, the seven-day average of daily new cases per 100,000 people was 5.9, just above the 5.2 low set on Oct. 17.

Despite the good news, some counties are still not close to their pre-winter surge lows. Napa County reported a rate of 6.8 on March 16, but that jumped to 8.7 by Sunday, much higher than the 3.5 rate on Labor Day.

Some counties, including Napa, do not report data on weekends or on holidays, when chances for reporting of outlier numbers is greater. Gaps in reporting can create artificially low numbers for certain days, and inflated numbers on the days when the reporting catches up. Lapses by the state reporting agency could also contribute to these single-day lows if they create gaps a few days without new cases reported.

Marin County saw a 2.5 per 100,000 case rate on Oct. 3, but was able only to get down to 3.9 by Thursday, before going up to 5.2 on Sunday. San Mateo, Contra Costa, Alameda and Solano counties have all seen similar patterns.

Statewide, rates have declined dramatically, to 6.9 new cases per 100,000 residents as of Sunday. That not only bests the October low of 7.5 ahead of the winter surge, but marks the first time since prior to the summer surge that the number was below 7 — on June 8.

This “honeymoon period” of dwindling case rates after a surge has some experts concerned that a fourth spike could be on its way at the end of March or beginning of April, fueled by the coronavirus variants, namely the B.1.1.7 variant that originated in the UK and is 50% more transmissible. A pair of variants from California also are rapidly gaining ground in the state.

Lessons also remain from when the state began a new color-coded tier system at the end of August, tying reopening to metrics of progress against the virus. Most of the Bay Area started in the orange tier. Less than two months later, most were downgraded to purple after easing of restrictions was accompanied by backsliding on progress.

While unlikely to spur a spike as dramatic as what occurred during the winter months, the variants and complacency on the part of residents could push the curves back up again before a majority of the population is vaccinated. The U.S. Centers for Disease Control and Prevention has repeatedly raised this concern as worrisome variants spread and many states relax COVID-19 restrictions.

“Increasingly, states are seeing a growing proportion of their COVID-19 cases attributed to variants,” CDC director Dr. Rochelle Walensky said during a White House briefing on Monday. “I am worried that if we don’t take the right actions now, we will have another avoidable surge — just as we are seeing in Europe right now and just as we are so aggressively scaling up vaccination.”

Mike Massa contributed to this story. Dan Kopf designed the graphics in this article.

Kellie Hwang is a San Francisco Chronicle staff writer. Email: kellie.hwang@sfchronicle.com

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