Numbers behind outbreak demand an urgent response

Since midFebruary, the number of new coronavirus cases reported in the Bay Area each week has been doubling or tripling the total from the week before — a phenomenon known as exponential growth, which means the regional outbreak is on a steep, and concerning, upward trajectory.

The explosive numbers triggered the sixcounty shelterinplace orders Monday, and similarly alarming reports across California spurred a statewide order Thursday.

And when they finally start to fall, the numbers will tell public health officials that they can start to relax a little, though that could be months from now.

Because so much of the Bay Area’s future hinges on the case counts, it’s troubling that they’re not actually a very reliable marker for how widespread the virus is in the community, infectious disease experts say. Nationwide testing shortages mean that only certain highrisk people — those who are seriously ill, for example — are being counted, and many hundreds or thousands of cases are overlooked.

But official counts of people who have been infected are the best data public health officials have, so they’re driving policy decisions.

“Right now we have to take this very seriously, because we need to knock these cases down to a nonexponential growth,” said Dr. Shannon Bennett, a virologist and chief of science with the California Academy of Sciences.

But they’re still the best data public health officials have to follow the trajectory of the outbreak.

Extreme social distancing should substantially slow down the growth, but it probably won’t be apparent for at least two weeks, due to the incubation period of coronavirus and lags in testing. In fact, as testing becomes more widely available, case counts may climb dramatically in the near future.

But once things slow down and it looks like the region is hitting a plateau, public health

As of Saturday, 646 cases had been reported in the Bay Area — more than twothirds in the past week alone. California had 1,464 cases, 74% of them in the past week.

That’s staggering growth since the virus took off in California a month ago. In the last week of February, five cases were reported in the Bay Area, according to county reports. The next week — when state and local labs first started conducting their own coronavirus tests — cases shot up by 47.

The second week of March, 154 cases were reported in the Bay Area. This past week: more than 438.

The first sign that the shelterinplace orders are working will be a slowing of the pace of new cases. As testing increases — which should happen over the next few days and weeks — the case counts may seem to climb at an even more concerning rate.

But eventually the weektoweek numbers will steady — that’s known as reaching the inflection point — and then decline. That’s when the region may finally be able to ease up, if not abandon completely, some of the extreme social distancing measures, Bennett said.

“That’s the burning question for all of us: When will we detect the inflection point in new cases?” Bennett said. “And once we detect it, how long do we have to run over the crest of the wave before we’re on the other side?”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @erinallday

officials could decide people can resume some normal activities — maybe restaurants can reopen, but with limited capacity, and bars and movie theaters and other venues too. Some people may be able to return to work. But those are tough decisions to make, because easing up could leave communities vulnerable to reinfection, and in another few weeks or months the same situation could play out all over again.

That’s especially a concern if the rest of the country doesn’t also practice rigorous social

distancing. California isn’t an island, and all of the best mitigation efforts could be undone if visitors from other parts of the country “reseed” the state, infectious disease experts said.

“We can’t do this partially. The entire country should be doing what San Francisco is doing,” said Dr. Warner Greene, a senior investigator at the Gladstone Institutes who specializes in HIV research, but is currently studying the new coronavirus and how it infiltrates the body. “We can’t have anybody on the beaches in Florida for spring break.