The COVID-19 graph that keeps SFs health director up at night – SF Gate

The “reproductive rate,”  or Rt, is a key metric in measuring how fast a virus is growing, and San Francisco’s Director of Health Dr. Grant Colfax said it’s the indictor that he’s closely watching right now.

The Rt is the average number of people who become infected by a sick person. When it is above 1.0, a virus spreads quickly. A virus can die off when the Rt is below 1.0.

Colfax shared at press briefing Monday that San Francisco’s reproductive rate was below 1.0 through April and May and began to climb in June. The Rt currently stands at about 1.25 (based on data up to July 4), and could be a little lower or even higher.

“We were doing so well that our reproductive rate fell below 1.0 and got to as low as .85 and, if that had stayed, the virus would basically burned out in the city,” Colfax said.

While San Francisco is experiencing a less severe pandemic compared to other parts of the state and country, such as Los Angeles and Florida, the rising reproductive rate reveals the potential for more severe conditions.

If it remains at 1.25 or climbs even higher, Colfax shared a projection showing the surge in hospitalizations that could unfold and overwhelm the city’s health care system (see graph above as well as more slides from the presentation.)

“This is the slide that keeps me up at night,” Colfax said. “This is what happens if the reproductive rate stays at that 1.25 number. If everything stays the same, we would hit a peak of 830 hospitalizations in our city. In April, we peaked at 94 and thanks to all the work that people did in San Francisco and sheltering in place, we drove that number down.”

Projections for hospitalizations go all the way up to over 6,000. Colfax calls it “a situation that reflects the challenges that New York had.”

“Certainly we hope that this doesn’t happen,” he added. “It is so key here. We have a window here to do better.”

If the Rt persists at 1.25, S.F. projects there could be about 890 deaths by the end of 2020, and as many as nearly 3,000. The city death toll is currently 50.

Colfax, along with Mayor London Breed, provided a deep dive into the S.F. Department of Public Health COVID-19 data that guides decisions the city makes around reopening the economy and society.

The city rolled back some planned changes to its shelter-in-place order last week. The opening of hair and nail salons, tattoo parlors and massage services was delayed.

“We know we’re going to be living with COVID for some time and the goal is to adapt using this data…” said Breed. “We know there are a lot of numbers out there, like our key health indicators available online, and it can be difficult to track what it all means. I know at the end of the day everyone wants to understand exactly what this means and when will we will open.”

Since the start of the pandemic, the city has recorded 4,950 cases of the coronavirus. Some 1,000 of those total cases were detected in the past two weeks alone, particularly in the southeast part of the city that has experienced the most transmission.

Both Colfax and Breed emphasized the need for people to wear masks and socially distance.

“These times are hard but having a mass pandemic in our city is even worse,” Colfax said.

MORE CORONAVIRUS COVERAGE:

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Amy Graff is the news editor for SFGATE. Email her: agraff@sfgate.com.