Why Covid took off in California, again

Between reinstated mask mandates and spiking coronavirus case numbers, this summer is starting to feel a little too much like 2020.

Even in California, a state with a vaccination rate well above average, the number of people hospitalized with Covid-19 has nearly doubled in the past two weeks, according to a New York Times database. The Centers for Disease Control and Prevention’s map of coronavirus spread shows California bathed in orange and red, signaling the highest levels of transmission.

So how did we get here?

Well, what’s happening in California is a story playing out across the country. Summer ushered in more socializing and fewer restrictions, just as the extremely contagious Delta variant gained a foothold.

Andrew Noymer, a public health professor at the University of California, Irvine, employed a very California analogy to explain it to me: “Delta is a lightning strike and loosening restrictions is the wind” — and they have joined forces to create a threat like a dangerous wildfire.

Although 53 percent of California residents are fully vaccinated, better than most states, it has not been enough to prevent Delta from spreading. Even in San Francisco, which at 70 percent has one of the best vaccination levels among big cities, new coronavirus cases have increased 141 percent over the past two weeks.

“The Delta variant isn’t hyperbole. It isn’t public health people wringing their hands,” Dr. Kirsten Bibbins-Domingo, an epidemiologist at the University of California, San Francisco, told me. “It’s a game-changer.”

The Delta variant is so contagious that it has caused cases to spike like never before, Bibbins-Domingo said. Its proliferation also means that achieving herd immunity — the threshold needed to halt a virus’s spread — would most likely require vaccinations of at least 95 percent of people, she said.

But that doesn’t mean vaccinations aren’t helping now. People who already have received shots are far less likely to end up in a hospital if they contract the coronavirus. Nationwide, 97 percent of people hospitalized with Covid-19 are unvaccinated, according to the C.D.C.

So even as Delta spreads in California, the number of hospitalizations and deaths will be much lower than previous surges because more than 21 million Californians are vaccinated, said Dr. Timothy Brewer, an infectious-disease expert at the University of California, Los Angeles.

“The good news is: The vaccines are working,” Brewer said.

First published in the New York Times