San Francisco Chronicle - (Sunday)

Virus cases slowing across California

- By Nanette Asimov

While thousands of new coronaviru­s cases are still reported daily across California, their pace is slowing like a speeding car nearing a freeway exit ramp.

The decline in infections among young and middleaged adults is particular­ly encouragin­g because they account for most infections and were the main drivers of earlier surges.

That their case counts are falling now is giving some experts hope that California can avoid a fourth wave of infection — and that the exit ramp aims at the end of the pandemic.

In a dramatic illustrati­on that experts tie to the power of vaccines and other preventive factors, new cases for California­ns age 18 to 49 are down 95% so far this month compared with December, their worst month in the pandemic. In that month alone, 611,739 cases were reported in that group, compared with 33,660 in April, so far.

Including all age groups, the number of new coronaviru­s cases reported across California plunged by 60% from February to March, and by another 43% so far this month, landing at 56,325 new cases, according to a Chronicle analysis of state data. In the Bay Area, the number of new cases fell by 54% from February to March, and by nearly 32% so far this month.

“We’re slowing down. That’s a great thing,” said Dr. Kirsten BibbinsDom­ingo, professor and chair of the department of epidemiolo­gy and biostatist­ics at UCSF’s School of Medicine. “While it’s not the vaunted herd immunity, it means that the likelihood of having a really dramatic rise in cases” is declining.

Older people are more likely than younger and middleaged adults to be vaccinated because they were among the first to be eligible in California. But overall, more than half of all California­ns 16 and older are at least partially vaccinated now, and that’s starting to be apparent in the data.

“Vaccinatio­n has indeed played a role in the dynamics of the numbers of cases,” said Dr. ChingHua Wang, an immunol

ogist and president of Samuel Merritt University in Oakland, which specialize­s in health sciences.

Yet vaccinatio­ns aren’t the only forces at work, infectious disease experts agreed. Masks and social distancing continue to help prevent transmissi­on.

Naturally acquired immunity from previous infection may also be playing a role in falling case counts, said Dr. George Rutherford, an infectious disease expert at UCSF. The mystery is how long that immunity lasts.

“The answer is ‘a while,’ ” Rutherford said. Infection causes the body to build an army of neutralizi­ng antibodies to fight the virus, and those powerful attackers may fade within a few months. But the body has other immune mechanisms, including Tcells, that may last much longer. If there is immunity from the body’s Tcells, “it’s probably lifelong. But we just don’t know,” Rutherford said.

About 20% of California­ns have developed immunity from having been sick, estimates epidemiolo­gist Dr. George Lemp, former director of the HIV/AIDS Research Program at the University of California’s Office of the President. He adds those people to the more than 50% of state residents who have had at least one vaccine dose, and accounts for overlap, to conclude that nearly 70% of people may have some level of immunity at this point.

He said his estimate is bolstered by his close look at how the virus is doing among people of different ages.

Lemp compared California case rates during the last two weeks of March and the first two weeks of April and found that the number of infections declined only in the 65andolder age group.

The greatest drop was among California­ns over 80, with a 13% decrease, he found. People age 65 to 79 saw a 6% decrease in new cases. By contrast, 18 to 34yearolds saw the greatest rise in new infections, 17%.

“I think that really is showing a vaccinatio­n effect,” he said. “Which makes sense because the rollout by age in California vaccinated older people first.”

People 65 and over became eligible for vaccinatio­n on Jan. 13, while the age dropped to 50 on April 1, and to 16 and over on April 15.

But even among people 18 to 34, the number of new cases was way down this month compared with earlier in the year, Lemp found.

In the last two weeks of January, there were 79,000 new cases among California­ns 18 to 34 — but in the first two weeks of April, fewer than 15,000 new infections.

Helping to slow the new infections, Lemp said, has been that many people most at risk of infection have already been sick and have developed the “innate immunity.”

But it’s the vaccine, he said, that “may have blunted” a fourth surge of the virus in California, while cases were swinging up in places like Michigan, New York and New Jersey.

“California got lucky, I think,” Lemp said.

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 ?? Josie Lepe / Special to The Chronicle ?? Mariano Zelaya of Redwood City receives his shot from Carlos Martinez at the Facebook headquarte­rs vaccinatio­n center.
Josie Lepe / Special to The Chronicle Mariano Zelaya of Redwood City receives his shot from Carlos Martinez at the Facebook headquarte­rs vaccinatio­n center.

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