Delta Coronavirus Strain Is Growing Fast In California, With Risk For Unvaccinated People

The highly infectious delta coronavirus variant is rising fast in California, with cases more than doubling in the past month and tripling in one Bay Area county, according to genomic sequencing results reported by state and local public health departments.

The first California cases of delta, a variant that emerged in India, were reported in early May. As of last week, 349 cases had been identified, and the variant made up about 5% of all coronavirus cases that underwent genomic sequencing for the week. The alpha variant, first identified in the United Kingdom, continues to dominate statewide, making up just over half of all cases reported last week.

In the United States, the delta variant makes up roughly 10% of all cases that have been sequenced, according to federal authorities. Some models predict it will become the dominant strain by the end of the summer.

The delta variant is more infectious than alpha, and may be twice as transmissible as the original strain of the coronavirus from Wuhan, China, according to preliminary studies. But infectious disease experts say it’s unlikely to cause as much chaos in California as it has in other parts of the world, including the U.K. and India, because of the state’s high vaccination rates.

“The Bay Area in particular has been doing quite well on the vaccination front. Hopefully we’ll see enough of the community covered that we won’t be seeing big surges. I’m optimistic on that front,” said Patrick Ayscue, an epidemiologist and senior fellow with the Chan Zuckerberg Biohub. “But that optimism is for people who are vaccinated. Unfortunately there’s no good news on the delta variant for the unvaccinated populations.”

The concern among many experts is that delta is so easily transmissible that it will quickly burn through populations that are not vaccinated, or even just partially immune. The U.K., for example, delayed reopening its economy by a month because of a spike in delta cases last week.

Delta is so infectious, said Dr. Eric Topol, executive vice president of Scripps Research in La Jolla (San Diego County), that nearly everyone who remains vulnerable due to lack of vaccination is precariously at risk now. “You have three choices: You get fully vaccinated, you had prior COVID infection and maybe a reduced risk, or three, you get delta COVID,” he said.

The concern in California and the Bay Area is the potential for delta to spread in communities where vaccination rates overall are low. Statewide, 48% of Californians are fully vaccinated and 13% more have had at least one shot of a two-course vaccine, but those rates aren’t even across counties. The Bay Area generally has very high vaccination rates; in San Francisco, 65% of residents are fully vaccinated.

But some counties have much lower rates — only about 20% are fully vaccinated in Lassen County, for example — and Black and Latino residents are less likely to be fully vaccinated than white and Asian residents statewide.

Nationwide, a handful of states have very low vaccination rates overall. And among all adults, vaccination coverage is lowest among young people ages 18 to 29, according to the Centers for Disease Control and Prevention. Young adults are less likely than older adults to have serious complications from COVID-19, but as a group they may be especially susceptible to the delta variant, infectious disease experts say.

“Our whole region is one of the safest places in the United States at this point,” said Dr. Sara Cody, the Santa Clara County health officer, at a news briefing Monday. “At the same time, we are still in the middle of a global pandemic. We still have variants that are emerging, and not everywhere is as safe as we are.”

Santa Clara County also has undervaccinated pockets, Cody said. And though the county’s overall high vaccination rates will confer some protection on all residents, those pockets could see outbreaks.

The county is one of a few in California that tracks variants locally. Delta cases have increased more than fourfold over the past month in Santa Clara County, from 12 in mid-May to 58 as of last week, and the variant now makes up 7% of all cases that undergo genomic sequencing.

Much about delta remains unclear, including whether it causes more serious illness than earlier strains, how it affects children who are too young to be vaccinated, and how well all of the vaccines hold up to it. Studies show that the two-dose course from Pfizer or Moderna offers strong protection against the variant. More research is needed about the one-dose Johnson & Johnson vaccine, though a similar vaccine being used in the U.K. offers decent protection too.

There have been signs that delta can cause more serious illness, but there are few studies proving that either way. Most of the evidence indicating it’s more virulent than other variants comes from India, where the surge in cases has put so much stress on hospital systems that it may have impacted the quality of health care overall.

Most infectious disease experts say the variant doesn’t appear to cause more serious illness in children. But if it spreads among unvaccinated groups, including children, teens and young adults, that could lead to a rise in hospitalizations and instances of long COVID, the syndrome that produces a range of debilitating symptoms for some people long after they have recovered from the initial infection, Topol said.

“I’m not worried about big increases in deaths, but I’m concerned about what we’ll see in hospitalizations and long COVID,” he said.

Infectious disease experts said it’s critical that older teens and young adults get vaccinated, and make sure they get both doses for maximum protection.

Federal authorities recently have reported several cases of heart inflammation among 12- to 15-year-olds after a second vaccine dose, causing some experts to reconsider fully vaccinating that group, at least for now. A CDC advisory panel is expected to address the topic later this week.

But children in that age group, as well as those under 12 who aren’t yet authorized for any vaccines, should get decent protection from delta and all variants of the virus if they’re surrounded by adults who are fully vaccinated, said Dr. George Rutherford, an infectious disease expert with UCSF.

“Kids get coronavirus from adults, they don’t get it from other kids,” Rutherford said. “We have ample evidence of that. And that’s the way to protect them. We need to have all the teachers vaccinated and all the parents and the other adults and older siblings.”

Ayscue and others who have been tracking variants throughout the pandemic noted that delta likely won’t be the world’s last mutation of the coronavirus. And even as California and the rest of the United States start to recover from the pandemic, this pattern of identifying new variants and figuring out how to fight them will continue as long as the virus is spreading widely elsewhere in the world.

“Delta is definitely the biggest threat we’ve faced so far, but the virus continues to look for new ways to thwart our vaccines, our interventions,” said Dr. Warner Greene, an infectious disease expert with the Gladstone Institutes in San Francisco. “We didn’t even know about this variant until February of 2021 and it’s going to become the predominant variant in the world shortly.”


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