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Highly contagious Delta coronavirus variant spreading fast in California

Microscopic virus particles.
This transmission electron microscope image shows the coronavirus that causes COVID-19.
(National Institute of Allergy and Infectious Diseases, Rocky Mountain Laboratories)

The Delta coronavirus variant is now the third-most common in California, new data show, underscoring the danger of the highly contagious strain to people who have not been vaccinated against COVID-19.

The variant makes up 14.5% of California coronavirus cases analyzed so far in June, up from 4.7% in May, when it was the fourth-most identified variant in California, according to data released by the California Department of Public Health.

Experts say the Delta variant poses a greater chance of infection for unvaccinated people if they are exposed. The variant, first identified in India, may be twice as transmissible as the conventional coronavirus strains. It has been responsible for the rise in cases recently in India, the United Kingdom and elsewhere.

But vaccinated people are well protected against infection and illness from the Delta variant. One recent study found that the full two-dose course of the Pfizer-BioNTech vaccine was 88% effective against symptomatic disease caused by the Delta variant and 96% protective against hospitalization.

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There is no widespread scientific consensus on whether the Delta variant is more likely to cause more serious illness than other strains.

With the highly contagious Delta variant of the coronavirus continuing to spread statewide, the Los Angeles County Department of Public Health is recommending that all residents wear masks in public indoor spaces — regardless of whether they’ve been vaccinated for COVID-19.

Delta’s rise comes as California’s dominant strain, Alpha, first identified in the United Kingdom, may have peaked.

In May, the Alpha variant made up 58.4% of coronavirus cases that were analyzed in California. Alpha’s share fell in June, now making up 37.7% of analyzed cases — still the top variant but with a much smaller proportion.

The Gamma variant, first identified in Brazil, is also being seen more often in California. In May, the variant represented 10.1% of analyzed cases. It now makes up 21.6% of analyzed cases in the state, but Delta is still growing at a more rapid rate.

Los Angeles County, the nation’s most populous, has confirmed 123 Delta variant cases — 49 of them among residents of Palmdale and Lancaster. Fourteen cases of the Delta variant were in people from a single household.

L.A. County data suggest that vaccines are still overwhelmingly effective in protecting people against the Delta variant, as well as other known variants.

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Of those 123 confirmed cases of the Delta variant in the county, 89% occurred among people who were not vaccinated against COVID-19, and 2% among those who were partially vaccinated.

No one has died from the Delta variant in L.A. County.

COVID-19 vaccines appear to protect against the Delta variant, but there’s real concern about the vulnerability of people who aren’t yet vaccinated.

The few fully vaccinated people who have been infected with the Delta variant “experienced relatively mild illness,” L.A. County Public Health Director Barbara Ferrer said.

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Almost everyone who has died in L.A. County of COVID-19 has been unvaccinated.

Data released by the county showed that 99.8% of COVID-19 deaths from Dec. 7 to June 7 occurred among unvaccinated people.

Cases, hospitalizations and deaths by vaccination status in L.A. County (June 24, 2021)

“If you are fully vaccinated, you have a lot of protection,” Ferrer said, adding that for the “very small numbers” of people who contracted the Delta variant despite vaccination, “they really did not have serious illness. … This is a pandemic of unvaccinated people.”

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The results of outbreaks of the Delta variant elsewhere also support the vaccines’ effectiveness. In Israel, an outbreak of the Delta variant is “driven primarily by the unvaccinated,” tweeted Dr. Ashish Jha, dean of the Brown University School of Public Health.

Some vaccinated people in Israel are still getting infected “because no vaccine is 100%,” Jha wrote. But the infections that are breaking through the immunity provided by the vaccines are causing mild disease.

“What’s happening in Israel is vaccines working exactly as we all expected,” Jha wrote.

In South Africa, President Cyril Ramaphosa on Sunday announced a sweeping set of new coronavirus restrictions, including curfews and school closures, as the country faces another surge in cases that threatens to outpace two previous spikes. The Delta variant, first discovered in India, appears to be driving South Africa’s new increase, Ramaphosa said.

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South Africa recorded more than 15,000 new coronavirus cases Sunday, including 122 deaths, bringing its total number of fatalities to nearly 60,000.

It’s unclear whether Delta-plus is more transmissible than the original, highly transmissible Delta variant. Without more research, it shouldn’t yet be cause for alarm, experts say.

Meanwhile, data released by California show that the percentage of the tested population who have antibodies to the coronavirus — a sign of immunity to COVID-19 — is also increasing.

From May 16 to June 12, 85.9% of Californians who were tested for coronavirus antibodies had them — a promising sign of growing immunity, either because of immunization or past exposure to the virus. That’s up from 76.6%, calculated during a four-week period in May.

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Experts have estimated that 70% to 85% of a population needs to have immunity for a region to develop “herd immunity” to COVID-19, which interrupts the sustained transmission of the virus.

Officials are continuing to urge everyone to be vaccinated against COVID-19, including people who survived a previous infection. Experts say immunity provided by the vaccination is more robust and longer-lasting than immunity from an infection.

The highly infectious Delta variant is making the task of getting to herd immunity more difficult.

With conventional coronavirus strains, it could take perhaps 71% of the population to be immune for a region to reach herd immunity and interrupt the virus’ transmission, said Dr. George Rutherford, a UC San Francisco epidemiologist. But a variant such as Delta — because it is so transmissible — would increase that threshold to, say, 84%, he said.

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The Delta variant is also spreading nationwide.

From May 9 to May 22, the Delta variant made up less than 3% of analyzed coronavirus samples nationwide. But from June 6 to June 19, that proportion rose to more than 20%.

Dr. Anthony Fauci, the U.S. government’s top infectious-disease expert, called the strain “currently the greatest threat in the U.S. to our attempt to eliminate COVID-19.”

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