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Delta variant is rising. Breakthrough infections, too. How should I protect myself?

Karen Kaplan, Melissa Healy and Deborah Netburn, Los Angeles Times on

Published in News & Features

The CDC cited new data from an outbreak this month in Barnstable County, Massachusetts, where 69% of eligible residents are vaccinated. Out of 469 cases tied to gatherings at restaurants, bars and vacation homes, 74% were breakthrough infections in people who had been fully vaccinated. Among the 133 viral samples that were genetically sequenced, 90% were caused by the delta variant or an offshoot of the strain.

The confidential CDC document also cites discouraging reports about breakthrough cases in India, where the delta variant was first detected. The viral loads of vaccinated people who became infected with delta were higher than the viral loads of vaccinated people who became infected with other coronavirus strains, those reports found.

The growing evidence of delta’s propensity to cause breakthrough infections is sure to complicate efforts to persuade vaccination holdouts to roll up their sleeves for COVID-19 shots — a challenge anticipated by the authors of the CDC document.

Going forward, health officials will have to “improve [the] public’s understanding of breakthrough infections” and stop “describing breakthrough cases as ‘rare’ or as a ‘small percentage’ of cases,” the document says.

How much does the new data change things?

Overall, it now appears that the delta variant poses more risks than initially thought but that the vaccines available in the U.S. still offer strong protection.

“Delta is basically coronavirus on speed,” said Dr. Gregory Poland, an immunologist at the Mayo Clinic in Rochester, Minnesota.

Scientists have established that people infected with delta have about 1,000 times more viral particles in their upper respiratory systems than people who are infected with earlier coronavirus strains.

That difference allows delta to jump from person to person just four days after an initial infection, said Chunhuei Chi, director of the Center for Global Health at Oregon State University. With previous strains, it took about six days for that to occur, he said.


This rapid spread helps explain why the delta variant, which is thought to have arrived in the U.S. in March, now accounts for 82% of recent coronavirus infections in the United States, according to CDC estimates.

The data on whether delta causes more serious illness is less certain. The CDC document cited several studies, some of them preliminary, suggesting that delta infections were more likely to lead to serious illnesses that require hospital care.

Not everyone has found that data persuasive.

Neeraj Sood, a health policy expert at USC’s Sol Price School of Public Policy, said the CDC document relies heavily on studies that have small sample sizes and use viral load as a surrogate for disease severity, which may not necessarily be an accurate substitution.

“I didn’t look and say, ‘There’s a pretty bulletproof case that delta is more virulent,’” he said.

Sood said the CDC has also made the mistake of focusing too heavily on the rising number of breakthrough infections, which are to be expected when an increasing share of the population is vaccinated. Researchers should instead focus on how deeply the vaccines have driven down rates of severe illness and hospitalization among people who do get infected, he said.

“If it’s not going to kill you, not going to put you in the hospital, I’m less worried,” he said.

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