The highly transmissible delta variant is a more formidable foe than previously believed, largely due to its ability to infect and be spread by people who are fully vaccinated, according to data gathered by the Centers for Disease Control and Prevention.

A confidential document prepared by the agency cites evidence from a recent outbreak in Massachusetts involving at least 145 people who were infected with the delta variant, which was first detected in India. In the Massachusetts outbreak, the viral loads of the 80 people who were vaccinated were essentially the same as the viral loads of the 65 people who were not vaccinated.

The CDC document also cites reports about so-called breakthrough cases in India. The viral loads of vaccinated people who nonetheless became infected with delta were higher than the viral loads of vaccinated people who were infected with other coronavirus strains, those reports found.

Some of the data in the document was described Tuesday by CDC Director Dr. Rochelle Walensky when she announced new guidance regarding the use of face masks. In parts of the country where coronavirus transmission rates are “substantial” or “high,” everyone — regardless of vaccination status — should wear face coverings when they’re in indoor public spaces, the agency now says.

The confidential document was obtained by the Washington Post and posted on its website.

The delta variant was already known to be about 50% more transmissible than the Alpha variant, which was first detected in the United Kingdom and is itself about 56% more transmissible than the original coronavirus that sparked the global pandemic.

Scientists have also 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 an estimated 82% of recent coronavirus infections in the United States, according to CDC estimates.

The CDC document suggests that breakthrough cases are expected to rise — not only because of delta's enhanced transmission powers, but because the number of vaccinated people is rising as well.

Though delta's superspreading abilities have been clear for some time, scientists have not been sure whether infections caused by the variant are more dangerous. The CDC document cites three reports that link the variant with more serious health outcomes:

— In a preprint study from Ontario, Canada, people infected with delta were more than twice as likely to be admitted to the hospital, nearly four times more likely to be treated in the intensive care unit, and more than twice as likely to die of COVID-19 compared with people infected with earlier versions of the coronavirus.

— In a preprint report from Singapore, people infected with delta were 4.9 times more likely to require oxygen treatment, be admitted to an ICU or die of COVID-19 than were people infected in the initial months of the pandemic.

— In a peer-reviewed study from Scotland, the odds of needing hospital care were 85% greater for people with delta infections than for people infected with other versions of the virus.

In another potentially ominous sign, the CDC document also noted that among people who had already weathered a coronavirus infection at least 180 days earlier, the risk of a subsequent infection with the Delta variant was about 46% higher than the risk of a subsequent infection with the Alpha variant.

Studies from Britain, Canada and Israel suggest the Pfizer-BioNTech vaccine loses some effectiveness when it goes up against delta. Although its ability to reduce the risk of coronavirus infections and COVID-19 cases was diminished, the vaccine was highly effective at reducing the risk of hospitalization — by 96% in England and Scotland, by 100% in Canada and by 93% in Israel.

Overall, the COVID-19 vaccines authorized for use in the U.S. are making an impact, the CDC document says.

As of July 24, there were 178.6 new cases per week per 100,000 unvaccinated people, compared with 21.4 new cases per week per 100,000 vaccinated people — an eightfold reduction. The weekly incidence of COVID-19 hospitalization and death were both 25 times lower in those who were vaccinated than in those who were not.

But with only 49.4% of the U.S. population fully vaccinated, other measures are necessary to curb the delta variant’s spread, the CDC document says.

"Given higher transmissibility and current vaccine coverage, universal masking is essential to reduce transmission of the Delta variant," it says.

The document also calls on officials to "consider vaccine mandates for [health care professionals] to protect vulnerable populations."

———

Los Angeles Times staff writer Deborah Netburn contributed to this report.

———

©2021 Los Angeles Times. Visit latimes.com.