Covid’s Delta Variant: What We Know


The super-contagious Delta variant of the coronavirus is now responsible for about one in every five Covid-19 cases in the United States, and its prevalence has doubled in the last two weeks, health officials said on Tuesday.

First identified in India, Delta is one of several “variants of concern,” as designated by the Centers for Disease Control and Prevention and the World Health Organization. It has spread rapidly through India and Britain.

Its appearance in the United States is not surprising. And with vaccinations ticking up and Covid-19 case numbers falling, it’s unclear how much of a problem Delta will cause here. Still, its swift rise has prompted concerns that it might jeopardize the nation’s progress in beating back the pandemic.

“The Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate Covid-19,” Dr. Anthony S. Fauci, the nation’s leading infectious disease expert, said at the briefing. The good news, he said, is that the vaccines authorized in the United States work against the variant. “We have the tools,” he said. “So let’s use them, and crush the outbreak.”

The Delta variant is unlikely to pose much risk to people who have been fully vaccinated, experts said.

“If you’re fully vaccinated, I would largely not worry about it,” said Dr. Ashish K. Jha, dean of the Brown University School of Public Health.

According to one recent study, the Pfizer-BioNTech vaccine was 88 percent effective at protecting against symptomatic disease caused by Delta, nearly matching its 93 percent effectiveness against the Alpha variant. But a single dose of the vaccine was just 33 percent effective against Delta, the study found.

“Fully immunized individuals should do well with this new phase of the epidemic,” said Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. “However, the protection offered by a single dose appears low, and of course if you are not at all vaccinated, consider yourself at high risk.”

Delta is likely to infect “large numbers” of unvaccinated people, he said.

The pandemic is waning in the United States, with cases, hospitalizations and deaths all on the decline. The seven-day case average, roughly 10,350 a day, is the lowest since March 2020, Dr. Rochelle Walensky, director of the C.D.C., said at the briefing on Tuesday. “These numbers demonstrate the extraordinary progress we’ve made against a formidable foe,” she said.

So while Delta may account for an increasing percentage of cases, it is not yet clear whether it will drive the total number of cases higher.

“I think we are not going to see another big, national surge in the United States because we have enough vaccination to prevent that,” Dr. Osterholm said.

Still, vaccination rates have been highly uneven, and are lower in certain states and demographic groups. Delta could fuel outbreaks in the South, where vaccinations lag, or among young people, who are less likely to be vaccinated than their elders.

“In places where there’s still a lot of susceptibility to the virus, it opens a window for cases to start going up again,” said Justin Lessler, an infectious disease epidemiologist at Johns Hopkins University. “But even in those states, and certainly nationally, we’re probably not getting back to the numbers we were seeing last winter.”

Still, he said, it could prolong our path out of the pandemic. “It continues the doldrums,” he said.

Get vaccinated. If you’re already vaccinated, encourage your family, friends and neighbors to get vaccinated. Vaccination is likely to slow the spread of all the variants and reduce the odds that new, even more dangerous variants emerge.

“I encourage people who are vaccinated to trust in the vaccines but be cognizant that new variants will continue to occur where transmission exists,” said Saskia Popescu, an infectious disease epidemiologist at George Mason University. “So it’s really about ensuring local, national and global vaccination.”



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