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The Delta Variant: What Scientists Know

The variant is spreading rapidly worldwide and fueling new outbreaks in the U.S., mainly among the unvaccinated.

Deserted streets in the central business district in Sydney, Australia, as lockdowns were reinstated in recent weeks.Credit...Joel Carrett/EPA, via Shutterstock

The spread of the super-contagious Delta variant has prompted new restrictions around the world and spurred stark new warnings from public health officials.

“The Delta variant is more aggressive and much more transmissible than previously circulating strains,” Dr. Rochelle P. Walensky, director of the Centers for Disease Control and Prevention, said at a White House briefing last week. “It is one of the most infectious respiratory viruses we know of and that I have seen in my 20-year career.”

An internal C.D.C. document, a copy of which was obtained by The New York Times, said that Delta was as transmissible as chickenpox. Although the vaccines continue to provide strong protection against severe disease and death, fully vaccinated people can be infected by, and transmit, Delta, the agency noted. Given these facts, it was time to “acknowledge the war has changed,” the document said.

This week, these observations prompted the C.D.C. to change the recommendations it released in May, which had said that fully vaccinated people did not need to wear masks indoors. The agency now says that in virus hot spots, everyone, regardless of vaccination status, should wear masks in indoor public spaces. It also recommended universal masking in schools this fall.

The World Health Organization, citing the rise of Delta, the dearth of vaccines and high rates of community transmission in many parts of the world, has also encouraged fully vaccinated people to continue wearing masks.

First identified in India, Delta is one of several “variants of concern,” as designated by the C.D.C. and the World Health Organization. It has spread rapidly across the world and poses a particular threat in places where vaccination rates remain low.

The variant is now responsible for more than 80 percent of infections in the United States, largely among unvaccinated people, according to the C.D.C. It is already fueling new outbreaks in states with low vaccination rates, including Missouri, Nevada and Arkansas.

The Delta Variant

An internal document from the Centers for Disease Control and Prevention notes that the Delta variant is much more contagious than other known versions of the virus.

Fatality rate

(log scale)

100%

Bird flu

Ebola

50

Smallpox

More

deadly

MERS

20

SARS

10

5

1918 flu pandemic

2

Spreads faster

1

Delta variant

More transmissible than Ebola or smallpox, and as contagious as chickenpox.

Original

version of

coronavirus

Measles

0.1

Seasonal

flu

Polio

Chickenpox

2009 flu

Common cold

0

1

5

10

15

Average number of people infected by each sick person

Fatality rate

(log scale)

100%

Bird flu

Ebola

50

Smallpox

MERS

More

deadly

20

SARS

10

5

1918 flu pandemic

2

Spreads faster

1

Delta variant

More transmissible than Ebola or smallpox, and as contagious as chickenpox.

Original version of corona-

virus

Measles

0.1

Seasonal

flu

Polio

2009

flu

Common

cold

Chickenpox

0

1

5

10

15

Average number of people infected by each sick person

Fatality rate

(log scale)

100%

Bird flu

50

Ebola

More

deadly

Smallpox

MERS

20

SARS

Spreads faster

10

5

1918 flu pandemic

2

Delta

variant

As infectious as chickenpox.

1

Original

version of

coronavirus

Measles

0.1

Seasonal

flu

Polio

Common

cold

Chickenpox

0

1

5

10

15

Avg. number of people infected by each sick person

By The New York Times | Note: Average case-fatality rates and transmission numbers are shown. Estimates of case-fatality rates can vary, and numbers for the coronavirus are preliminary estimates.

Australian cities facing outbreaks of the variant instituted lockdowns, Malaysia extended stay-at-home orders, Ireland delayed plans to reopen indoor dining and Hong Kong restricted incoming flights from Britain, where Delta is widespread. On Thursday, Italy announced that it would begin requiring either proof of vaccination or a recent, negative test in order to dine indoors, visit museums or participate in other activities — a measure prompted, in part, by the Delta variant.

Roughly 60 percent of American adults have been fully vaccinated, and several widely used vaccines in the United States appear to provide good protection against Delta. But children under 12 are not yet eligible for the vaccine, and vaccination rates have been highly uneven across the country.

“If you are not vaccinated, please take the Delta variant seriously,” Dr. Walensky said at the briefing. “This virus has no incentive to let up, and it remains in search of the next vulnerable person to infect. Please consider getting vaccinated and take precautions until you do.”

Here are answers to some common questions about the Delta variant.

Delta, formally known as B.1.617.2, is believed to be the most transmissible variant yet, spreading more easily than both the original version of the virus and the Alpha variant first identified in Britain. It is believed to be roughly twice as contagious as the original virus.

People who are infected by Delta may carry 1,000 times more virus, and for a longer period of time, than those infected by the original virus.

Other evidence suggests that the variant may be able to partially evade the antibodies made by the immune system after a coronavirus infection or vaccination. And the variant may also render certain monoclonal antibody treatments less effective, the C.D.C. notes.

Delta is also “likely more severe,” the agency said in its internal document.

A recent Scottish study, for instance, found that people infected by the Delta variant were roughly twice as likely to be hospitalized as those infected with Alpha. Studies in Canada and Singapore have turned up similar findings.

Delta has been reported in 182 countries, and is now the most common variant in many of them, including in India and Britain. Over the past month, infections have nearly doubled in most regions of the world, the W.H.O. said at a news briefing on Friday. “Much of this increase is being driven by the highly transmissible Delta variant,” Dr. Tedros Adhanom Ghebreyesus, the director general of the agency, said at the briefing.

Delta was first identified in the United States in March. It spread quickly. In early April, Delta represented just 0.1 percent of cases in the United States, according to the C.D.C. By early May, the variant accounted for 1.3 percent of cases, and by early June, that figure had jumped to 9.5 percent. The C.D.C. now estimates that the number has hit 82.2 percent.

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A pedestrian walks past an electronic information board warning of a “variant of concern” circulating in Blackburn, England.Credit...Oli Scarff/Agence France-Presse — Getty Images

It’s not clear yet. “We’re hurting for good data,” said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, in June.

In Britain, where the variant is widespread, reports have emerged that Delta may cause different symptoms than other variants do. Researchers conducting the Covid Symptom Study, which asks people with the disease to report their symptoms in an app, have said that the most common symptoms of Covid have changed as the variant has spread through Britain.

“What we’ve noticed is the last month, we’re seeing different sets of symptoms than we were seeing in January,” Tim Spector, a genetic epidemiologist at King’s College London who leads the study, said in June.

Headaches, a sore throat and a runny nose are now among the most frequently reported symptoms, Dr. Spector said, with fever, cough and loss of smell less common.

These data, however, have not yet been published in a scientific journal, and some scientists remain unconvinced that the symptom profile has truly changed. The severity of Covid-19, regardless of the variant, can vary wildly from one person to another.

“I’ll wait for published data before I make a conclusion,” Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, said last month. “The fact is Covid is generally associated with a wide variety of symptoms, so it’s hard to say if this is truly unusual or if this is anecdotal.”

Even if the data hold up, it does not necessarily mean that Delta itself causes different symptoms than other variants do. A milder symptom profile could be a result of the fact that the variant is primarily infecting younger people, who are the least likely to be vaccinated, or those who may already have some immunity to the virus from a previous infection, for example.

Although there is not yet good data on how all of the vaccines hold up against Delta, two doses of several widely used shots, including those made by Pfizer-BioNTech, Moderna and AstraZeneca, appear to retain most of their effectiveness against the Delta variant, research suggests.

Delta may cause more frequent breakthrough infections, or infections in people who have been fully vaccinated, than other variants. However, these infections tend to be mild or asymptomatic. The vaccines prevent more than 90 percent of severe disease, the C.D.C. document notes.

“Being fully vaccinated gives you a high degree of protection against infection, and an even higher degree of protection against severe illness, hospitalization and death,” Dr. Walensky said at last week’s briefing. “That is what these vaccines were designed for.”

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A man is given the a Covid-19 vaccine in Chelsea, Mass. Credit...Joseph Prezioso/Agence France-Presse — Getty Images

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 and 95 percent against the original version of the virus. 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,” Dr. Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, said in an interview last month. “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.”

Data on Johnson & Johnson’s single-shot vaccine have been more limited and mixed. Earlier this month, the company reported the results of a small study indicating that its vaccine retained most of its effectiveness against Delta.

But a recent study posted online, which has not yet been peer-reviewed, suggested that the company’s vaccine may be much less potent against Delta than against the original strain of the virus. Boosters may ultimately be needed, the study’s authors said.

American health officials have said that boosters are not recommended at this time but are increasingly acknowledging that people over 65 and those with compromised immune systems may ultimately need another shot, even if they received the Pfizer or Moderna vaccines.

The C.D.C. believes it’s possible. Although vaccinated people are less likely to become infected by Delta than those who are unvaccinated, those who do contract the virus may carry just as much of the virus in their noses and throats as unvaccinated people, Dr. Walensky said this week.

“Delta variant vaccine breakthrough cases may be as transmissible as unvaccinated cases,” the internal C.D.C. document said.

A new C.D.C. study, published on Friday, documented a Covid-19 outbreak among people who attended summer events or large public gatherings in Barnstable County, Ma. Among the 469 cases identified among Massachusetts residents, 74 percent occurred in people who had been fully vaccinated. (Nearly all of the infections were caused by the Delta variant, sequencing suggests.) On average, they carried roughly the same amount of virus as unvaccinated people, researchers found.

After a long and steady decline, cases are on the rise again in the United States, likely fueled by Delta. Roughly 71,000 new infections are reported each day, up from about 11,000 a day a little over a month ago.

But case numbers remain far below last winter’s peak, and experts do not expect them to rise that high again. “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 last month.

However, Delta is driving outbreaks in areas where vaccination rates are low.

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

And because many of the most vulnerable Americans — including older adults and those with chronic health conditions — have been vaccinated, hospitalization rates are increasing more slowly than the number of total infections.

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,” Saskia Popescu, an infectious disease epidemiologist at George Mason University, said last month. “So it’s really about ensuring local, national and global vaccination.”

Face masks, which remain a particularly important tool for those who are ineligible for or do not have access to vaccines, can provide additional protection. The C.D.C. now recommends that even fully vaccinated people wear masks in some circumstances: in public indoor spaces in areas of the country in which cases are surging; if they have compromised immune systems; or if they live with young children, older adults or others who may be especially vulnerable to the virus.

“We are fighting the same virus, but a virus that’s become fitter and better adapted to transmitting among us humans,” Dr. Michael Ryan, executive director of the W.H.O. health emergencies program, said at a news briefing on Friday. “That’s the change. But the way in which we fight the virus hasn’t changed. And vaccines and all of the other measures still remain hugely effective if applied in a comprehensive fair, equitable, and rapid manner.”

A version of this article appears in print on  , Section D, Page 4 of the New York edition with the headline: The Delta Variant: What Experts Know. Order Reprints | Today’s Paper | Subscribe

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