Covid’s Delta Variant: What We Know

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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 on Thursday. “It is one of the most infectious respiratory viruses we know of and that I have seen in my 20-year career.”

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.

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.

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

The C.D.C. has given no indication that it will change the recommendations it released in May saying that fully vaccinated people do not need masks. But Dr. Walensky has said that local governments can adopt stricter measures, and some are. Los Angeles County, for instance, has reinstated its indoor mask mandate.

Nearly 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.

Why are people worried about the Delta variant?

Delta, formerly 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.

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

But much is still unknown or unproven, including whether this variant may cause more severe illness. 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. More research and data are needed to assess its effects in comparison to those of other variants.

“The severe disease piece I think is the one question that really hasn’t been answered yet,” Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, said last month.

Where is it spreading?

Delta has been reported in 124 countries, and is now the most common variant in many of them, including in India and Britain.

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 83.2 percent.

Does the Delta variant cause different symptoms?

It’s not clear yet. “We’re hurting for good data,” Dr. Osterholm said.

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.

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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.

If I’m vaccinated, do I need to worry?

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.

Even with Delta, breakthrough infections, or infections in people who have been fully vaccinated, remain relatively rare, scientists believe, and tend to be mild or asymptomatic.

“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 the briefing. “That is what these vaccines were designed for.”

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,” 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.”

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 new study posted online this week, 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, however, have said that boosters are not recommended at this time. “There is no reason to believe right now that people who have taken the J.&J. vaccine are in need of a booster dose,” Dr. Anthony S. Fauci, the nation’s leading infectious disease expert, said at the White House briefing.

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Will Delta return us to last year’s pandemic peak?

After a long and steady decline, cases are on the rise again in the United States, likely fueled by Delta. Roughly 41,000 new infections are reported each day, up from about 11,000 a day 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.

So far, the rising case numbers have been driven primarily by outbreaks in areas where vaccination rates are low. Delta could also prompt outbreaks 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,” Justin Lessler, an infectious disease epidemiologist at Johns Hopkins University, 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.

What can I do?

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.

“Mask wearing remains an effective tool for reducing transmission, especially indoors where the virus may be easily spread through inhalation of aerosols emitted by an infected person,” Barbara Ferrer, the director of public health for Los Angeles County, said in a statement in June.

In a recent interview with The Times, Dr. Ferrer said that the county’s new mask mandate had been prompted by concerns about Delta.

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