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7 Things to Know About the Delta Variant

A new coronavirus mutation is spreading in the U.S. Here's what that means for you.

computer illustration of coronavirus delta variant mutation concept

Bill Oxford/Getty Images

En español | Cases of COVID-19 are once again on the rise in the U.S., with close to 100,000 new infections being reported daily. And a new strain of the coronavirus is at the center of this summer surge.

All viruses mutate and they mutate often, says Andrew Pekosz, professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health. But when a mutation causes a virus to change its behavior — maybe the virus becomes more contagious or more lethal than previous versions — it gets classified as a variant.

Delta — the World Health Organization (WHO) names coronavirus variants after Greek letters — is the latest variant of concern in the U.S. Here's what you need to know about it.

1. Delta is spreading quickly

One thing that sets the delta variant apart from other coronavirus strains is the speed at which it's spreading. In just a few months’ time, the delta variant went from being nonexistent in the U.S. to now making up the vast majority (about 83 percent) of new COVID-19 cases, according to data from the Centers for Disease Control and Prevention (CDC).


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It's a pattern that's played out in other countries, as well — most notably India, where delta was first identified, and the United Kingdom. “As soon as [the delta variant] enters a country, it starts a real upward trajectory and eventually becomes the dominant virus, or at least the majority of virus circulating in that country,” Pekosz says. “So that’s telling us that there’s something about this virus that’s making it better to transmit in the population, because it’s given [other variants] a head start, and it’s still able to catch up and overtake them in terms of the numbers of infected people.”

2. Delta may cause different symptoms

Some reports suggest that the delta strain could cause different symptoms than other variants. A U.K. study that tracks COVID-19 symptoms through an app, for example, reported a change in the top-ranked symptoms since delta starting dominating.

Cold-like symptoms, including headache, runny nose and a sore throat, now top the list in the ongoing study, while more traditional COVID-19 symptoms — loss of smell, shortness of breath, fever and persistent cough — have since moved down. And doctors in the U.S. are noting a similar trend, especially in areas that have some of the highest rates of delta cases, NBC News reports.

One possible explanation for the shake-up in symptoms: Younger people are less likely to get severe disease from a coronavirus infection than older adults, says Lisa Gralinski, an assistant professor of epidemiology at the University of North Carolina's Gillings School of Global Public Health. They are also less likely to be vaccinated. About 80 percent of Americans 65 and older are fully vaccinated, whereas about 8 percent of 18- to 24-year-olds and about 20 percent of 25- to 39-year-olds are fully vaccinated.

Then there’s the question of whether the new set of symptoms is “something that's more intrinsic to this variant that's on the rise,” Gralinski notes. “I don't think we have enough information to really know that yet.”

3. The variant could be to blame for severe disease in some communities

There isn't solid evidence to indicate that the delta variant is deadlier than other coronavirus strains, says Wafaa El-Sadr, M.D., professor of epidemiology and medicine at Columbia University. Some research suggests it may be, including a study out of Scotland that found risk for COVID-related hospitalizations from a delta infection was nearly double compared to an infection from the alpha variant in unvaccinated individuals. And in the U.S., hospitalizations and deaths are increasing in unvaccinated communities, largely driven by the spread of delta. Still, the research is ongoing.

4. COVID vaccines provide strong protection against delta

The delta variant “evades a little bit of the immune response that's generated after vaccination,” Gralinski says. Even so, it’s no match for the authorized vaccines, which experts say provide a high level of protection from severe illness from delta and other variants circulating in the U.S. So far, less than 1 percent of fully vaccinated individuals have been hospitalized or have died from COVID-19, CDC data show.

Variants of concern in the U.S.

  • Alpha (B.1.1.7)
  • Beta (B.1.351)
  • Gamma (P.1)
  • Delta (B.1.617.2)

These variants seem to spread more easily than other coronavirus strains, according to the CDC. And ease of transmission may lead to more cases of COVID-19, which will put more strain on health care resources and lead to more hospitalizations and potentially more deaths, the CDC says.

Pointing to a handful of studies in a recent news briefing, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a recent briefing that after both doses, the mRNA vaccine from Pfizer-BioNTech is about 80 percent effective at preventing infection from the delta variant, 88 percent effective at preventing symptomatic disease and 96 percent effective at preventing hospitalization caused by delta. New data out of Israel, however, shows lower rates of effectiveness — around 64 percent protection from infection and 93 percent protection from serious illness and hospitalization. (Moderna's vaccine also uses mRNA technology and has shown similar rates of overall effectiveness to the Pfizer-BioNTech in previous studies.)

The key to this protection, however, is full vaccination, Pekosz says. “If you've only gotten one of your mRNA shots, that’s where you can really see the delta variant being able to evade some of those immune responses,” he says. In fact, one dose of the two-dose mRNA vaccines was found to be only 33 percent effective at preventing symptomatic disease caused by the delta variant in a preprint study out of the U.K.

"So if you've got a good strong immune response generated by both doses of the mRNA vaccine, then you should be in a good place,” Pekosz says. “But if you're only partially immune — if you're in between doses or if you take in the first dose and you decide to skip the second dose — then you're in an area where the vaccine may protect you against older strains, but it may not be enough immunity to protect you against the delta variant.”

Johnson & Johnson released a statement on July 1 noting that its one-shot vaccine “generated strong, persistent activity against the rapidly spreading delta variant and other highly prevalent SARS-CoV-2 viral variants.” The studies containing this data are currently being submitted for publication. Hours before J&J's announcement, Fauci said it's reasonable to assume the effectiveness of J&J's vaccine is on par with AstraZeneca's vaccine, which uses the same viral vector technology. The AstraZeneca vaccine, which isn’t authorized for use in the U.S., has been shown to be 60 percent effective at preventing infection and 92 percent effective at preventing hospitalization caused by the delta variant.

Experts will be keeping a close eye on the strength and duration of these vaccines — especially in more vulnerable populations. Although it’s not recommended yet, “highly vulnerable populations might be asked to go in and take a booster that consists of the delta variant or maybe the next variant that comes by, just to make sure that they're maintaining that level of immunity," Pekosz says.

5. You’ll need to keep your mask handy

The CDC announced on July 27 that fully vaccinated individuals should wear a mask in public indoor settings in areas where COVID-19 transmission is “high or substantial” —about 85 percent of the country falls into this category. The new recommendation is a reversal from guidance issued in mid-May. And the reason has to do with delta.

Emerging research shows that while still rare, it is possible for vaccinated people to contract COVID-19. And if they’re infected with the delta variant, they may carry as much virus as an unvaccinated individual. This high viral load, as it’s called, suggests “an increased risk of transmission,” CDC Director Rochelle Walensky said in a statement. Meaning that while a fully vaccinated person with a delta infection may not show symptoms of illness, they could transmit the virus to others, including more vulnerable populations.  

“If you know you're in a situation where you'll be coming in contact with people who might be carrying the virus, it's always best to have another layer of protection, like a mask or some level of social distancing,” Pekosz says.

Timothy Brewer, M.D., a professor of medicine and epidemiology at the University of California, Los Angeles (UCLA), says there's no good evidence to suggest that vaccinated individuals need to mask up when they're outside. And so far, no guidance from the CDC suggests this is necessary.

A key point, experts say, is to pay attention to what's happening in your area. If vaccination rates are high where you are, there's less of a risk. “The more you're exposed to people who are unvaccinated, that possibility of getting infected increases,” El-Sadr says.

Keep this in mind, too, as you travel this summer — especially considering that a significant share of U.S. counties have vaccination rates under 30 percent, according to CDC data.

"If the risk increases, why not do something to help mitigate that risk? Mask wearing is an easy” solution, Pekosz says.


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6. Delta won't be the last variant to pop up

Not only does vaccination help prevent infection and illness from the delta variant and others out there (there are currently four “variants of concern” circulating in the U.S.), but vaccination also helps to keep new and potentially more dangerous variants from popping up.

That's because every time the virus jumps to a new person, its chances of a mutation increase. “But if we break the cycle of transmission and decrease the number of infections in a community, that is the best defense against the evolution and the development of these new variants,” El-Sadr says.

7. Be vigilant, not alarmed

While the delta variant’s rise to dominance is concerning, “there's no reason to be alarmed,” El-Sadr says — especially since the vaccines can protect people from infection and prevent serious illness. “That's important. I don't want people to be absolutely panic stricken about this,” she adds.

The advice of experts: Get vaccinated if you haven't already, and if you're holding out on your second shot in a two-dose series, go back and complete it. Also, if you're sick, stay home — even if you are vaccinated, UCLA's Brewer says. “And remember to wash your hands after you're been out and try to maintain your physical distancing.”

Finally, “use your judgment,” El-Sadr says. “If you're in a crowd that makes you feel uncomfortable, especially when you don't know whether people are vaccinated or not, it's prudent to put a mask on to protect yourself.” Doing so will also protect others around you, should you get infected and unknowingly pass it on, El-Sadr adds.

Editor’s note: This article, originally published July 2, 2021 has been updated to reflect new information.

Rachel Nania writes about health care and health policy for AARP. Previously she was a reporter and editor for WTOP Radio in Washington, D.C. A recipient of a Gracie Award and a regional Edward R. Murrow Award, she also participated in a dementia fellowship with the National Press Foundation.

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