What experts know so far about the delta variant

The variant is rapidly outcompeting other versions of the coronavirus around the world

a train conductor monitors people boarding a train car

Passengers board a Missouri River Runner Amtrak train on June 11, 2021. The spread of the delta variant is causing concern, especially in places with low vaccination rates such as Missouri.

Charlie Riedel/AP Images

Yet another coronavirus variant has public health officials around the globe scrambling to control its spread.

The delta variant, which first emerged in India, has now spread to more than 80 countries and is quickly becoming the dominant version of the virus (SN: 5/9/21). In places like the United Kingdom, delta has dethroned the highly transmissible alpha variant, which was first identified in that country, as the most common form of the virus.

That rapid spread of the delta variant has forced health officials to react. U.K. officials, for instance, delayed plans to reopen the country, pushing the date back to mid-July. And health officials in Israel, a nation where nearly 60 percent of the population is fully vaccinated, reinstated its requirement that residents wear masks indoors — a public health measure that had been lifted 10 days before. In the United States, places like Los Angeles County recommend that even vaccinated people still wear masks indoors. The World Health Organization also urges everyone to continue wearing masks, though the U.S. Centers for Disease Control and Prevention’s guidelines that vaccinated people can go without masks in most situations remain in place.     

Delta poses the biggest threat to unvaccinated people, the latest studies suggest. In the United States, delta is responsible for an estimated 26.1 percent of cases across the country. Its prevalence is doubling every two weeks. Narrowing in on regions that include states with low vaccination rates like Missouri and Wyoming reveals that delta is already causing the majority of infections in some places. On July 1, the Biden administration announced that teams of experts equipped with testing supplies and therapeutics would be sent to U.S. hot spots to control outbreaks of delta.

The concern is even greater globally. Just 23.4 percent of people around the world have received at least one dose of a COVID-19 vaccine, most of whom reside in wealthy countries. Less than 1 percent of people in lower-income countries have gotten a shot.

As the delta variant takes center stage amid the pandemic, here’s what researchers know so far.

Delta spreads easily.

The coronavirus that causes COVID-19 is still around because it’s been able to adapt well to spread among humans, says Ravindra Gupta, a virologist at the Cambridge Institute of Therapeutic Immunology & Infectious Disease in England.

While the alpha variant is somewhere around 50 percent more contagious than previous versions of the virus, delta appears to have beaten that benchmark (SN: 4/19/21). Data from Public Health England, a U.K. government health agency, suggest that delta may be 60 percent more transmissible than alpha.

“That’s pretty concerning,” says Ravina Kullar an epidemiologist at UCLA and a spokesperson for the Infectious Diseases Society of America.

People who are unknowingly infected with the delta variant are more likely to pass the virus on to someone else, perhaps seven to eight others, Kullar says. “You can just see an outbreak occurring pretty rapidly if someone harbors the delta variant” but is not isolated from others.

The variant can evade parts of the immune system.

The higher chances of spreading delta to other people isn’t the only concern. With delta, “we have a virus that has all these transmission advantages that alpha did,” Gupta says. But delta can also dodge parts of the immune system, which gives it an extra advantage over alpha. “That explains, in our view, why it’s causing problems everywhere,” Gupta says.

For instance, antibodies from both recovered and vaccinated people were less potent at stopping delta from infecting cells than alpha or the original version of the virus from Wuhan, China, Gupta and colleagues report in a preliminary study posted June 22 on Research Square. And when the team analyzed a cluster of COVID-19 cases in health care workers who had been vaccinated with AstraZeneca’s shot at a hospital in India in April, the researchers found that most were infected with delta.

The same was true at two other health care centers in Delhi, a hint that delta may be more likely to infect some vaccinated people, called breakthrough infection, than variants like alpha (SN: 5/4/21).

On the whole, vaccines still seem do their job.

Even amid the threat of breakthrough infections, vaccinations are so far still protecting people from the worst of COVID-19. One preliminary study, for instance, found that COVID-19 vaccines appear to be less effective against delta than some other variants. But two shots are better than one. A single dose of Pfizer or AstraZeneca’s vaccines is around 33 percent effective at preventing symptomatic disease for delta infections three weeks after the shot, researchers report May 24 at medRxiv.org. That’s compared with 55 percent effectiveness against alpha.

A second dose of Pfizer’s jab, however, raised effectiveness against delta to nearly 88 percent against delta, down from 93.4 percent against alpha. A second dose of AstraZeneca’s shot is around 60 percent effective, down from 66 percent against alpha.

Protection from hospitalization is even better, researchers report June 21 in a separate preliminary study from Public Health England. A single dose of Pfizer’s COVID-19 vaccine was 94 percent effective at keeping people out of the hospital after being infected with delta and one dose of AstraZeneca’s was 71 percent effective. Two doses bumped those numbers up to 96 and 92 percent, respectively.

And so far, in highly vaccinated places like the United Kingdom and Israel, for instance, the rise in COVID-19 cases hasn’t yet been linked to a large spike in hospitalizations or deaths. But hospitalizations and deaths tend to lag a couple of weeks behind case increases, so time will tell whether those numbers will go up.

There’s also not yet much information on delta and the effectiveness of vaccines like Johnson & Johnson’s COVID-19 shot, leaving lots of people waiting, Kullar says. One hopeful sign: A preliminary study posted July 1 at medRxiv.org suggests that antibodies sparked by that vaccine still recognize the variant. So the shot should still be effective.

The key point, however, is that the more vaccinated people there are, the less likely it is that delta will cause problems in a community. 

But vaccines don’t protect everyone equally.

The good news is that young, relatively healthy people who are vaccinated are probably going to be OK. But “we are seeing hospitalizations, and we will see deaths, in people who have been vaccinated who are older, who have underlying conditions,” Gupta says. Not all individuals have the same level of protection from the vaccines. What’s more, children younger than 12 still aren’t eligible for vaccination.

Kullar agrees, noting that there are still lots of people who are immunocompromised, such as organ transplant recipients or people on cancer treatments, or elderly people who might still be at risk. Many of these people have “gotten vaccinated, they’ve done all that they can. Now, they’re relying on those other people around them to protect them.”

Experts are watching and waiting for the next variant to appear.

Delta likely won’t be the last variant to pop up amid the pandemic (SN: 5/26/20). While vaccines still protect people now, the chances that a variant that might render them far less effective will emerge goes up as the virus circulates among the unvaccinated.

Variants will continue to emerge as the coronavirus spreads, Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, said at a June 25 news conference. “That’s what viruses do, they evolve, but we can prevent the emergence of variants by preventing transmission. It’s quite simple. More transmission, more variants. Less transmission, less variants.”

Dampening spread to give the virus fewer opportunities to mutate is crucial, Kullar says. “Before we thought [alpha] was concerning, now there is the delta variant, which puts [alpha] to shame. What’s to come next?”

The time to plan for the future of vaccines amid the spread of new variants that can possibly evade the immune system much more effectively than delta or other current forms of the virus may already be here, Gupta says. “This is not the end of the road.”

Erin I. Garcia de Jesus is a staff writer at Science News. She holds a Ph.D. in microbiology from the University of Washington and a master’s in science communication from the University of California, Santa Cruz.

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