The Delta variant appears to be more transmissible than any other coronavirus strain.
Some experts worry the variant could result in more breakthrough cases in vaccinated people, especially those who have had only one dose of vaccine.
Right now, the Delta variant does not seriously threaten anyone who has had two vaccine doses.
Scientists have long worried about a coronavirus variant that's more dangerous than the original virus in three key ways: It would be more transmissible, result in more serious illness, and evade protection from existing vaccines.
"The nightmare here is a variant that checks off all three boxes," said Bob Wachter, the chair of the Department of Medicine at the University of California, San Francisco.
No prior variant, he said, has checked more than one or two. But the Delta variant, first identified in India in February, has come closest to checking all three.
Right now, two doses of vaccine are at least greater than 88% effective at preventing serious cases of COVID-19, of the type that might put you in hospital, even from the Delta variant. However, a single shot is only about 33% effective in protecting patients from that level of harm, according to studies of Delta variant.
"The data today says that this variant gets a full checked box for more infectious, probably gets a checked box for more serious, and at least gets a partial checked box for immune evasion. And that's scary," Wachter said.
The Centers for Disease Control and Prevention labeled Delta a "variant of concern" on Tuesday.
"Delta is a superspreader variant, the worst version of the virus we've seen," Eric Topol, the director of the Scripps Research Translational Institute, tweeted on Tuesday.
For the most part, however, Delta hasn't drastically challenged vaccines. Public Health England analyses have found that two doses of Pfizer's vaccine are still 96% effective at preventing hospitalizations - and 88% effective at preventing symptomatic COVID-19 - from Delta cases. Two doses of AstraZeneca's vaccine, meanwhile, are around 92% effective at preventing hospitalizations and 60% effective at preventing symptomatic COVID-19 from Delta.
But that efficacy does not come after just one dose: A single shot of either Pfizer's or AstraZeneca's vaccines were just 33% effective at preventing symptomatic COVID-19 from Delta.
"The fact that three weeks after your first dose you're only 30% protected - versus, in the original, you were 80% - says that this thing has figured out how to at least partly evade the immune system," Wachter said.
It's also possible, he added, that vaccine protection could "wear off more quickly."
Delta is the most transmissible strain yet
While Delta undoubtedly poses the biggest threat to unvaccinated people, some experts worry that it may result in more breakthrough infections - cases of COVID-19 diagnosed at least two weeks after someone is fully vaccinated.
"That's the concern - that you're more likely to get COVID from the same exposure than you would have been before," Wachter said. "And you're more likely, if you have COVID, to have a more serious case."
Other experts are also afraid the strain may further evolve into something more dangerous, since Delta's high transmissibility enables it to spread easily among unvaccinated people, and therefore to keep replicating and mutating.
"The worst-case scenario is if Delta mutates into something completely different, a completely different animal, and then our current vaccines are even less effective or ineffective," said Vivek Cherian, an internal-medicine physician in Baltimore.
Research from Public Health England suggests that the Delta variant is associated with a 60% increased risk of household transmission compared to Alpha - the variant discovered in the UK. Alpha is already around 50% more transmissible than the original strain, the CDC said.
In other words, "Alpha is to the original as Delta is to Alpha," Wachter said.
Researchers in Scotland, meanwhile, found that getting infected with Delta doubles the risk of hospital admission relative to Alpha.
Does Delta make breakthrough infections more likely?
Although variants are responsible for the majority of breakthrough infections, it's very rare to get COVID-19 after being fully vaccinated: A May CDC report found that just 0.01% of vaccinated Americans got sick.
Even when it comes to Delta, Cherian said, "my guess is you don't really have to worry about breakthrough infections."
But Wachter worries that Delta could turn a mild breakthrough case into a more serious one.
"It increases the risk that we're going to see more breakthrough infections and maybe more serious breakthrough infections than I would have worried about a few weeks ago," he said.
The biggest risk may be for elderly or immunocompromised people, he added.
"The 80-year-old who's been fully vaccinated - their level of immunity is not the same as a 30-year-old," Wachter said.
Delta could threaten our return to normal life
At the moment, Delta accounts for 10% of US coronavirus infections, but scientists expect it to become the dominant strain within weeks. Wachter said he would "start acting much more carefully" if Delta came to represent one out of every three or five COVID-19 cases in a given region.
"If I had gotten comfortable with being inside without a mask in a place where I wasn't sure that everybody's vaccinated, I would now be uncomfortable," he added.
Cherian, on the other hand, doesn't think Delta warrants that level of caution yet - though most experts still worry that a more concerning variant could arise out of the fast-mutating strain.
"It is a perfectly human instinct to feel now we have weathered this terrible 18 months, and now we are out of it and over it," Wachter said. "I hope that's true, and it may turn out to be true. But the chances of that not being true, and that we're going to have more in our future to deal with, have gone up considerably in the last few weeks because of Delta."
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