What to Know About the Delta Variant

While cases of COVID-19 have fallen significantly since the beginning of the year, the coronavirus is still circulating in the U.S. And a new strain is gaining a foothold. Delta (the World Health Organization names coronavirus variants after Greek letters) is the latest variant of concern in the U.S.

All viruses mutate and they mutate often, says Andrew Pekosz, professor of molecular microbiology and immunology at 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 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 majority of new COVID-19 cases, according to data from the Centers for Disease Control and Prevention (CDC). In some areas of the country, it's responsible for nearly three-fourths of new infections.

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, where it is also the dominant variant. “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.”

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. Doctors in the U.S. are noting a similar trend, especially in areas that have some of the highest rates of delta cases.

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 against delta and other variants circulating in the U.S.

Pointing to a handful of new studies, 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. Pekosz doesn't see the need for a national booster campaign yet but says it's not outside the realm of possibilities that “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.”

Be vigilant, not alarmed – Get fully vaccinated
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.