What We Know About the Dangerous Delta Variant

A treatment tent outside the emergency department at Holmes Regional Medical Center in Melbourne, Florida, on July 29. The tent was set up to serve as an overflow area as the number of COVID-19 infections surges throughout Brevard County because of the Delta variant and large numbers of unvaccinated residents. Photo: Paul Hennessy/SOPA Images/LightRocket via Getty Images

The Delta (B.1.617.2) coronavirus variant originally discovered in India last December has now become the most dominant — and worrisome — strain of the coronavirus circulating globally. The most transmissible variant yet, Delta is roughly twice as contagious as the original COVID strain, and as much as 60 percent more contagious than the Alpha (U.K./B.1.1.7) variant, which itself caused numerous waves of the pandemic around the world. As of the end of July, Delta has been detected in nearly every country in the world and has fueled rapid outbreaks in places where vaccination rates are low. Delta has also quickly become the dominant strain in the U.S., where it is estimated to account for at least 93 percent of all new sequenced cases in the country. Delta’s rapid spread among the unvaccinated in the U.S. has prompted the CDC to once again advise that everyone wear face masks indoors in public wherever there is significant or higher transmission of the variant. And there is evidence that Delta may come with a higher risk of severe illness for the unvaccinated. Though COVID vaccines continue to offer strong protection against Delta — and particularly against severe illness and death from the strain — the variant is believed to be at least partially able to avoid antibodies produced by a previous coronavirus infection or vaccination. Below is what we know about the Delta variant.

How is Delta different from other variants, and why is it more dangerous?

The Delta variant “is faster, it is fitter, [and] it will pick off the more vulnerable more efficiently than previous variants,” warned Dr. Mike Ryan, the executive director of the World Health Organization’s Health Emergencies Program, on June 21.

More specifically, Delta has multiple mutations that appear to give it an advantage over other strains, the most important of which is its higher transmissibility — roughly twice as contagious as the original strain of the coronavirus which originated in Wuhan, China. Data from the U.K.’s Delta outbreak indicated that the variant is as much as 60 percent more contagious than the Alpha variant, the previously dominant, most contagious strain in the U.K., U.S., and many other countries. A study published in the journal Nature found that the viral load in people infected with Delta, a measure of the density of viral particles in the body, is roughly 1,000 times higher than in people infected from previous versions of the coronavirus. “Putting it all together, Delta’s really difficult to stop,” Benjamin Cowling, an epidemiologist at the University of Hong Kong who was not involved in the study, told Nature.

The CDC has also expressed concern that in the unlikely event a fully vaccinated person develops a symptomatic so-called “breakthrough” infection from Delta, that person may be as contagious as an unvaccinated person with a Delta infection.

It is also possible that the Delta variant causes more severe illness than other variants for the unvaccinated. The CDC has said Delta is “likely more severe.” U.K. data has suggested Delta is more likely to lead to hospitalization than Alpha, but that could be due to increased transmissibility rather than it being more pathogenic. Per the Guardian:

An analysis of 38,805 sequenced cases in England revealed that the Delta variant was associated with a 2.61 times higher risk of hospitalisation within 14 days of specimen date than the Alpha variant. There was a 1.67 times higher risk of A&E care within 14 days. These figures take into account factors such as age, sex, ethnicity, area of residence and vaccination status. Data from Scotland supported the findings, also pointing to a more than twofold higher risk of hospitalisation for those infected with the Delta variant compared with the Alpha variant.

Here is how Delta compares with other variants of concern:

Vaccination provides the best protection against Delta, but vaccines appear to be slightly less effective against it

Research continues to indicate that full vaccination provides strong protection against the Delta variant, and is particularly effective at preventing serious illness or death from the strain.

One set of U.K. government research found that two doses of a COVID vaccine provided 81 percent protection against the B.1.617.2 variant (compared with 87 percent protection against the B.1.1.7 variant). One dose only provided 33 percent protection against symptomatic infection from B.1.617.2 (compared with 51 percent protection against B.1.1.7). That means, according to a Financial Times analysis, that a single dose is 35 percent less effective against Delta than it is against Alpha. In other words, Delta may not only pose the biggest threat to the unvaccinated, but to the partially vaccinated, as well.

Public Health England recently found that Pfizer’s COVID vaccine was 96 percent effective, and AstraZeneca’s vaccine 92 percent effective, at preventing hospitalization from the Delta variant.

In South Africa, a Sisonke trial found that the Johnson & Johnson vaccine provided a high level of protection against the Delta variant. The study included nearly 480,000 health-care workers. The study’s co-lead Glenda Gray said the single-shot vaccine was 71 percent effective in preventing hospitalization and potentially 96 percent effective against death.

Again, as with every known variant, full vaccination works against the Delta strain, preventing serious illness at the very least — but there are still signs that the variant marks a worrisome evolution in the coronavirus, and it seems likely it could raise the stakes for countries that continue to struggle to vaccinate their populations.

In a study released in Nature on July 8, French researchers tested unvaccinated people who had survived a bout of COVID-19 in order to study how well antibodies produced by natural infection, as well as vaccines, can neutralize Delta. Antibodies from those who were previously infected did not neutralize Delta very well, but a single dose of Pfizer or AstraZeneca dramatically boosted their antibody levels.

“This is an important study for confirming the immune evasiveness property of Delta, which is a feature that adds to its enhanced transmissibility, making it the most formidable version of the virus to date,” Eric Topol, director of the Scripps Research Translational Institute, told STAT News. “No surprises, but further characterization of the variant, which reinforces why it is so challenging.”

The U.K. — which has 75 percent of its total population fully vaccinated — recently experienced a drop in new case numbers, leading some scientists to hope that the country has reached the peak of this recent wave fueled by Delta.

Delta has become the dominant strain in the U.S.

The Delta variant has quickly become the dominant strain in the U.S., overtaking the Alpha variant, which has been the most prevalent COVID strain in the States for months. By July 31st, it accounted for more than 93 percent, up from 1.3 percent in early May and 9.5 percent in early June.

Though hundreds of millions of COVID vaccine shots have been administered in the U.S., full vaccination rates still vary across the country; areas where fewer shots have been administered could feel this highly transmissible strain’s impact. “I’m really holding my breath about the South and what happens over the summer,” Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children’s Hospital, recently told CNBC. There is also some evidence that young Americans are beginning to lose interest in getting vaccinated. By mid-July, hospitalizations began to increase in several states hit hard by Delta.

Low-income and minority communities have a deficit in vaccinations as well, making them all the more vulnerable.

Former FDA commissioner Scott Gottlieb has noted that he is particularly worried that Delta might fuel an epidemic come the fall. And here is what Ashish K. Jha, the dean of Brown’s School of Public Health, warned in a June 16 Washington Post op-ed:

We are entering a time when being unvaccinated is going to become exceedingly more dangerous. Society is open. Distancing is a thing of the past, and mask-wearing is declining. All of the public health protections that kept unvaccinated people safe are disappearing, but the delta variant is gaining momentum. In some states, such as Mississippi and Wyoming, vaccination rates mean that covid cases are likely to spike this summer and fall. Even in highly vaccinated places, the delta variant may trigger the occasional outbreak. The difference will be in hospitalizations and deaths. For the vaccinated, breakthrough infections will be inconvenient, annoying or maybe even miserable. But rarely deadly. The unvaccinated will be far more vulnerable.

In an interview with New York’s David Wallace-Wells, Scripps Research Institute director Eric Topol, emphasized that vaccines have been shown effective against the Delta variant in the U.S.:

While American caseloads have been growing at alarming rates in certain parts of the country, the effects of vaccination are just as striking here. In recent weeks, amid all the alarm over Delta, the seven-day rolling average of American deaths per million has, according to Our World in Data, ticked up from a nadir of 0.66 on July 8 to 0.74 today. It was over two as recently as late May, and over ten in January. In Los Angeles, where new mask mandates are being issued and in-door dining has been temporarily suspended, new cases have grown tenfold since mid-June. The seven-day average of deaths has in fact fallen from 6.3 to 5.9, though some amount of future rise is likely, since death peaks invariably trail case peaks. An NYU modeling team recently predicted that New York City might see 900 deaths in the Delta wave; in its horrific first wave, last spring, the city recorded more than 20,000.

Delta is spreading in New York City

The Delta variant appears to be causing an uptick in cases in the former epicenter of the global outbreak, where much of daily life has returned to something resembling pre-pandemic norms. Delta now accounts for at least 83 percent of new cases in New York City, as of August 6. While the number of new daily COVID cases has more than doubled across recent weeks, so far the trend of new daily hospitalizations and deaths from COVID-19 have remained mostly flat. After the supercharged variant spurred new cases, New York City Mayor Bill de Blasio announced what is believed to be the first government-issued mandate in the United States requiring proof of COVID-19 vaccination for people taking part in indoor activities in the city.

Why is the B.1.617.2 variant now being called ‘Delta’?

On May 31, the World Health Organization announced that it would give new designations to COVID variants of concern using the Greek alphabet, both because of confusion over the “alphabet soup” names currently in wide use and to prevent variants from being referred to based on where they were first discovered (i.e., the U.K., South Africa, or India variants), a practice that runs the risk of creating harmful stigmas about specific countries and that may become confusing if more than one variant of concern originates in a single country. The WHO has designated the B.1.617.2 variant as Delta.

Scientists will continue to use the more complicated alphanumeric names for variants, as they always have, but the WHO hopes that the Greek-letter-based names will become the widely used ones among nonscientists.

What is the Delta Plus variant?

The Delta variant spawned a new, slightly altered variant of its own, dubbed Delta Plus. India was the first country to find the mutated version — just as it was the first to find the original Delta variant — but it’s spreading in a number of countries including the United States and the United Kingdom. Data from Scripps Research’s Outbreak.info tracker suggests 509 Delta Plus cases have been detected worldwide. As of July 23, there were 70 cases of the Delta Plus variant in India, according to the Hindustan Times. At least 39 have been discovered in the United Kingdom. Last week, South Korean officials said they confirmed two Delta Plus cases. One death linked to the Delta Plus variant has been reported so far, in India, where authorities have deemed the strain a “variant of concern.” As of August 6, the Santa Clara County Public Health Department has reported 46 cases of Delta Plus in its region of California.

Delta Plus contains an additional mutation called K417N, raising concerns that it could be more transmissible. But that may not be the case: “I predict 417 is not an important enough mutation. Delta is bad enough as it is, and I don’t think 417 will change [it] that much or become dominant,” Ravindra Gupta, a professor of clinical microbiology at the Cambridge Institute for Therapeutic Immunology and Infectious Diseases who has been genetically sequencing COVID-19, told Time magazine.

Why is a more transmissible variant more dangerous?

In a May 28 op-ed for the New York Times, Zeynep Tufekci succinctly broke down the threat of increased transmissibility:

A variant with higher transmissibility is a huge danger to people without immunity either from vaccination or prior infection, even if the variant is no more deadly than previous versions of the virus. … A more transmissible variant can burn through such an immunologically naïve population very fast.

Increased transmissibility is an exponential threat. If a virus that could previously infect three people on average can now infect four, it looks like a small increase. Yet if you start with just two infected people in both scenarios, just 10 iterations later, the former will have caused about 40,000 cases while the latter will be more than 524,000, a nearly 13-fold difference.

This is why allowing the coronavirus to spread and evolve unchecked is so dangerous

Delta is yet more proof of both how SARS-CoV-2 continues to evolve and how that evolution is continuing to produce variants that are more dangerous than those that came before them.

The best way to prevent new variants from evolving is to give the coronavirus fewer opportunities to evolve by preventing and containing outbreaks with effective precautions like vaccination, face masks, and proper ventilation.

This post has been updated throughout to include new information.

What We Know About the Dangerous Delta Variant