explainer

What We Know About the Dangerous Delta Variant

Pedestrians walk past a sign warning members of the public about the spread of coronavirus in Hounslow, West London, on June 1. Photo: Justin Tallis/AFP 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. Research indicates that it is the most transmissible variant yet — as much as 60 percent more contagious than the Alpha (U.K./B.1.1.7) variant, which itself fueled numerous waves of the pandemic around the world. Delta has already spread to at least 124 countries and prompted explosive outbreaks in countries or areas of countries with low vaccination rates. In the U.S., Delta has quickly become the dominant strain: On July 20, the CDC estimated that Delta now accounts for 83 percent of all new sequenced cases in the country — up from 50 percent at the beginning of the month. On top of all that, Delta may be more likely to infect people who are only partially vaccinated than other strains, and may also come with a higher risk of hospitalization. Below is what we know about the Delta variant.

How is Delta different from other variants, and why may it be 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.

Delta has multiple mutations that appear to give it an advantage over other strains. The most important apparent advantage is that the mutations may make the strain more transmissible than any other variant, which would also make it the most dangerous variant yet. Professor Neil Ferguson, a leading epidemiologist at Imperial College London and one of the chief pandemic advisers to the U.K. government, said on June 4 that Delta is estimated to be 60 percent more transmissible than Alpha, which is itself more transmissible than the original strain of the coronavirus that emerged in China in late 2019 — and that is why scientists believe it became a dominant variant globally. Other estimates from the U.K. have said that Delta may be 40 or 50 percent more transmissible than Alpha.

A new study in the journal Nature found that the viral load, a measure of the density of viral particles in the body, is roughly 1,000 times higher in people infected with the Delta variant than those 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.

There is limited research regarding whether or not the Delta variant causes more severe illness than other variants. According to Public Health England, early data suggests that 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.

This chart, created by cardiologist Eric Topol, provides a simple breakdown of how Delta compares to other variants of concern:

Vaccines are effective against Delta — but may be slightly less effective

Research by the U.K. government has found that full vaccination is effective against the Delta strain but may be slightly less effective than against other variants, particularly after only one dose.

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 B.1.617.2 than it is against B.1.1.7.

If that is accurate, it means that Delta may be the variant that currently poses the biggest threat to partially vaccinated populations worldwide.

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.

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 53 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 7, it accounted for more than 51 percent of sequenced COVID cases in the U.S. By July 20, it accounted for 83 percent.

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. “Less than 10 percent of adolescents are vaccinated in many of these southern states, so we have a real vulnerability here.” 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 69 percent of new cases in New York City, and 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.

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. About 200 cases have been spotted in 11 countries, reports CNN. 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.”

Delta Plus contains an additional mutation called K417N, raising concerns that it could be more transmissible. But that might 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, 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. Residents of countries like Taiwan or Vietnam that had almost completely kept out the pandemic, and countries like India and Nepal that had fared relatively well until recently, have fairly little immunity, and are largely unvaccinated. 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. From the available evidence, B.1.617.2 may be the most transmissible variant yet to spread throughout the world, and thus poses the biggest risk to unvaccinated populations, and possibly also populations where most vaccine recipients have only received one dose. Scientists have good reasons to sound the alarm over it.

And more dangerous COVID strains raise the threat of increasingly threatening variants which may evolve from it. Vietnam’s health ministry has announced that it has detected a variant which appears to be a hybrid of both B.1.1.7 and B.1.617.2 variants. The country has only been able to give at least one dose of a COVID vaccine to about one percent of its population thus far, leaving it highly vulnerable to the new variants, despite faring much better than most of the world at preventing the spread of COVID-19. Now Vietnam is racing to do more testing to see how far the hybrid strain has spread and how it differs from its predecessors. (It should be noted that some scientists are urging restraint before jumping to any conclusions about how bad this — or any — new variant is.)

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 face masks and proper ventilation, and by vaccinating people before they can be exposed to infection in the first place.

This post has been updated throughout to include new information.

What We Know About the Dangerous Delta Variant