explainer

What We Know About the Dangerous COVID B.1.617.2 (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 B.1.617.2 coronavirus variant originally discovered in India last December has now become one the most — if not the most — worrisome strain of the coronavirus circulating globally. Recent research suggests it may be the most transmissible variant yet and has fueled numerous waves of the pandemic around the world. B.1.617.2 has already spread to at least 62 countries. The strain undoubtedly contributed to the massive wave of cases that has inundated India in recent months, and makes up more than 90 percent of new cases in the U.K. and about 10 percent of new infections in the U.S. It’s driven a resurgence of infections, with the U.K. racking up more than 11,000 daily cases for the first time in nearly four months on June 17. On top of that, it may be more likely to infect people who are only partially vaccinated than other strains; low-income and minority communities have a deficit in vaccinations, making them all the more vulnerable. Below is what we know about B.1.617.2 — also known as the Delta variant.

How is B.1.617.2 different from other variants, and why may it be more dangerous?

The Delta variant 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, which would also make it the most dangerous variant yet. One study indicated B.1.617.2 may be up to 50 percent more transmissible than the B.1.1.7 (U.K./Alpha) variant. 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 the “best estimate at the moment” is that Delta is 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. As a wave of infections hit the U.K. in May, Britain’s health minister said the Delta variant was estimated to be 40 percent more transmissible than the Alpha variant.

A study by the Indian government has found that Delta has become, by far, the most dominant strain in that country. The Chinese province of Guangdong has locked down areas to try to control a flare-up of cases largely driven by the highly transmissible variant.

The bottom line is that if the preliminary research is accurate, the Delta variant may soon become the most dominant COVID strain in the world and lead to rapid outbreaks in countries without high vaccination rates.

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. 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 B.1.617.2 compares to other variants of concern:

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.

Vaccines appear to be slightly less effective

Recent research by the U.K. government has found that full vaccination is still largely effective against the Delta strain but may slightly be less effective than against other variants, and even less so after only one dose. The 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 B.1.617.2 may be the variant that currently poses the biggest threat to partially vaccinated populations worldwide.

Again, as with every known variant, full vaccination works against the Delta strain — 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.

The Delta variant may soon be the dominant strain in the U.S.

The Delta variant is quickly on its way to becoming the dominant strain in the U.S., overtaking the Alpha variant, which has been the most prevalent in the States for the past two months. As of June 13, the Delta variant accounted for about ten percent of new infections in the country. According to Scott Gottlieb, former commissioner of the Food and Drug Administration, that share is doubling every two weeks. “That doesn’t mean that we’re going to see a sharp uptick in infections, but it does mean that this is going to take over,” Gottlieb told CBS’s Face the Nation. “I think the risk is really to the fall – that this could spike a new epidemic heading into the fall.”

The former head of the FDA also noted that while the supercharged variant appears to be more contagious, the good news is that the vaccines we have available appear to work well against it. “We have the tools to control this and defeat it, we just need to use those tools,” Gottlieb added.

Still, vaccination rates 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, told CNBC. “Less than 10 percent of adolescents are vaccinated in many of these southern states, so we have a real vulnerability here.”

The Delta strain has already become dominant in the U.K. — and delayed the end of the country’s lockdown

As of June 11, the Delta variant accounted for more than 90 percent — and as much as 96 percent — of all new COVID-19 cases in the U.K. Adds the Guardian, a recent Public Health England report “further revealed that cases of the virus are doubling between every 4.5 and 11.5 days, depending on the region of England, and that it has about a 60 percent increased risk of household transmission compared with the Alpha variant.”

The rise of the variant has also delayed the country’s full reopening. Prime Minister Boris Johnson announced on June 14 that the easing of coronavirus restrictions originally scheduled for June 21 was being pushed back to July 19 (at the earliest).

The rapid spread of B.1.617.2 has led the U.K. to attempt to speed up its vaccination campaign, particularly the second doses that many adults in Britain still have not received.

The good news is twofold. First, since a majority of the U.K. population is fully vaccinated, the number of people the B.1.617.2 strain is infecting remains small. Second, because the U.K. is a world leader in the genome sequencing of variants, it is providing the clearest picture yet of B.1.617.2 capabilities — to the benefit of scientists worldwide.

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

B.1.617.2 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 to yet spread in 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 the threat of any more dangerous COVID variant also raises the threat of more dangerous COVID 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 1 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 COVID B.1.617.2 Variant