The Delta variant of coronavirus is now the dominant lineage in the US, parts of Europe and elsewhere.
Also known as B.1.617.2, it is clearly more transmissible, but it is unclear if it causes more severe disease.
It’s taken over from the B.1.1.7 or Alpha variant in most of the countries where it is spreading fastest, but most also still have a mix of variants. Here’s what scientists know about the most common among them:
Delta
As of July 3, the Delta variant accounted for 51.7% of cases in the US, according to the US Centers for Disease Control and Prevention.
By mid-June, the Delta variant accounted for 99% of Covid-19 cases in the UK, according to Public Health England, and it is set to account for 90% of cases in Europe by the end of August, according to the European Centre for Disease Prevention and Control.
The World Health Organization reports the Delta variant has been detected in 100 countries. It has overtaken the worrying Beta, or B.1.351 variant, in South Africa.
“Delta is the most transmissible of the variants identified so far,” WHO Director-General Tedros Adhanom Ghebreyesus said last month.
“We know that the Delta variant has increased transmissibility and it is currently surging in pockets of the country with low vaccination rates,” CDC Director Dr. Rochelle Walensky told a White House briefing Thursday.
“In parts of the Midwest and upper mountain states, CDCs early sequence data suggests the Delta variant accounts for approximately 80% of cases,” she added.
What makes it so much more transmissible? It carries a cluster of mutations on the spike protein—the part of the virus used to attach to the cells it attacks – that appear to make it more effective at getting into cells, where it can replicate.
On Wednesday, a team of Chinese researchers reported people infected with Delta carried viral loads – the amount of virus in their bodies – that were 1,200 times higher than people infected with older lineages.
It also appears to reach a point where it can be transmitted two days earlier, on average, after infection, the researchers said in their report, posted online.
However, multiple studies indicate all three vaccines authorized for use in the US can protect people against Delta. They all show the vaccines generate a cushion of protection that more than makes up for the variant’s ability to compete with the immune response. They don’t prevent all infections, but greatly reduce the likelihood of severe disease, hospitalization and death.
Delta does evade the immune system, which may mean people who have been infected once with an older variant may be more likely to catch it again. It also eludes the effects of a monoclonal antibody treatment called bamlanivimab, made by Eli Lilly and Co, but is vulnerable to the protection offered by other monoclonal antibody treatments.
A French study published in Nature Thursday showed the Delta variant was four times less sensitive than the Alpha variant to blood collected from people who had recovered from Covid-19 up to 12 months ago.
The same researchers found a single dose of either the Pfizer/BioNTech or AstraZeneca Covid-19 vaccines was not effective against either the Beta or Delta variants. Antibodies in blood from just 10% of people in the study neutralized the Delta variant after a single vaccine dose. But a second dose of vaccine elicited an immune response in 95% of people in the study, though antibodies were significantly less potent against the Delta variant compared to the Beta variant.
“Early data from Scotland, from individuals who tested positive from 1 April to 21 June 2021, showed an increased risk of hospitalization among cases infected with the Delta variant compared with those infected with the Alpha variant,” WHO noted.
Delta carries a distinct cluster of mutations compared to other variants, including one called L452R known to increase transmissibility. It does not carry two other worrying mutations known as E484K and N501Y – which are seen in the B.1.1.7 or Alpha variant first seen in Britain, which swept many countries at the end of 2020 and the beginning of 2021, and also in the B.1.351 variant of Beta variant first seen in South Africa, and well as the P.1 or Gamma variant.
Alpha
This past spring, it was the B.1.1.7 or Alpha variant of coronavirus that was worrying public health officials. It swept across England quickly and then out into the world, quickly becoming the dominant lineage in the US.
As of Sunday, it accounted for just 28.7% of cases in the US, the CDC estimates, as it was replaced by Delta.
It was shown to be at least 50% more transmissible than older lineages. It carries 23 mutations, including one called N501Y that increases transmission.
It’s fully susceptible to monoclonal antibody treatments and vaccines.
Beta
First seen in South Africa, the B.1.351 or Beta variant has both the E484K mutation that is linked with immune escape and the N501Y mutation suspected of helping make many other variants more contagious. It has been shown to be 50% more transmissible than older strains and evades Lilly’s dual monoclonal antibody treatment but not others.
Blood tests and real-life use both suggest it can infect people who have recovered from coronavirus and also people who have been vaccinated against Covid-19.
Vaccine makers trying to get out ahead of the new variants by developing booster shots have focused on B.1.351, as it’s the variant scientists most fear could elude vaccine protection. But partial escape doesn’t mean full escape, and vaccines are still expected to protect people to some degree.
It’s being overtaken by Delta in South Africa and have never gained much of a foothold in the US, where it currently accounts for 0.2% of new cases, according to CDC.
Gamma
The P.1 or Gamma variant that swept Brazil currently accounts for 8.9% of new infections in the US, according to CDC.
Gamma carries both the E484K and N501Y mutations, with more than 30 others. It has been demonstrated to evade the effects of Lilly’s monoclonal antibody treatment but not one made by Regeneron. Blood tests show it might partly escape both natural and vaccine-elicited immune responses.
Epsilon
The B.1.427 and B.1.429 variants are usually lumped together and known as Epsilon.
First seen in California, this one has the same L452R mutation carried by Delta, but not some of its other mutations and has not taken off in the way Delta has.
“Epsilon (B.1.427/B.1.429) has been associated with increased transmissibility, a modest decrease in susceptibility to some antibody treatments, and reduced neutralization by convalescent and post-vaccination sera,” WHO says.
Iota
First seen in New York last November, the B.1.526 or Iota variant accounts for 3% of US cases, CDC estimates. That’s down from 9% of samples last April.
It has what’s called a 484 mutation that should help the virus attach more easily to the cells it infects and also makes the virus less recognizable to the immune system.
Eta
First seen in the UK and Nigeria, Eta, also known as B.1.525, carries the E484K mutation. It’s decreasing in frequency in the US, now accounting for almost no samples tested.
Zeta
Circulating in Brazil since last year, this variant, also known as P.2, also carries the worrying E484K mutation and has not been found widely globally. It’s almost disappeared in the US, according to the CDC.