A coronavirus variant first detected in India in February has now gone global, popping up in dozens of countries and raising fears that the strain may spearhead a wave of infection that could overwhelm health care systems, reverse reopening plans and even potentially undermine the rollout of vaccines.
The B.1.617.2 strain, officially known as the Delta variant, is worrying health officials across the world, including in the United States. The Delta variant now accounts for more than 6% of sequenced virus samples in the US, according to data from the US Centers for Disease Control and Prevention.
While that might seem a relatively small share, the speed of its growth is worrying. A month ago, the strain accounted for just over 1% of sequenced virus samples, according to the CDC data.
Experts believe the Delta variant sparked the huge wave in infections seen across India over the past two months. It is now causing concern in the United Kingdom, where it now comprises 91% of new cases, according to Health Secretary Matt Hancock.
The spread of the variant spread came at the same time as a considerable spike in case numbers in the UK in recent days, a spike that prompted the government to deploy the military in the hardest-hit areas to help run the test-and-trace program.
The World Health Organization (WHO) designated B.1.617 and its sublineages, including B.1.617.2, as "variants of concern" on May 10. That classification means a variant may be more transmissible or cause more severe disease, fail to respond to treatment, evade immune response or fail to be diagnosed by standard tests.
The Delta variant was the fourth to be declared a "variant of concern" by the WHO; the others are B.1.1.7, which was first seen in the UK and is now known as the Alpha variant; B.1.351, or Beta, first detected in South Africa; and P.1, first found in Brazil and now called Gamma.
Here's what you need to know.
Is it more contagious?
Experts now believe the Delta strain is likely more transmissible.
Hancock said last weekend the strain is "around 40% more transmissible" than the formerly dominant Alpha variant, which was already more transmissible compared to the original strain of the virus.
Speaking at a White House Covid-19 briefing on Tuesday, National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci said studies support the idea that the strain is more transmissible.
"Clearly now its transmissibility appears to be greater than the wild type," Fauci said, adding that the 6% share the strain now has in the US is similar to a tipping point previously seen in the UK.
"This is a situation, the way it was in England where they had a B.1.1.7 dominant, and then the [B.1.] 617 took over. We cannot let that happen in the United States," Fauci said.
Is it more deadly?
Early evidence suggests the Delta variant could cause an increased risk of hospitalization in comparison to the Alpha strain, according to Public Health England (PHE).
While PHE cautioned that more data is needed, its early findings showed that people infected with the variant were more likely to suffer serious illness. An analysis of 38,805 sequenced cases in England showed that people infected with the Delta variant carried 2.61 times the risk of hospitalization within 14 days compared with the Alpha variant, when variables such as age, sex, ethnicity and vaccination status were taken into consideration, the PHE said last week.
Fauci echoed the worry, saying the variant "may be associated with an increased disease severity."
Do vaccines work against it?
There is evidence the existing Covid-19 shots are working against the Delta variant.
A team of researchers at BioNTech and the University of Texas Medical Branch reported Thursday they had found evidence the Pfizer/BioNTech vaccine would protect against infection with the Delta variant and others.
They tested the blood of 20 fully vaccinated volunteers against lab-engineered versions of several virus variants and found evidence the immune system should neutralize them.
Researchers based in the UK reported last week that most people who receive two doses of the Pfizer/BioNTech coronavirus vaccine still would have protection against the new variant, although they said the antibodies appear to be significantly reduced.