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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.

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 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?

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.

The United Kingdom, which has world-class virus sequencing capabilities, warned that B.1.617.2 – a subtype of the variant first found in India – would likely overtake others, such as the highly transmissible B.1.1.7 first found in the UK last year, to become the most dominant variant of the virus in the country.

“The thing which has changed is the very clear view now that everyone has looked at it, that [B.1.617.2] is more transmissible than the B.1.1.7 and we expect, over time, this variant to overtake and come to dominate in the UK, in the way that B.1.1.7 took over and indeed other variants have taken over prior to that,” England’s chief medical officer Professor Chris Whitty said Friday.

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).

“There is currently insufficient evidence to indicate that any of the variants recently detected in India cause more severe disease,” Public Health England said on May 7.

Do vaccines work against it?

Lab research into the efficacy of the Pfizer and Moderna vaccines against the variants first seen in India showed that the shots appear to protect against variants. The findings were reported in a pre-print paper on biorxiv.org on Sunday, which has not yet been peer-reviewed.

The new research involved serum samples collected from eight people who had recovered from Covid-19, six people fully vaccinated with the Pfizer vaccine and three people fully vaccinated with Moderna’s. The researchers analyzed in lab experiment how the serum samples neutralized lentiviruses – a type of retrovirus – equipped with the same mutations as the B.1.617 and B.1.618 coronavirus variants.

They found some decreases in neutralization, but overall, antibodies from people who had been vaccinated appeared to work “well above” the serum from people who had recovered from Covid-19 caused by earlier versions of the coronavirus.

More research is needed to determine just how effective the vaccines are against those variants in the real world, they said.

Their findings appear to confirm what researchers from Oxford University are seeing in early real-life data, which give “a degree of confidence” that vaccines work against the variants, UK heath secretary Matt Hancock told Sky News on Sunday.

He echoed other scientists who have been cautiously optimistic about the efficacy of vaccines. Sir John Bell, Oxford’s Emeritus Professor of Medicine, told Times Radio that early lab results suggest the B.1.617.2 variant “will be susceptible to the vaccine in the way that others are.”

While the variant is now the dominant strain in some parts of the UK, Hancock said “the people who are ending up in hospital are largely people who are eligible for the vaccine but have not taken the vaccine.”

Early data published by PHE showed similar results for the AstraZeneca and Moderna vaccines. They, too, appeared to be effective against the Delta variant once both doses have been administered.

Which countries have detected the variant?

The variant has been identified in 74 countries, on every continent apart from Antarctica, WHO said in its latest weekly epidemiological update published on Tuesday.

The United Kingdom has reported the largest number of cases of the strain outside India, it said.

Meanwhile, in the United States – where B.1.617 is also present – the Centers for Disease Control and Prevention (CDC) is still classifying it as a “variant of interest,” but noted that this classification could change, based on scientific evidence.

What does it mean for global roadmaps out of lockdown?

If India’s coronavirus outbreak can’t be contained and continues to spread to neighboring countries with low vaccine supplies and weak health systems, experts warn the world risks replicating scenes witnessed in India.

The UK, where the Delta variant is now dominant, is providing something of a cautionary tale for the rest of the world. Neil Ferguson, an epidemiologist at University College London, said on Wednesday that the variant could lead to a “substantial third wave” of Covid-19 infections in the UK.

And the more the virus spreads, the more chances it has to mutate and evolve into new variants that could eventually resist current vaccines, threatening to undermine other countries’ progress in containing the pandemic.

CNN’s Maggie Fox, Kara Fox, Ivana Kottasová, Robert Iddiols and Aditi Sangal contributed to this report.