The coronavirus variant labeled “delta” was first recorded in India, where a slow vaccination drive and complacency about pandemic rules helped spark a record-breaking surge in cases this spring.

Since then the variant has spread, and as new cases rise in Britain, it has become dominant, despite one of the most successful vaccination programs anywhere. Its newfound prevalence could upend plans for a return to normalcy.

On Monday, British Health Secretary Matt Hancock said the government was still evaluating the data to see whether the country could reopen fully by June 21 as planned, with a deadline to decide by next Monday.

“I know that these restrictions have not been easy, and with our vaccination program moving at such pace, I am confident that one day soon freedom will return,” he told Parliament.

Britain has fully vaccinated more than 41 percent of its population, while more than 60 percent have received at least one shot. But over recent weeks, the number of new cases recorded daily has ticked slowly but surely up, while health authorities confirmed last week the delta variant had come to dominate new infections.

The situation could have a significant impact on vaccination and reopening plans around the world — including in the United States, where rates of vaccination have slowed despite the Biden administration’s target of having 70 percent of the country vaccinated by July 4.

What is the delta variant?

Also known by the scientific name B.1.617, the variant was first identified in Maharashtra, India, in October. It became the “delta” variant after the World Health Organization implemented a naming system based on Greek letters earlier this month.

Though it is just one of many variants to have arisen during the pandemic, it is considered one of the most alarming. The World Health Organization has labeled it a “variant of concern.”

It has splintered into several sub-variants — including one, B.1.617.2, that is widespread in Britain.

While scientists are still studying the variant to better understand it, its impact can be seen in India, where it contributed to a surge in cases in recent months that led to daily death tolls topping a record 4,500.

On Monday, Hancock said before Parliament that the British government believed that the delta variant was 40 percent more transmissible than the alpha variant, also known as B.1.1.7, which was first detected in Britain. Scientists are still studying the matter.

A model released by a team of researchers at the University of Warwick last month warned that if the delta variant were 50 percent more transmissible than the alpha variant, it could lead to Britain’s biggest wave of hospitalizations yet, with around 10,000 a day.

What about vaccinations?

British data shows that the majority of new cases in the country are among those not yet vaccinated. Nearly all serious cases have been recorded among the unvaccinated or partially vaccinated.

As of Thursday, according to data Hancock cited, only three fully vaccinated people infected with the new variant had been hospitalized.

“The jabs are working,” Hancock said. “We have to keep coming forward to get them, and that includes vitally that second jab, which we know gives better protection against the delta variant.”

A pre-print publication by Public Health England released last month found that one dose of the Pfizer-BioNTech or AstraZeneca vaccines was only 33 percent effective against the delta variant, compared with 50 percent for the alpha variant. That gap closed with a second dose.

In Britain, where two-dose vaccines produced by Pfizer, Moderna and AstraZeneca are commonly used, the government has focused on getting first shots out to a wide number of people, with second doses sometimes becoming available months after the first.

The concern is bigger, however, in countries vaccinating less swiftly than Britain or using vaccines with lower rates of efficacy, such as the Chinese-produced Sinopharm vaccine.

As new variants of coronavirus continue to be discovered, here's what you need to know about how these mutations work and how they spread. (The Washington Post)

Where has the delta variant been found?

Britain is among the world leaders in sequencing viruses, which is one reason it is so focused on variants. Other nations, including the United States and India, have lagged behind in this aspect of virus tracking.

As of last month, according to the WHO, cases of the delta variant had been confirmed in 62 countries. That includes the United States, where it made up 3 percent of coronavirus cases as of May 8, according to the Centers for Disease Control and Prevention.

WHO officials have warned that variants, combined with reopening plans, could lead to outbreaks.

“Relaxation of public health and social measures, increased social mobility, virus variants and inequitable vaccination are a very dangerous combination,” Maria Van Kerkhove, the WHO’s covid-19 technical lead, said at a briefing last week.