LONDON — The B.1.617.2 coronavirus variant that was first identified in India is now believed to be dominant in the United Kingdom, Public Health England (PHE) said on Thursday, adding that early evidence suggests it could cause an increased risk of hospitalization in comparison to the strain that was first detected in England.

While PHE cautioned that more data is needed, its early findings showed that the B.1.617.2 or "Delta" variant was more likely to cause serious illness than the B.1.1.7 or "Alpha" variant, which had been dominant in the United K since it was first detected in southeast England last fall.

An analysis of 38,805 sequenced cases in England showed that 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.

This week, at least 278 people with the Delta variant attended hospital emergency departments across the UK, resulting in 94 people being admitted to hospital overnight -- an increase from the 201 people with the Delta variant who attended emergency departments last week, including 43 overnight admissions, PHE outlined. "The majority of these had not been vaccinated," PHE highlighted.

Grant Shapps, the UK's transport secretary, said the decision was based on rising positivity rates in Portugal and a growing concern that returning travelers could bring in more coronavirus variants, specifically mutations to the fast-spreading Delta variant.

"We just don't know the potential for that to be a vaccine-defeating mutation, and we don't want to take the risk as we come up to 21 June and the review of the fourth stage of the unlock," Shapps said Thursday in an interview with the BBC.

Portugal's President Marcelo Rebelo de Sousa has criticized the UK's decision, warning that a balance needed to be found in order to avoid "health fundamentalism" and stressed the need to "move forward."

Coronavirus cases in England are continuing to climb, with the number of people testing positive hitting the highest level in six weeks on Thursday. A total of 17,162 people tested positive for the coronavirus in England in the week up to May 26, a 22 percent increase compared to last week, according to the UK's National Health Service Test and Trace program.

Speaking to CNN, Hancock said that while the UK will remain cautious, the uptick in cases will not necessarily lead to a higher mortality rate due to vaccination levels.

"The big change, of course, is the vaccine ... the number of cases doesn't automatically lead to hospitalizations and sadly deaths — as it automatically did in the past — because we've broken that link, but it's not completely broken so we're watching this very, very carefully," he said.

Half of all adults in the UK are now fully vaccinated, UK Vaccines Minister Nadhim Zahawi announced in a tweet on Thursday, describing it as an "important milestone."

Vaccination still appears to be making an impact on the spread of the virus.

"One shining light however is that vaccination still appears to be making an impact on spread," Meaghan Kall, an epidemiologist with PHE, said on Twitter. According to PHE, 73 percent of Delta cases are in unvaccinated people, compared to only 3.7 percent in people who have received both doses.

Similarly, only 5% (seven infections) of the confirmed Delta cases that were admitted to hospital overnight — and two deaths — were people who had received both doses.

The PHE data also reveals the spread of variants within schools in England. In the first week of June, there had been 140 outbreaks of the Delta variant within educational settings. Delta cases also made up for about half of known school outbreaks in the most recent week.

Reacting to the PHE report on Twitter, Christina Pagel, director of University College London's clinical operational research unit, said she was astonished at the "rosy" interpretation of the PHE data.

"I get the desire to be optimistic (honestly) but minimizing delays action and ultimately makes things worse. Exponential growth is a red flag," Pagel said.

"We *now* have evidence that delta is (a lot) more transmissible, partially vax resistant & more severe. It's now 80% of our cases. The genie is out of the bottle." — CNN