LONDON — The U.K. on Saturday recorded its most new coronavirus infections since early February as the National Health Service ran a "grab a jab" initiative to further drive up vaccination rates.

Government figures showed that another 18,270 people tested positive for the virus across the U.K, the highest daily number since Feb. 5. Over the past week, nearly 100,000 have tested positive, around 50% increase up on the week before. That has raised questions over whether lockdown restrictions will end as planned.

Daily cases have risen fairly sharply over the past few weeks as a result of the delta variant, which was first identified in India and is considered by government scientists to be between 40% to 80% more transmissible than the previous dominant strain. It accounts for nearly all the new cases in the U.K. 

Most of the new confirmed cases are among younger age groups which haven't yet received COVID-19 vaccines. The latest spike came as hundreds of walk-in vaccination sites, including at stadiums and shopping centers, opened in England over the weekend in a bid to boost vaccine numbers, particularly among those younger age groups.

"This is a phenomenal achievement and it's fantastic to see so many young people coming forward for their jabs, doing their bit to protect themselves and their loved ones," Vaccines Minister Nadhim Zahawi said.

The spread of the variant upended the Conservative government's plans to lift all remaining restrictions on social contact in England this week. The plan is to lift those restrictions on July 19, but whether it will do so could largely depend on whether the vaccine rollout has created a firewall that protects the most vulnerable. The other parts of the U.K. — Scotland, Wales and Northern Ireland — are following similar plans.

The hope is that the link between infections and those needing hospitalization and subsequently dying has broken because of the rapid rollout of vaccines. As of Saturday, nearly two-thirds of the U.K. population have received at least one vaccine dose while 48% have had two. 

Recent analysis from Public Health England showed that two doses of the main vaccines the U.K. is using are highly effective against hospitalization from the delta variant — 96% in the case of the Pfizer-BioNTech vaccine and 92% for the AstraZeneca jab.

Though the number of people in hospitals and dying have been edging up over the past couple of weeks, they haven't risen at the same rate as infections. On Saturday, the government said another 227 people have been hospitalized, taking the total to 1,505, nowhere near the 40,000 levels recorded earlier in the year during the peak of a second surge. Virus-related deaths also remained relatively low at 23, taking the death total to 128,089.

Worries about the vaccines were heard as thousands of anti-lockdown protesters marched through central London on Saturday. Some even threw tennis balls into Downing Street, where Prime Minister Boris Johnson has his office and residence. "Shame on you," some chanted.

Meanwhile, Health Secretary Matt Hancock resigned after breaking social distancing rules with an aide he was allegedly having an affair with. The tabloid Sun newspaper had run images appearing to show the married Hancock and senior aide Gina Coladangelo kissing in an office at the Department of Health.

In a letter to Johnson, Hancock said the government owes "it to people who have sacrificed so much in this pandemic to be honest when we have let them down."

Johnson had been facing widespread calls to fire Hancock, who had apologized for breaching social distancing rules. Coladangelo is a friend of Hancock's from their days together at Oxford University and was appointed to his department last year.

"The last thing I would want is for my private life to distract attention from the single-minded focus that is leading us out of this crisis," Hancock said in his letter of resignation.

"I want to reiterate my apology for breaking the guidance, and apologize to my family and loved ones for putting them through this," he said. "I also need (to) be with my children at this time."