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As the U.K. eased a months-long lockdown this week -- and eager crowds -- health officials there called their outlier approach to COVID-19 vaccination a success.

Britain has continued to stretch out the interval between first and second doses of COVID-19 vaccines for up to 12 weeks in order to prioritize first doses for as many people as possible. Initially, the move was made in response to a dramatic uptick in cases at the end of December and into January -- spurred by the highly contagious B.1.1.7 variant.

But now, new cases and hospitalizations have continued to hold at levels not seen since last summer, seemingly a nod to how well the strategy has worked thus far.

Another potential factor: concerns about rare and unusual blood clots tied to the AstraZeneca vaccine, developed on home soil, didn't trigger any pauses or slowdown in vaccinations (unlike many nations in continental Europe), nor did it prompt vaccine hesitancy.

Yet concerns linger about delaying second doses as the economy opens back up.

The U.K. government earlier this week pushed forward of its COVID-19 vaccination program after successfully completing phase 1. This first phase targeted people 50 and older, the clinically vulnerable, and health and social care workers. It met the April 15 target ahead of time, the government said, noting that the group accounts for 99% of all COVID-19 deaths.

About 95% of Britons 50 and older have now received a first dose of the vaccine, and 92% of people who are clinically extremely vulnerable to COVID-19 got theirs, according to the government. This week, people age 45 to 49 became eligible, and the U.K. expects to be able to offer a vaccine to all adults by the end of July.

The National Health Service will continue to offer people their second doses within 12 weeks, the government said, from the the Joint Committee on Vaccination and Immunisation -- an independent advisory committee that advises British health departments on vaccination.

Vaccines Minister Nadhim Zahawi, speaking in the House of Commons earlier this week, said he was "absolutely delighted" that the country has met its vaccination targets ahead of schedule.

"Over 32 million people have had their first dose and more than 7.6 million have had their second dose," Zahawi said. "We are making phenomenal progress, but we remain focused on ensuring that no one gets left behind."

The Joint Committee on Vaccination and Immunisation said this week in its of the vaccination program that Public Health England has estimated that more than 10,000 deaths have been averted in the first three months of efforts.

The committee did advise that for phase 2, it's preferable for people age 18 to 29 without underlying conditions to be offered an alternative to the AstraZeneca vaccine if available. It added there have been no such issues reported with the Pfizer or Moderna vaccines.

Great Britain, with a population of about 68 million, has secured access to 40 million doses of the Pfizer vaccine, and 17 million doses of the Moderna vaccine.

The latest figures reported by the British government show just 2,672 new COVID-19 cases on April 15, comparable to numbers . New daily cases reached a high of more than 68,000 on Jan. 8 as the B.1.1.7 variant took hold.

Hospitalizations have also decreased and remained relatively low since vaccination began. There were 201 COVID-19 admissions on April 11, according to the latest government data, down from a peak of more than 4,000 several days in January.

Government data further show that the daily number of deaths within 28 days of a first positive COVID-19 test reported on April 15 was 30. Daily deaths topped 1,000 most days at the beginning of the year. Deaths from COVID have been averaging about 35 per day for several weeks, even as France, with a similar population, is seeing a daily toll nearly 10 times greater.

"The U.K. COVID-19 picture looks very encouraging right now," Jessica Justman, MD, associate professor of medicine and epidemiology at Columbia University's Mailman School of Public Health, told MedPage Today in an email. She offered two explanations for the success.

"First, the U.K. is just coming out of its third lockdown," she said. "We know that lockdowns serve as a major brake on transmission. At the same time, about 32 million, or half of the U.K. population, has received at least one jab of a COVID vaccine, based on a delayed (12 week) second dose strategy."

Justman noted that while the delayed second dose strategy "seems quite logical ... we need to remember there is a stronger evidence base for not delaying the second dose."

Peter Marks, MD, PhD, director of the FDA's Center for Biologics Evaluation and Research, this week during an American Medical Association webinar.

"People are worried about the COVID-19 variants," Marks said during the virtual session. "My topline message about these variants is that all of them to date -- at least in the laboratory -- seem to be susceptible to the immune response by the currently deployed vaccines, granted some of them have a greater resistance to that neutralization."

"One of the rationales for giving both doses of the mRNA vaccines is that, when you get one dose, your antibody titers are actually not very high," Marks said. "It's the second dose that gets the very high titers that are capable of neutralizing things like the South African variant."

Delaying the administration of second doses is "not a good idea," Marks said. Neither is administering just a single dose of a two-dose vaccine, he said, "because once somebody is vaccinated they change their behavior, and we'd like to see them protected so they don't take risks when they shouldn't."

Marks also warned against half doses of certain vaccines, and mixing and matching vaccines.

"Now that the lockdown restrictions are being lifted," Justman said, "we will see whether the cases go back up, especially cases caused by existing (B.1.1.7) or other more concerning variants (B.1.351), or whether the vaccinated part of the population will help to hold the line."

, pubs and restaurants in England opened up for outdoor drinking and dining for the first time since January; "non-essential" shops and "close contact" services such as hair salons were also allowed to resume operation. Wales, Scotland, and Northern Ireland are expected to follow suit later this month. Indoor dining, however, won't be allowed until May 17 at the earliest, and June 21 is the current target for a complete end to restrictions.

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