The new strategy, announced Wednesday by the head of the UK's medicines regulator MHRA, means that the interval between doses could be extended to up to 12 weeks, instead of the three weeks previously stipulated.
It has prompted a debate among experts, with the British Medical Association (BMA), a body representing UK doctors, criticizing the move to postpone appointments for the very vulnerable patients currently awaiting their second shots.
The Pfizer/BioNTech vaccine has been in use in the UK since early December, when the country became the first in the world to approve it, but supplies are limited.
The argument over the vaccination strategy comes as infection rates soar in much of the UK, thanks in part to a new, more infectious variant of the virus. Most of England is now under the toughest level of restrictions to try to limit the virus' spread.
"This group of very elderly patients is at the highest risk of death if they contract Covid-19, which is why GPs are so concerned for them. It is grossly and patently unfair to tens of thousands of our most at-risk patients to now try to reschedule their appointments," Dr. Richard Vautrey, chair of the BMA General Practitioners Committee, said in a statement Thursday.
The Doctors' Association UK also raised "real and grave concerns" over the new vaccination strategy, warning Friday that it could undermine the National Health Service's patient consent process, "as well as completely failing to follow the science."
Meanwhile, Pfizer said it did not have data to demonstrate that just a single dose of its vaccine would provide protection against the disease after more than 21 days.
"Pfizer and BioNTech's Phase 3 study for the Covid-19 vaccine was designed to evaluate the vaccine's safety and efficacy following a 2-dose schedule, separated by 21 days," Pfizer said in a statement on Thursday. "There are no data to demonstrate that protection after the first dose is sustained after 21 days."
But the chief medical officers for England, Scotland, Wales and Northern Ireland defended the move in a letter to healthcare professionals published Thursday, saying it was based on the "balance of risks and benefits," and that the "great majority" of initial protection came from the first jab.
"The second vaccine dose is likely to be very important for duration of protection, and at an appropriate dose interval may further increase vaccine efficacy," they said.
"In the short term, the additional increase of vaccine efficacy from the second dose is likely to be modest; the great majority of the initial protection from clinical disease is after the first dose of vaccine."
'Terrible impact' on emotional well-being
The BMA warned in its statement that the delay could have a "terrible impact on the emotional well-being" of vulnerable and at-risk patients.
"The BMA believes these are patients that have already been promised, by the NHS and local clinicians, that they will receive a second dose of Pfizer vaccination next week; they have given their consent to receive it and, quite rightly, are expecting to have it," the BMA said.
Vautrey, the BMA chair, told CNN Friday that healthcare professionals were particularly concerned about the "practicality of doing this so quickly," with little warning given to practitioners on the revised guidance.
"We were only told in the last day that we are expected, next Monday, to re-change all of the appointments that we made for next week... it was simply not practical for our practice staff to do that in such a short space of time," Vautrey said.
"We wanted the commitments that we made to our elderly patients to give their vaccine to be honored, certainly in the next few days."
Helen Salisbury, a general practitioner in the English city of Oxford, told the BBC's Today program Friday that her practice had not as yet canceled existing second appointments next week. This was because, firstly, she had been unable to find any data on immunity after the first dose beyond the 21 days when the booster was given in trials, and secondly, because the practice wanted to protect its most vulnerable patients, the elderly, and maintain their trust in the vaccine.
"When you have started a patient on a course of treatment and you have said, this is what the plan is, here's one jab, please come back in three weeks, it's really important that you have the second jab to be fully protected -- and then to turn round five minutes later and say no, don't worry about that, you can have it in 12 weeks rather than three weeks -- I don't think that's good enough, actually," she said.
In their letter, the chief medical officers said they recognized the operational difficulties and potential distress involved in rescheduling second appointments at short notice.
"However, we are all conscious that for every 1,000 people boosted with a second dose of Covid-19 vaccine in January (who will as a result gain marginally on protection from severe disease), 1,000 new people can't have substantial initial protection which is in most cases likely to raise them from 0% protected to at least 70% protected," they said.
Pfizer said it had not evaluated different dosing schedules because "the majority of trial participants received the second dose within the window specified in the study design."
In its open letter, the Doctors' Association UK wrote: "Protection provided by the first dose of the Pfizer vaccine is considerably lower at 52.4% compared to 95% if two doses given three weeks apart."
Vautrey told CNN that greater assurances would be needed from both the UK chief medical officers and Pfizer in order to give healthcare professionals and patients confidence in the government's strategy.
"We need Pfizer themselves to be confident that this new dosage regime is going to deliver effective coverage and protection to our patients, particularly our most vulnerable patients," Vautrey said.
UK regulators have also advised giving the second dose of the newly approved Oxford/AstraZeneca vaccine, which is due to be rolled out from Monday, four to 12 weeks later.
US also considering spacing out jabs
The strategy of extending the interval between first and second vaccine doses is also "under consideration" in the United States, National Institute of Allergy and Infectious Diseases Director Anthony Fauci said Thursday.
Two vaccines have so far been approved for use in the US, the Pfizer/BioNTech shot and another made by Moderna, which requires a second dose 28 days later.
"I still think, if done properly, you can do a single dose, reserve doses for the second dose, and still get the job done," Fauci said on NBC's Today Show, "but there's a lot of discussion about whether or not you want to spread out the initial vaccination by getting more people vaccinated on the first round."
Fauci said it could be debated either way, but a potential problem would be if a person didn't get the second dose in time and there was a lag period.
He said it was known from the clinical trials that "the optimal time is to give it on one day and then for Moderna 28 days later and for Pfizer 21 days later, that's what the data tells us is the best way to do it."
If you want to stick with the data, that's how it should be done, he said, "but you can make an argument, and some people are, about stretching out the doses by giving a single dose across the board and hoping you're going to get the second dose in time to give to individuals."
UK Prime Minister Boris Johnson struck an optimistic note on vaccines in an address on New Year's Eve.
"We have a hard struggle still ahead of us for weeks and months because we face a new variant of the disease that requires a new vigilance," he said.
"But as the sun rises tomorrow on 2021, we have the certainty of those vaccines. I believe 2021 is, above all, the year when we will eventually do those everyday things that now seem lost in the past, bathed in a rosy glow of nostalgia -- going to the pub, concerts, theaters, restaurants, or simply holding hands with our loved ones in the normal way."
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