The UK’s chief medical officers have defended plans to delay giving the second dose of a Covid vaccine to more than 500,000 people who have received the first jab, saying they believed the public would “understand and thank us for this decisive action” despite an outcry among doctors. Recipients of the Pfizer/BioNTech vaccine had previously been told there would be a three-week gap between doses, with many already booked in for their second jab. But the government announced a change to its Covid vaccination strategy on Wednesday, saying second doses of the newly approved Oxford/AstraZeneca vaccine and the previously approved Pfizer/BioNTech jab would now be given up to 12 weeks after the first dose. The move applies to people scheduled to have their second dose of the Pfizer jab after 4 January, as well as those yet to receive either jab. The government said it hoped the approach would free up more doses and mean as many people as possible would soon get their first jab and have some protection against the disease. The new regime was backed by the chief medical officers of the four UK nations, with instructions given to health service leaders in England by NHS bosses on Wednesday to postpone many of the scheduled second Pfizer/BioNTech jabs. The announcement caused controversy, however. Pfizer and BioNTech warned that two doses of their vaccine were required for maximum protection against Covid and that they did not have evidence that the first dose alone offered protection after three weeks. Azeem Majeed, a professor of primary care and public health at Imperial College London and a practising GP, said he was shocked by the change of plan. “We’ve got thousands or perhaps tens of thousands of people who have already booked their second jabs, and these are people who are all elderly, so they often have to make special arrangements for their care or have someone to bring them down. Someone is going to have to ring all these patients to cancel their appointments and rebook them, so it is going to create a lot of work for people as well,” he said. “Clearly the people making these rules are not the ones having to enforce them. I understand the rationale that the government wants to increase the vaccine supply to make sure more people are immunised, but I think that could have been done for people starting from now, rather than cancelling the ones who have already been booked in, which is very disruptive.” In a joint statement on Thursday night, Chris Whitty, the chief medical officer of England, and his counterparts in Wales, Scotland and Northern Ireland stood by their decision, although they acknowledged that it would “distress patients who were looking forward to being fully immunised”. They added: “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 added: “We have to follow public health principles and act at speed if we are to beat this pandemic which is running rampant in our communities, and we believe the public will understand and thank us for this decisive action.” Earlier, one doctor who spoke to the Guardian on condition on anonymity said they hoped to continue with the original schedule for the second dose rather than cancelling appointments, saying that to do otherwise would put their most vulnerable patients at risk and break promises to patients. Alison Bolam, a GP at Horfield health centre in Bristol, said her practice would take a similar approach. “If we try to ring and cancel 1,000 80-year-olds, and then ring 1,000 75-year-olds and ask them to come in instead, we risk wasting vaccine because we don’t have the staff capacity to do that with the bank holiday in the way,” she said. “If anyone gets the first vaccine now, the second one will be 10 to 12 weeks later, but for anybody who has already had it, they will get their second vaccine as originally scheduled.” The British Medical Association said it was unreasonable and unfair to expect practices to cancel and rebook appointments for patients to have their second dose. The chair of the BMA’s GP committee, Dr Richard Vautrey, said: “Local leaders are telling us that is unprofessional and impractical to amend the appointments for thousands of frail, elderly patients, particularly those booked and who have already made arrangements to have their second vaccination in the next two weeks.” Vautrey called on the government to publish a scientifically validated justification for the new approach, adding that rebooking appointments would cause huge logistical problems. The Guardian understands a number of NHS clinical commissioning groups were holding meetings on Thursday on the issue, to decide how to proceed. Dr Helen Salisbury, a GP and medical adviser to the health experience research group at the University of Oxford, tweeted that the health secretary Matt Hancock, should help to phone her elderly patients and explain why their second jab had been delayed. She said her primary care network needed to cancel and rebook 1,160 appointments. “At 5 mins per phone call, that’s 193 hours’ work. Not to mention the grief & anger,” she tweeted. Her comments unleashed a flurry of responses from other GPs, with some questioning whether it was ethical to delay a second dose when patients had given consent for the first dose on the premise that they would receive the second three weeks later. The Doctors’ Association UK said it had sent a letter to Hancock and others involved in the change of strategy regarding the Pfizer vaccine. “A patient can’t consent for a treatment then have the treatment changed without their permission, especially when the evidence for change is lacking,” it said. Dr Vinesh Patel, a GP partner and spokesman for the DAUK’s GP committee, said using the Pfizer jab outside the trialled vaccination regime was a huge gamble. Others, however, welcomed the change in approach. Dr Ed Turnham, a GP in Norwich, said the MHRA should be congratulated for its “brave, pragmatic decision”. “I think most of our elderly patients will be happy that this will reduce the delay in protecting their friends and relatives, and reduce the risk of hospitals being overwhelmed,” he tweeted. Prof Martin Marshall, chair of the Royal College of GPs, sympathised with GPs and their patients but said the new strategy had merits. The Joint Committee on Vaccination and Immunisation had “put forward a compelling case that this is the best way to protect as many people as possible from the virus, and this is the decision that has been made,” he said. But he added: “NHS England’s guidance does allow for some clinical discretion for practices to go ahead with second vaccinations where necessary, and if a practice uses this it is important it is respected.”