Medical personnel is given the Pfizer-Biontech Covid-19 corona virus vaccine at the Favoriten Clinic in Vienna, Austria, on December 27, 2020 on the occasion of the Pfizer-BioNTech Covid-19 corona virus vaccine rollout.Georg Hochmuth | AFP | Getty ImagesThe U.K.’s decision to delay giving people the second dose of a coronavirus vaccine is proving controversial, with experts, advisors and the vaccine producers all weighing in on the strategy.Britain was among the first countries in the world to kick off a mass vaccination campaign against the coronavirus, after approving the Covid-19 vaccine developed by Pfizer and BioNTech at the start of December. On Monday this week, it started rolling out the University of Oxford and AstraZeneca’s vaccine, having approved it for use just ahead of the new year.As both vaccines require two doses per person, the U.K. government said initially that a second dose would be given either three or four weeks after the first dose, depending on which vaccine was being given and in line with the dosing regimens tested in clinical trials.However, it is now recommending a gap of up to 12 weeks, in an effort to give more people a first dose – and some initial protection against Covid-19.Vaccine maker concernsBioNTech and Pfizer have responded to the decision, saying there is no evidence that their vaccine will continue to protect against Covid-19 if the second shot is given more than 21 days after the initial dose.”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. The safety and efficacy of the vaccine has not been evaluated on different dosing schedules as the majority of trial participants received the second dose within the window specified in the study design,” the companies said in a statement to CNBC Tuesday.”Although data from the Phase 3 study demonstrated that there is a partial protection from the vaccine as early as 12 days after the first dose, there is no data to demonstrate that protection after the first dose is sustained after 21 days.”The companies said it was now “critical to conduct surveillance efforts” on any alternative dosing schedules.Final data analysis from Pfizer/BioNTech’s clinical trials showed that the vaccine was 95% effective in preventing Covid-19 seven days after the second dose was given.For the University of Oxford/AstraZeneca candidate, interim analysis of late-stage trial results were a bit more nuanced with an anomaly in the vaccine doses given to trial participants. Essentially, when the vaccine was given as two full doses, its efficacy was found to be 62.1% but when some trial participants received a half dose followed by a full dose it was found to be and 90%. Both dosing regimens had the two shots given a month apart. AstraZeneca was not immediately available for comment on the U.K.’s decision to delay the second doses.Reasons behind the decisionThe decision to extend the dosing window comes as British hospitals struggle with increasing admissions. The coronavirus is running amok in the U.K. with a new, more transmissible strain of the virus spreading exponentially in London and the South-East, and now appearing in other parts of the country.To date, the country has recorded over 2.6 million cases of coronavirus and more than 75,000 related deaths, according to data compiled by Johns Hopkins University. On Monday, the U.K. recorded 58,784 new cases, and has now reported more than 50,000 new coronavirus cases for seven days in a row. On Monday the U.K.’s Prime Minister Boris Johnson announced a third nationwide lockdown for England.It is against this grim backdrop that the U.K. drug regulator, the Joint Committee on Vaccination and Immunisation and the four U.K. chief medical officers agreed to delay the gap between the first and second dose of vaccines in order “to protect the greatest number of people in the shortest amount of time.”There are signs others could follow Britain too, with Germany’s health ministry now seeking the advice of an independent vaccination commission on whether to follow the U.K.’s dose delay tactic. Denmark has reportedly already approved a delay of up to six weeks between the first and second shots of the vaccine.’Finely balanced’So far, more than a million people in the U.K. have already been vaccinated with the Pfizer/BioNTech vaccine, according to the government, and some, like the first ever recipient of that vaccine outside of a clinical trial, have received their second dose.But now, thousands of others in the top priority category are now being told they will have to wait up to 12 weeks for their second dose.The British Medical Association described the move as “grossly unfair” to thousands of at-risk patients in England, but the U.K.’s independent Scientific Advisory Group for Emergencies (SAGE) said in a statement published Sunday that it was a “very difficult and finely balanced decision.”Responding to the BMA’s concerns, SAGE said that “in normal circumstances, we would argue for continuation of the earlier plans to administer two doses of the Pfizer-BioNTech vaccine 21 days apart. However, these are not normal circumstances and there are other important public health considerations.”The U.K. is prioritizing vaccinating elderly residents in care home, their carers, people over 80 years of age and, frontline health and social care workers.The country has pre-ordered 40 million doses of the Pfizer/BioNTech vaccine, enough for 20 million people, and has agreed a deal with AstraZeneca for 100 million doses, enough for 50 million individuals. The U.K.’s population is around 66 million.