A first dose offers "substantial" protection, said the UK’s chief medical officers as they declared their support for the advice of the Joint Committee on Vaccination and Immunisation (JCVI).
The JCVI has outlined a new dosing regimen aimed at providing a speedier rollout. It means that in the UK, the second dose of both vaccines will be given within 12 weeks of the first.
But Alejandro Cravioto, chairman of WHO’s Strategic Advisory Group of Experts on Immunization (SAGE), said the two doses of the Pfizer jab should be administered within 21 – 28 days.
He told an online news briefing on Tuesday: “We deliberated and came out with the following recommendation: two doses of this (Pfizer) vaccine within 21-28 days.”
Mr Cravioto added: “SAGE made a provision for countries in exceptional circumstances of (Pfizer) vaccine supply constraints to delay the administration of the second dose for a few weeks in order to maximise the number of individuals benefiting from a first dose.
“I think we have to be a bit open to these types of decisions which countries have to make according to their own epidemiological situations.”
England’s Chief Medical Officer Professor Chris Whitty told a Downing Street press conference on Tuesday that extending the gap between the first and second jabs would mean the number of people vaccinated can be doubled over three months.
“If over that period there is more than 50 per cent protection then you have actually won. More people will have been protected than would have been otherwise,” said Professor Whitty.
“Our quite strong view is that protection is likely to be lot more than 50 per cent.”
Professor Whitty said the vaccine timetable was “realistic but not easy”.
He added: “The NHS is going to have to use multiple channels to get this out but they are very determined to do this, but that does not make it easy.
“And, of course, in the case of the Pfizer vaccine, as I think is widely reported, it’s more difficult to handle because of the complicated cold chain model.
“We also, with both vaccines, wanted to be very careful in the first two or three days that we went a little bit slowly just in case there were some initial unexpected problems.”
Pfizer has said it only tested the vaccine's efficacy when the two jabs were given up to 21 days apart. Data from its clinical trials showed the vaccine was 95 per cent effective in preventing Covid-19 seven days after the second dose was given.
In a statement to CNBC, Pfizer said: “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.
“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.”
In a joint statement, the chief medical officers for England, Scotland, Wales and Northern Ireland acknowledged that the decision to reschedule appointments for second jabs was “operationally very difficult”.
They said: "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. We hope this has your support."