The World Health Organisation (WHO) has joined Pfizer and BioNTech in warning against delaying a second dose of their Covid vaccine.

UK officials announced last week that the vaccination policy would change to increase the period between the prime and booster jabs from the recommended three-to-four week interval to 12 weeks.

Healthcare workers were ordered to reschedule appointments for the second dose to widen the vaccine programme to as many people as possible, even though the initial shot offers less protection on its own.

But Alejandro Cravioto, chairman of the WHO’s Strategic Advisory Group of Experts on Immunisation (Sage), said the second dose should only be delayed in “exceptional circumstances”.

He said: “We deliberated and came out with the following recommendation: two doses of this [Pfizer] vaccine within 21-28 days.

“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.”

Mr Cravioto added: “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.”

BioNTech and Pfizer have also said there is no evidence that their vaccine would continue to protect against Covid if the booster shot is given later than tested in trials.

”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 joint statement.

”There is no data to demonstrate that protection after the first dose is sustained after 21 days.”

It comes after the the Joint Committee on Vaccines and Immunisation (JCVI) last week proposed changing the approved dosing schedules, saying that “initially vaccinating a greater number of people with a single dose will prevent more deaths and hospitalisations than vaccinating a smaller number of people with two doses”.

People queue at an NHS Covid-19 vaccination centre for the Pfizer-BioNTech vaccine in LondonAFP via Getty Images
People queue at an NHS Covid-19 vaccination centre for the Pfizer-BioNTech vaccine in LondonAFP via Getty Images

England’s Chief Medical Officer Chris Whitty said during a news conference on Tuesday that there were unknowns in extending the gap between vaccine doses, and the chances of it leading to a new mutant of the virus were a small but real concern.

Increasing the length of time between doses was a sensible balance of risk, he said.

However five UK medical scientists have criticised the plan, saying proven dosing schedules should not be altered “without solid scientific support or evidence”.

In an opinion piece published online by the British Medical Journal, the scientists said the change was based on “assumptions” rather than scientific evidence or trial data.

They also questioned the rationale behind prolonging the time between first and second doses.

The scientists from the universities of Nottingham, Manchester and De Montfort also wrote that suggestions by officials on the JCVI that the strategy was due to vaccine shortages were disputed by the manufacturers.

”While assumptions can be useful for generating a hypothesis, alone they are not a sufficient reason to alter a known effective dosing regimen,“ they wrote in the BMJ.

The proposals have prompted similar considerations by other governments and generated fierce debate among scientists around the world.

Denmark has approved a delay of up to six weeks between the administration of the first and second shots of the vaccine.

Germany is also considering whether to implement the strategy.

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