There is “very little data” to underpin the UK’s recommendation to delay the second dose of the Pfizer Covid-19 jab for up to 12 weeks, global health leaders have said.

The second dose of the vaccine should only be delayed for up to six weeks, according to the World Health Organisation (WHO).

WHO experts met to discuss policy recommendations for the use of the Pfizer/BioNTech vaccine on Tuesday.

Alejandro Cravioto, chair of the WHO’s Strategic Advisory Group of Experts on Immunisation – also known as Sage but not to be confused with the British Sage group of scientists – said that in “exceptional” circumstances the second dose of the vaccine could be delayed.

The group concluded that it could be delayed for up to six weeks, which was the “outer limit” observed in the clinical trials for the vaccine.

Experts in the UK have said that the second jab can be delayed for up to 12 weeks in a bid to get more people vaccinated sooner.

Dr Cravioto told a WHO press briefing on Tuesday: “Sage recommend the administration of two doses of this vaccine within 21 to 28 days.

“While we acknowledge the absence of data on safety and efficacy after one dose beyond the three/four weeks studied in the clinical trials, Sage made a provision for countries in exceptional circumstances of vaccine supply constraints and epidemiological settings 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.”

Dr Kate O’Brien, another member of the WHO’s working group, said: “The recommendation that came out of Sage was to allow for this somewhat extended interval that went up to a six-week period in settings where (there is) concern about the epidemiology and the prediction of supply.”

She added: “That was allowing for a period of time that was an outer limit in the clinical trials.”

Dr Joachim Hombach, executive secretary of the group within the WHO, said the recommendation is to deliver the dose within 21 to 28 days where the bulk of the clinical trial data was generated, with 42 days being the upper limit of the data seen by experts.

“The JCVI (Joint Committee on Vaccination and Immunisation) of the UK has given more flexibility – up to 12 weeks – in consideration of the specific circumstances that the country is currently facing.

“We feel that we need to be grounded in evidence in relation to our recommendations, but totally acknowledge that countries may see needs in order to be even more flexible in terms of the administration of the second dose.

“But it is important to note that there is very little data from the trials that underpin this type of recommendation.”

Pfizer has said previously that its clinical trials tested the vaccine’s safety and efficacy following the two-dose schedule, separated by 21 days.

In a statement released when British authorities made the decision on December 30, the pharmaceutical giant added: “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.”