WHO weighs in on COVID-19 vaccine second dose delay

The UK's chief medicial officers - faced with the increased threat from the more transmissible variant and a health service under pressure - now recommend that the gap between the first and second vaccine doses should be lengthened to allow more people to receive the first dose and therefore a certain level of protection. It says a second dose of the vaccine can be administered within 12 weeks.

The US, in contrast, has stated that it will not make any changes​​ to the 21 day spacing between doses. Denmark is allowing up to six weeks between doses, as is France.

The WHO’s Strategic Advisory Group of Experts on Immunization (SAGE) puts the maximum delay between doses at six weeks: given that clinical data is available for up to 42 days between doses.

2nd dose delay acceptable 'for a few weeks'

The committee notes the difficulties countries may face in rolling out limited doses of vaccine and their eagerness to vaccinate as many people as possible when faced with increasing case numbers.

“SAGE recommend the administration of two doses of this [Pfizer/BioNTech] vaccine within 21 to 28 days,”​ said Dr Alejandro Cravioto, SAGE chair, at a press briefing this week.

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

While it recommends the maximum delay between doses should be six weeks, the SAGE committee also says that an individual who has had a delay in getting the second dose – for whatever reason – can still receive a second dose: “There’s no outside limit at which point we say ‘you can’t get your second dose’.”​

Responding specifically to the UK’s strategy of spacing out doses, Dr Kate O’Brien, Director, Immunisation, Vaccines and Biologicals at the WHO, says each country is weighing the risks and rewards of the strategy in the context of their own situation.

“I think everybody realises that we're operating in a space where the amount of evidence to drive policy decisions is evidence that is evolving. ​

"What many communities are doing is weighing the evidence that we do have about the efficacy and the immunogeneicity that has [been] derived from clinical trials where the timing of doses is delivered in a specified timing, against what to do around the possibility and some evidence that shows the possibility that single-dose efficacy between the timing of the first and the second dose is quite high."​

Countries with constrained supply have to consider how they give out the first doses of the vaccine in the context of how supply is rolling in; weigh up broader rollout of a first dose against a second dose delay; and whether they want to risk administering the vaccine outside the parameters of clinical trials without evidence for efficacy; 

"What we're seeing around the world is different committees are weighing those risks and discussing those risks in different ways.”​