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Covid-19 vaccination: What’s the evidence for extending the dosing interval?

BMJ 2021; 372 doi: (Published 06 January 2021) Cite this as: BMJ 2021;372:n18

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Covid-19 vaccines: to delay or not to delay second doses


How do you take your vaccine—one lump or two?

  1. Gareth Iacobucci,
  2. Elisabeth Mahase
  1. The BMJ

On 30 December the four UK chief medical officers announced that the second doses of the covid vaccines should be given towards the end of 12 weeks rather than in the previously recommended 3-4 weeks. Gareth Iacobucci and Elisabeth Mahase look at the questions this has raised

Why has the government taken the step to delay the second dose?

In a letter sent to healthcare staff on 30 December NHS England said the decision had been taken to prioritise giving the first doses of vaccine (whether the Pfizer and BioNTech one or that of Oxford University and AstraZeneca) to as many people as possible on the priority list to “protect the greatest number of at-risk people overall in the shortest possible time.”1 Delaying the second dose meant that the prioritisation process “will have the greatest impact on reducing mortality, severe disease and hospitalisations and in protecting the NHS and equivalent health services,” it said.

Why was this decision taken?

In a letter to the profession sent on 31 December laying out the “scientific and public health rationale” for the change to the dosing schedule,2 the chief medical officers said that vaccine shortages were a major reason for the shift in approach. “We have to ensure that we maximise the number of eligible people who receive the vaccine. Currently the main barrier to this is vaccine availability, a global issue, and this will remain the case for several months and, importantly, through the critical winter period. The availability of the AZ [AstraZeneca] vaccine reduces, but does not remove, this major problem. Vaccine shortage is a reality that cannot be wished away.”

What’s the evidence for changing the schedule?

There isn’t much for the Pfizer-BioNTech vaccine, as trials did not compare different dose spacing or compare one with two doses. The trials of the Oxford-AstraZeneca vaccine did include different spacing between doses, finding that a longer gap (two to three months) led to a …

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