UK preparing to study 'mix-and-match' approach to COVID-19 vaccination

Pilot trials are being planned in the UK to investigate a regimen using two different COVID-19 vaccines to see if a "mix-and-match" approach gives better protection against SARS-CoV-2 infection. The studies could start in January, pending approval of a second vaccine, following the UK's recent temporary authorisation of Pfizer and BioNTech's BNT162b2.

The trial will begin if AZD1222, a vaccine being developed by AstraZeneca and the University of Oxford, is approved in the coming weeks. Rather than receiving two doses of the same vaccine, participants in the study will receive one dose of either BNT162b2, an mRNA-based candidate, or AZD1222, which uses a replication-deficient chimpanzee viral vector, followed a few weeks later by a single dose of the vaccine they did not receive initially.

Heterologous boost approach

Although late-stage studies have shown that two doses of BNT162b2 given 21 days apart have an efficacy rate of 95%, researchers want to explore whether the immune response can be increased by using this "heterologous boost" approach. Kate Bingham, chair of the UK's vaccine taskforce, said "it's to do with trying to trigger the immune response and the durability," adding "you do a prime with one vaccine and then the second – whether it's 28 days or two months or whatever the agreed periods would be – would be with a different vaccine."

While the UK government has ordered 40 million doses of BNT162b2, compared to 100 million doses of AZD1222, Bingham explained that the combination studies are "not being done because of supplies." Clive Dix, deputy chair of the taskforce, added "there is a slight benefit to it, too, in that if prime and boosting either way around work, it may help with the deployment, because it might just be simpler to deploy that way round, but the main reason is to get a stronger immune response."

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