Covid vaccine mixing: How does it work and why is the UK expanding trial?

A major “mix and match” Covid-19 vaccine study in the UK is being expanded as part of crucial research into the effects of receiving two different kinds of jabs.

It looks at whether giving a first dose of one type of Covid shot and a second dose of another elicits an immune response that is as good – or potentially even better – than as using two doses of the same vaccine.

First launched in February, more than 800 people have already taken part in the mix and match trial by taking doses of the Pfizer and AstraZeneca vaccines.

The scientists behind the Com-Cov study will now include the Moderna and Novavax jabs as part of their experiments.

While the Moderna vaccine has been approved, the Novavax jab is still waiting for approval from the UK medicines regulator before it can joins the rollout.

Why are scientists interesting in mixing different jabs?

Matthew Snape, the Oxford University professor leading the trial, said the idea is to explore whether the multiple coronavirus vaccines that are available “can be used more flexibly”.

Britain and many other countries in Europe are currently using AstraZeneca’s and Pfizer’s Covid vaccines in nationwide immunisation campaigns against the pandemic.

But reports of very rare blood clots have prompted some governments to say the AstraZeneca shot should only be given to certain age groups, or that people who have had a first dose of AstraZeneca’s vaccine should switch to a different one for their second dose.

If different doses can be mixed safely, it would allow the NHS and other national health bodies to switch freely between whatever vaccines are available.

“If we can show that these mixed schedules generate an immune response that is as good as the standard schedules, and without a significant increase in the vaccine reactions, this will potentially allow more people to complete their Covid-19 immunisation course more rapidly,” Prof Snape said.

Could it create an even stronger immune response?

It has been suggested that combining different vaccines might offer more longer-lasting immunity against Covid-19 and its variants.

Professor Anthony Harnden, deputy chairman of the UK government’s Joint Committee on Vaccination and Immunisation (JCVI), said: “Mixed schedules may – and this is a big may – but they may give better longer-term protection, and that will be very interesting to see.”

Testing on mice gives suggests that combining doses may provide a better immune response, Prof Snapes said on Thursday.

“There’s some suggestions from animal studies in mice that actually you may get a better immune response if you, for example, combine the AstraZeneca-type vaccines with an RNA-type vaccine, and that actually seems to generate in some aspects a better immune response,” he said.

“So it will be interesting to see if we see that in humans also.” But the lead scientist emphasised that the main purpose of the study is to demonstrate that mixing is not substantially worse than not mixing.

“What I’m hoping is that we won’t rule out any combinations,” said Prof Snape. “That’s how we need to look at it – are there any combinations we shouldn’t be giving, because they don’t generate a good immune response and I’m hoping that won’t be the case.”

Will we all be getting different Covid jab doses in future?

Professor Jeremy Brown, a member of the Joint Committee of Vaccination and Immunisation, said people will eventually “have to” mix Covid-19 jabs.

“It’s practically going to have to be that way because, once you’ve completed a course of, say, the Moderna or Pfizer or the AstraZeneca with two doses, in the future it’s going to be quite difficult to guarantee you get the same type of vaccine again,” he told the BBC.

“So there will be a mix-and-match occurring just by the sort of practicalities of doing a third or fourth vaccination over the next few years.”

How big is the expanded study?

Prof Snape said the Oxford team will seek to recruit adults aged over 50 who have received their first, or “prime” vaccination in the past 8-12 weeks.

These volunteers, who will have received either the AstraZeneca or Pfizer vaccine, will be randomly allocated to get either the same vaccine, or the Moderna or Novavax vaccine, for a second dose.

More than 800 people have already taken part. The six new parts of the trial will each involve 175 people, adding a further 1,050 recruits in total, Prof Snape said.

Results from the original mixing trial, using AstraZeneca and Pfizer shots only, are expected as early as late April or May, while results of this new phase should come in July.