The research, led by the University of Oxford, will now include the Moderna and Novavax vaccines as well, as part of the ongoing investigations into alternating the first two jabs.
Testing multiple vaccines
The Com-Cov study, launched in February, is looking to recruit adults aged over 50 who have received their first vaccine dose in the past eight to 12 weeks.
The volunteers, who will have received either the AstraZeneca or Pfizer vaccine, will be randomly allocated to receive either the same vaccine for their second dose, or a dose of the jabs produced by Moderna or Novavax.
The Moderna jab has now started being rolled out across the UK, while the Novavax jab manufactured by GlaxoSmithKline (GSK) is under rolling review by the Medicines and Healthcare products Regulatory Agency (MHRA).
The six new arms of the trial will each recruit 175 candidates, adding 1,050 recruits into the programme, and researchers will study any adverse reactions and immune responses to these new combinations of vaccines.
The study is designed as a so-called non-inferiority study, which means its intent is to demonstrate that mixing is not substantially worse than not mixing.
It will compare the immune system responses to the gold-standard responses reported in previous clinical trials of each vaccine.
Matthew Snape, associate professor in paediatrics and vaccinology at the University of Oxford, and chief investigator on the trial, explained: “The focus of both this and the original Com-Cov study is to explore whether the multiple Covid-19 vaccines that are available can be used more flexibly, with different vaccines being used for the first and second dose.
“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.
“This would also create resilience within the system in the event of a shortfall in availability of any of the vaccines in use.”
If the study shows promising results, the MHRA and Joint Committee on Vaccination and Immunisation (JCVI) would formally assess the safety and efficacy of any new vaccination regimen before it is rolled out to patients.
Prof Snape said he hoped the results of the second part of the study would be available in June or July, with the first part expected to report results next month.
He added: “What I’m hoping is that we won’t rule out any combinations.
“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.
“And that will give us lots of flexibility, not just in the UK, not just in Europe where we’re looking about restricting uses of some vaccines for some age groups, but across the world, where we have, perhaps, a little bit more intermittent supply of vaccines, not as reliable.
“But let’s hope that we can actually use this to get two doses of vaccine to as many people as possible.”
The study is being run across nine National Institute for Health Research-supported sites by the National Immunisation Schedule Evaluation Consortium, and is backed through funding from the Vaccines Taskforce and the Coalition for Epidemic Preparedness Innovations.
Professor Jeremy Brown, a member of the Joint Committee of Vaccination and Immunisation, said people will eventually “have to” mix Covid-19 jabs.
Speaking on BBC Radio 4’s Today programme, he said: “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.
“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.”