The trial, known as the Com-Cov study, was first launched in February to look at whether giving a first dose of one type of COVID-19 shot, and a second dose of another, elicits an immune response that is as good as using two doses of the same vaccine.
The idea, said Matthew Snape, the Oxford University professor leading the trial, “is to explore whether the multiple COVID-19 vaccines that are available can be used more flexibly”.
But reports of very rare blood clots have prompted some governments – including France and Germany – 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.
In a briefing about the expansion of the study to include Moderna’s and Novavax’s COVID-19 vaccines, Snape, an associate professor in paediatrics and vaccinology at Oxford, said it will seek to recruit adults aged over 50 who have received their first, or “prime” vaccination in the past 8-12 weeks.
The six new arms of the trial will each involve 175 people, adding a further 1,050 recruits in total, Snape said.
“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,” Snape said.
“This would also create resilience within the system in the event of a shortfall in availability of any of the vaccines.”
Reporting by Kate Kelland. Editing by Mark Potter