It is hoped the study will help policymakers understand whether mixing different Covid vaccines could be a viable route to increase the flexibility of vaccination programs and whether it could even provide better protection. "If we do show that these vaccines can be used interchangeably in the same schedule this will greatly increase the flexibility of vaccine delivery, and could provide clues as to how to increase the breadth of protection against new virus strains," Matthew Snape, chief investigator on the trial and associate professor in Paediatrics and Vaccinology at the University of Oxford, said on Thursday. The trial, which is formally known as the "COVID-19 Heterologous Prime Boost study" but has been dubbed the "Com-Cov" study, will recruit over 800 volunteers aged 50 and above in England to evaluate the four different combinations of prime and booster vaccination.