A new clinical study, based in the UK, will investigate alternating COVID-19 vaccine doses with the aim of examining whether different vaccines can safely be used for two-dose regimens.

The Com-Cov study will be the first in the world to evaluate the effects of using different vaccines for the first and second dose within a two-dose vaccination regimen.

This could, for example, use AstraZeneca/Oxford University’s vaccine for the first dose and Pfizer/BioNTech’s vaccine for the second dose.

It will be run by the National Immunisation Schedule Evaluation Consortium (NISEC) across eight National Institute for Health Research (NIHR) supported sites.

The study, which will run for 13 months, will monitor the impact of the different dosing regimens on immune responses, with initial findings expected to be published in the summer.

Researchers will also gather immunological evidence on different intervals between the first and second dose for a mixed-vaccine regimen against control groups when the same vaccine is used for both doses.

“Given the inevitable challenges of immunising large numbers of the population against COVID-19 and potential global supply constraints, there are definite advantages to having data that could support a more flexible immunisation programme, if needed and if approved by the medicines regulator,” said professor Jonathan Van-Tam, deputy chief medical officer and senior responsible officer for the study.

“It is also even possible that by combining vaccines, the immune response could be enhanced giving even higher antibody levels that last longer; unless this is evaluated in a clinical trial we just won’t know,” he added.

At the moment, a same-dose regimen is implemented for the UK’s COVID-19 vaccination programme, with no current plans for this to change.