A new trial testing whether giving people two different Covid vaccines for their first and second doses is as effective as the current approach of using the same vaccine for both has been launched.
The £7m trial is the first of its kind in the world, and comes as the UK reported that more than 10m people had received their first dose of a vaccine.
Scientists will also gather further evidence about the different intervals between receiving two doses of the vaccines.
The 800-person study will last for 13 months, with initial results expected to be released in the summer.
A same-dose regimen is currently implemented for the national COVID-19 vaccination programme, and there are no current plans for this to change.
Anyone who has received either the Pfizer or Astrazeneca vaccination as part of the UK-wide delivery plan will not be affected by this study.
They will receive their second dose from the same source and over the same 12 week interval.
Deputy chief medical officer Jonathan van Tam said it was “possible” that combining vaccines could give an “enhanced” immune response.
“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”, he said.
“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.”