Despite having no evidence about the extent of immunity offered by mixing COVID-19 doses, Britain says, it will allow people to be given shots of different COVID-19 vaccines on rare occasions.

In a departure from other strategies globally, the government said people could be given a mix-and-match of two COVID-19 shots, for example if the same vaccine dose was out of stock, according to guidelines published on New Year's Eve. "(If) the same vaccine is not available, or if the first product received is unknown, it is reasonable to offer one dose of the locally available product to complete the schedule," according to the guidelines.

Mary Ramsay, head of immunisations at Public Health England, said this would only happen on extremely rare occasions, and that the government was not recommending the mixing of vaccines, which require at least two doses given several weeks apart.

"Every effort should be made to give them the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all," she said.

COVID-19 has killed more 74,000 people in Britain - the second-highest death toll in Europe, and health officials are racing to deliver doses to help end the pandemic as fears grow that the health service could be overwhelmed.

(With input from agencies)