Britain will allow people to be administered a mix of vaccines, according to guidelines published Dec. 31.

The guidelines stated that "(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."

The guidelines depart from those of several other countries, including the US, who cite a lack of evidence as to why vaccines should not be mixed. 

Britain has said the mix-and-match approach to vaccine administering will be a very rare scenario, and that the country does not recommend it. 

Britain was the first country to approve the Pfizer-BioNTech vaccine, and recently approved its own AstraZeneca-Oxford University vaccine.