Britain to test mixing and matching of COVID-19 vaccines, and length of time between doses Frank Augstein/AP FILE - A nurse prepares to administer the Pfizer-BioNTech COVID-19 vaccine at Guyâs Hospital in London, Tuesday, Dec. 8, 2020. U.K. health authorities rolled out the first doses of a widely tested and independently reviewed COVID-19 vaccine Tuesday, starting a global immunization program that is expected to gain momentum as more serums win approval. (AP Photo/Frank Augstein, Pool) By: Scripps National & The Associated Press Posted at 1:33 PM, Feb 04, 2021 and last updated 2021-02-04 14:33:47-05 British scientists are starting a 13-month vaccine trial to test whether COVID-19 vaccines can be mixed and matched.