A new vaccine trial is underway in the U.K. to test whether mixing and matching different COVID-19 vaccines for the first and second doses is effective, BBC News reports.

The country is currently using the Pfizer-BioNTech vaccine and Oxford-AstraZeneca vaccine, which both require two doses. For the study, run by the National Immunisation Schedule Evaluation Consortium, patients will get one dose of each. 

The trial will involve 800 volunteers, age 50 or older, who haven't received a COVID-19 vaccine yet. They will receive their two vaccine doses as usual, four or 12 weeks apart, but one will be the Oxford shot and the other will be the Pfizer shot. If regulators approve other vaccines for use, they may be added to the study.

The study will last 13 months but scientists hope to have some initial findings by June. In the meantime, no changes are being made to the current vaccination process.

If effective, this might provide more flexibility in vaccine rollout. Mixing vaccines might also give a person better protection, scientists say. 

England's deputy chief medical officer, Professor Jonathan Van-Tam, said learning wether or not vaccines can be mixed will provide "definitive advantages," because of the large number of people that need the shots and the potential supply constraints, BBC News reports.

Vaccines Minister Nadhim Zahawi told BBC News that this approach of mixing vaccines has been used before for other diseases like hepatitis, polio, measles, mumps and rubella. Some Ebola immunization programs also mixed different vaccines to improve protection. 

Professor Matthew Snape from the University of Oxford, the chief investigator of the study, told BBC Radio that animal studies have shown "a better antibody response with a mixed schedule rather than the straight schedule" of vaccine doses.

He said this was a "tremendously exciting study" that will provide information for vaccine rollouts across the world. "It will be really interesting to see if the different delivery methods actually could lead to an enhanced immune response [in humans], or at least a response that's as good as giving the straight schedule of the same doses," he said. 

A similar study, conducted by U.K. and Russian scientists in Russia, is testing the combination of Oxford-AstraZeneca and Russia's Sputnik V vaccines, BBC News reported.

The U.S. is currently using the Pfizer-BioNTech and Moderna COVID-19 vaccines. The Advisory Committee on Immunization Practices (ACIP) says these two vaccines are not interchangeable with each other or with other COVID-19 vaccines, and notes that the safety and efficacy of a mixing vaccines has not been evaluated. 

However, the U.S. Centers for Disease Control and Prevention recently updated its guidelines to allow "in exceptional situations" getting a second dose of a different brand. For example, it says, "In extremely rare situations, some people simply may not have documentation or know which vaccine they got for their first dose.