As countries are scrambling for vaccines to protect their citizens from COVID-19, vaccine shortages and inequality have put countries, especially the poorer ones, under threat.

The UK is now undertaking a "world first" trial to evaluate the immune responses after people get mixed shots during the usually two-dose vaccine regimen, an attempt to increase the flexibility in vaccine supply and delivery.

Started in early February, the trial, dubbed "Com-Cov," is led by the University of Oxford and backed by 7 million pounds (about $9.68 million) from the British government's Vaccines Taskforce. 

More than 800 volunteers aged 50 and above will be recruited to the trial. They will be given four different combinations of the first dose and the booster one: a first dose of the Oxford-AstraZeneca vaccine followed by boosting with either the Pfizer-BioNTech vaccine or a further dose of the Oxford-AstraZeneca vaccine, and vice versa, said the University of Oxford.

"These will be evaluated at two different dosing schedules: at a four-week interval for an early interim data readout and at a twelve-week interval for comparison to current UK policy," the university added.

"If we do show that these vaccines can be used interchangeably in the same schedule, this will greatly increase the flexibility of vaccine delivery, and could provide clues as to how to increase the breadth of protection against new virus strains," said Matthew Snape, chief investigator on the trial and associate professor in Pediatrics and Vaccinology at the University of Oxford.

The strategy of mixing different vaccines was previously used in the prevention and control of infectious diseases such as Ebola.

Janssen Pharmaceuticals of Johnson & Johnson was granted by the European Commission in 2020 for its Ebola vaccine regimen, which consists of two doses, Zabdeno (Ad26. ZEBOV) and Mvabea (MVA-BN-Filo), with the former given first and the latter administered about eight weeks later.

"Given the inevitable challenges of immunizing 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 immunization program, if needed and if approved by the medicines regulator," said Jonathan Van-Tam, deputy chief medical officer and senior responsible officer for the study.

Initial trial data is expected to be obtained around June, and more vaccines will be added to the trial after they are approved and rolled out in the country.

Health authorities in other countries are cautious about mixing COVID-19 vaccines.

The latest update to the website of the U.S. Centers for Disease Control and Prevention said that the country's currently authorized mRNA COVID-19 vaccines "are not interchangeable with each other or with other COVID-19 vaccine products."

"The safety and efficacy of a mixed-product series have not been evaluated. Both doses of the series should be completed with the same product."