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Oxford University is set to begin a trial combining COVID-19 vaccines from AstraZeneca PLC and Pfizer Inc. that could enable greater flexibility in the use of scarce supplies globally.

The university will begin recruiting 820 participants over 50 years of age across eight U.K. sites this week, according to a statement Thursday. The AstraZeneca and Pfizer vaccines will be given in different orders and with two dosing intervals, four and 12 weeks apart.

The trial will allow researchers to see whether two shots of different vaccines produce better or worse results than two doses of the same product. The study could be key to relieving pressure on individual vaccine-makers to deliver shots if they run into manufacturing difficulties.

Supply delays from AstraZeneca and other drugmakers have prompted tensions with the European Union, spurring the EU to introduce export controls to help shore up its own vaccines. If shots can be combined, then countries may be more open to assisting one another with surplus supplies.

Enrollment should be completed this month, with initial data expected around June, according to Matthew Snape, the Oxford investigator leading the trial. The U.K. government contributed £7 million pounds ($9.6 million) to the study.

“It’s a great combination of science and policy,” Snape said at a press briefing Wednesday.

Mixing vaccines to create an enhanced immune response is common for inoculations targeting diseases such as hepatitis A and B. Combining shots can boost the immune response because the second shot won’t be limited by any immunity the body has built up to the platform delivering the first vaccine.

Both the AstraZeneca and Pfizer shots target the spike protein that the virus uses to enter cells. The British drugmaker’s vaccine is carried by a weakened chimpanzee adenovirus, while the U.S. company’s shot uses genetic material called messenger RNA.

The study organizers say theirs is the first combination trial to start globally, though Russia will also begin one next week mixing the AstraZeneca vaccine with its Sputnik V shot in Azerbaijan and the United Arab Emirates.

The study will look at the side effects and immune response the combinations produce, but not their efficacy. That was tested in large trials required for clearance and measured by infection rate.

The study will be the first to test the Pfizer vaccine, which is normally given in two doses 21 days apart, with a 12-week interval, Snape said in an interview. The U.S. company and German partner BioNTech SE conducted clinical trials with shots given three weeks apart, but U.K. health officials have extended the interval to allow more people to quickly get a first dose.

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