• A trial is being launched in the U.K. to explore whether using different Covid-19 vaccines for the first and second doses works.
• The aim is to make nationwide vaccination programs more flexible and to explore whether mixed vaccines can even enhance the immune response.
• The trial is being led by the University of Oxford and run by the National Immunisation Schedule Evaluation Consortium.

LONDON — A trial is being launched in the U.K. to explore whether using different Covid-19 vaccines for the first and second doses works in a bid to make nationwide vaccination programs more flexible.

The trial, being led by the University of Oxford and run by the National Immunisation Schedule Evaluation Consortium, will evaluate the feasibility of using a different vaccine for the initial "prime" vaccination to the follow-up "booster" vaccination. 

It is hoped the study will help policymakers understand whether mixing different Covid vaccines could be a viable route to increase the flexibility of vaccination programs and whether it could even provide better protection.

"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," Matthew Snape, chief investigator on the trial and associate professor in Paediatrics and Vaccinology at the University of Oxford, said on Thursday.

The trial, which is formally known as the "COVID-19 Heterologous Prime Boost study" but has been dubbed the "Com-Cov" study, will recruit over 800 volunteers aged 50 and above in England to evaluate the four different combinations of prime and booster vaccination.

It will trial 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. The research will also look at a first dose of the Pfizer-BioNTech vaccine followed by boosting with either the Oxford-AstraZeneca vaccine, or a further dose of the Pfizer-BioNTech vaccine.  The U.K. government has hailed the trial as a "world first."

These will be evaluated at two different dosing schedules: at a four-week interval to give an early interim data readout, and at a 12-week interval. This latter dosing interval is the current U.K. vaccination policy: A delay in the second dose means more people can get their first vaccines sooner amid a tight supply of shots.

Although the policy has been seen to be controversial, with some experts worrying that it could make the vaccines in use in the U.K. less effective; so far only the University of Oxford-AstraZeneca and Pfizer-BioNTech candidates are in use, with the Moderna shot set to be added to the basket of vaccines in use later in spring.

However, the University of Oxford published a study on Wednesday in which it showed that a delay of 12 weeks between the first and second doses of the AstraZeneca jab increases the efficacy of the vaccine.

The researchers found that the shot was 76% effective at preventing symptomatic infection for three months after a single dose, and that the efficacy rate rose to 82.4% when there was at least a 12-week interval before the second dose. When the second dose was given less than six weeks after the first one, the efficacy rate was 54.9%.

How the Com-Cov study will work

In the latest "Com-Cov" study, researchers will collect blood samples from trial volunteers and will monitor the impact of different dosing regimens on participants' immune responses, as well as looking for any additional adverse reactions to the new combinations of vaccines.

The study will last for 13 months and has been backed by £7 million ($9.5 million) of government funding from the Vaccines Taskforce, which was set up by the U.K. last April to co-ordinate efforts to research and produce a coronavirus vaccine.

Professor Snape said the study was "tremendously exciting," before adding that "it will provide information vital to the rollout of vaccines in the U.K. and globally."

Richer countries are scrambling to vaccinate as many people as possible as national lockdowns, aimed to limit the spread of infections and prevent hospitals from being overrun, damage economies.

The U.K. has been hit hard by the pandemic and has seen cases surge in the winter, aided by a more virulent variant of the virus that emerged in southeastern England that has now become a dominant strain in the country.

The U.K. has the fourth-highest number of cases in the world at present, with over 3.8 million confirmed infections, according to a tally from Johns Hopkins University, and has recorded 109,547 deaths.

The U.K. government was quick to pre-order coronavirus vaccines from a variety of makers early on last year, and to approve the vaccines currently in use. Its vaccination program has been widely praised for its agility and reach, and it's on target to have vaccinated 15 million people in its top four priority groups, including health and care workers, the elderly and over-70s and anyone deemed extremely clinical vulnerable, by mid-February.

The latest government data, as of Wednesday, shows that just over 10 million people have had their first vaccine dose, and just under 500,000 have had a second dose, too. The Oxford-AstraZeneca vaccine, which is produced in Britain, makes up the bulk of the U.K.'s vaccination program.  

Deputy Chief Medical Officer and Senior Responsible Officer for the study, Professor Jonathan Van-Tam, said that the research could even show that alternating vaccines could enhance the level of antibodies needed to fight potential Covid-19 infection.

"It is also even possible that by combining vaccines, the immune response could be enhanced giving even higher antibody levels that last longer; unless this is evaluated in a clinical trial we just won't know. This study will give us greater insight into how we can use vaccines to stay on top of this nasty disease," he said.

The U.K.'s vaccines minister Nadhim Zahawi told the BBC on Thursday that, for now, the country's vaccination program will continue as normal: "At the moment we will not change anything at all," Zahawi told the 'Today' program.

"If you've had a Pfizer BioNTech vaccine for your first dose, you'll have a Pfizer BioNTech vaccine for your second.  If you've had Oxford-AstraZeneca, you'll have Oxford-AstraZeneca for your second dose."