What do we know about mixing vaccines?

A major UK trial exploring the possibility of a mix-and-match approach to Covid vaccination is being expanded to include the Moderna and Novavax jabs.

Scientists behind the Com-COV study, led by the University of Oxford, hope that combining vaccines could give “broader, longer-lasting immunity” against the coronavirus and new variants, reports the BBC. Using different brands of vaccination for first and second doses would also give more flexibility to the vaccine rollout.

The study is looking for volunteers over 50 who have had a first dose of Pfizer or AstraZeneca.

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More than 800 people have already participated in the trial, but Professor Matthew Snape, from the Oxford Vaccine Group, is hoping to recruit a further 1,050 volunteers who have received one vaccine dose on the NHS in the past eight to 12 weeks.

Volunteers will receive a second dose, which could be the same as the first, or a shot of the Moderna or Novavax vaccine.

Researchers are looking for adverse reactions or immune system responses to new combinations of vaccine, rather than looking at whether the vaccines are effective at preventing disease. The study is designed to show that “mixing is not substantially worse than not mixing”, explains Sky News.

Most health experts agree that mixing and matching vaccines should be safe; the trial will try to determine if there are any possible combinations that should be avoided. Professor Snape told the BBC he hoped the trial “won’t rule out any possible combinations”.

“That’s how we need to look at it: Are there any combinations we shouldn’t be giving, because they don’t generate a good immune response?”

Danny Altmann, a professor of Immunology at Imperial College, who is not involved in the trials, told CNBC last week that he couldn't see “any argument why” mixing vaccines would be “unsafe or poor practice”.

“It would still induce great immunity. No problem with that whatsoever,” he said.

Andrew Freedman, a reader in infectious diseases at Cardiff University’s School of Medicine, also told the news organisation there was “no theoretical reason why that shouldn’t be feasible and safe, but we have to wait for these studies.”

Studies into the possibility of mixing different types of vaccine have become of particular interest after both Europe and the UK’s health regulatory agencies found a possible link between the Oxford-AstraZeneca vaccine and rare cases of blood clots, with young adults at a slightly increased risk.

The UK’s Joint Committee on Vaccination and Immunisation (JCVI) has emphasised that the benefits of the protection that the AstraZeneca vaccine offers against Covid “far outweigh” the risks of an adverse reaction in the vast majority of people. But the committee is advising that adults under 30 without underlying health conditions should be offered an alternative vaccine “if available”.

France’s health regulator recommends that people under the age of 55 who have received their first dose of AstraZeneca should have Pfizer or Moderna for their second dose, with Germany similarly advising that people under the age of 60 should opt for a different vaccine for their second dose.

The Centre for Disease Control in China says it is also considering mixing vaccines in order to boost efficacy, reports the BBC, with suggestions that some vaccines approved for public use in the country have efficacy as low as 50%. By contrast, the AstraZeneca vaccine is claimed to have 100% efficacy against severe or critical disease, and 76% efficacy against symptomatic Covid.

Russia’s Sputnik V vaccine involves using two slightly different doses to give immunity.

While some results from the first stage of the Com-COV study are expected next month, the trial will have reportable findings by the summer.

Professor Snape said mixing vaccines “will give us lots of flexibility, not just in the UK, not just in Europe – where we’re looking at restricting use of some vaccines for some age groups – but across the world, where we have, perhaps, a little bit more intermittent supply of vaccines.”

“Let’s hope that we can actually use this to get two doses of vaccine to as many people as possible.”