What happens if you mix the coronavirus vaccines? © Getty

Coronavirus vaccines: Why scientists think it’s safe to mix them

Most people will need two doses of a COVID-19 vaccine – but each jab may not need to be the same. Here's why some scientists see no harm in mixing them.

You know how it is: you wait around for the best part of a year and then three vaccines are approved all at once.

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In just over one month, the UK has given the green light for the Pfizer, Oxford and Moderna jabs to be administered across the country, with millions of people having received their first vaccine dose. But what would happen if their second jab was from a different manufacturer than the first? Is it really safe to mix coronavirus vaccines?

To prompt a strong immune protection against the coronavirus, all three vaccines need to be delivered in two parts – everyone should get a second jab a few weeks after their first. And, at the moment, the UK government says it will ensure people will receive the “same vaccine type” across both doses. If you get the Oxford vaccine as your first dose, your second will also be an Oxford jab – not Moderna or Pfizer. Easy.

However, there is a small – very small – possibility that vaccines may be mixed in select circumstances.

As Dr Mary Ramsay, head of immunisations at Public Health England has said, on “extremely rare occasions” the same vaccine is not available, or if there is no record which jab a person received first, a different vaccine may be administered for the second dose.

“Every effort should be made to give [patients] the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all,” she told the BMJ.

So, why is a combination of vaccines better than nothing at all? And how safe actually is this approach? You can find your complete dose of answers below.

Is it safe to mix the coronavirus vaccines?

At the moment, scientists have no reason to think a combination of vaccine doses from different manufacturers would cause harm. And, to confirm this, the UK’s Vaccine Taskforce has announced trials examining the interchangeability of the COVID-19 vaccines.

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The very fact these human trials have been greenlit is a good indication, says Dr David Matthews, virologist at the University of Bristol.

“You wouldn’t even be able to start them if a lot of people thought they’d be a problem,” he explains. “An ethics body, hospitals and other scientists will only go ahead with these trials if everyone is very confident that nothing horrible is going to happen.

“And if it turns out down the road there’s a problem then we’re going have to do a lot of science to work out why there’s an issue. But everything we know so far about vaccines and the way vaccines work says this shouldn’t be a problem. It’s not like such vaccines have been dropped down from a spaceship – we’ve had a long time to study them.”

Do both coronavirus vaccine doses need to be the same to work?

Although no studies have yet been published to confirm it, it’s extremely likely mixing vaccines will still produce a strong immune response, according to Matthews.

This isn’t to say the vaccines are identical – the Oxford adenoviral vaccine works differently from the Pfizer and Moderna mRNA jabs. While the Oxford vaccine prompts some of your cells to reproduce genetic instructions (codes on how to create the coronavirus ‘spikes’), the mRNA vaccines instead deliver these instructions straight to your cells.

“The bottom line of it is that all the approved vaccines are designed to make cells in your body to produce these spikes, which mimics how the coronavirus works. They all trick your immune system into thinking it’s infected with COVID-19, preparing it for a real thing. While their methods differ, the outcome is the same,” says Matthews.

“So, from a biologist’s perspective, there’s no reason why you couldn’t mix the two.

What happens if you mix the coronavirus vaccines? © Getty
Mass vacation sites, such as this one in Manchester, are being opened across the UK © Getty

“Sure, it’s not forbidden by the laws of physics that there might be some biology at play that we don’t know about. But, at the moment, you’d have to be on some really powerful hallucinogenic drugs to come up with why it wouldn’t work.”

In fact, some scientists predict two doses from different manufacturers will produce a better immune response than sticking to one type. After all, the idea of mixing vaccines (or “heterologous prime-boosting”) has been previously shown to boost immunity against diseases such as SARS.

As Professor Helen Fletcher from the London School of Hygiene & Tropical Medicine, previously said about combining coronavirus vaccines: “This is the logical next step in vaccine development for COVID-19 as priming with one type of vaccine and boosting with another is a commonly used strategy for increasing the level and duration of efficacy against infectious disease.

“Combining vaccines has the potential to be more effective through boosting immune responses to a higher level and maintaining immunity over a longer period of time.”

In short: not only is mixing vaccines probably safe, but it could soon be the norm.

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