Three Covid-19 vaccines are now approved for use in the UK, which is positive news given the huge increases in transmission – and subsequently, the rise in hospitalisations and deaths.

But there has been some confusion over the rollout of the vaccines, specifically whether people can have mixed vaccines – for example, one dose of the Pfizer vaccine followed by a second dose of the vaccine by Oxford/Astrazeneca.

The confusion was further compounded by a New York Times article, which reported Britain had “opened doors” for “mix-and-match vaccinations”. 

But the reality is, mixing and matching vaccines is not something that’s going to be rolled out in the UK. In fact, our own health officials advise against it except in very rare circumstances. Here’s what you need to know.

Submit a coronavirus health question to HuffPost UK.

Dr Mary Ramsay, head of immunisations at Public Health England (PHE), said in a statement: “We do not recommend mixing the Covid-19 vaccines – if your first dose is the Pfizer vaccine you should not be given the AstraZeneca vaccine for your second dose and vice versa.”

Her comments were echoed at a press briefing where Professor Sir Munir Pirmohamed, chair of the Commission on Human Medicines expert working group, said: “We’re not advising mixing different doses of the vaccines because we don’t have any data on that. 

“Our advice is that if you have the Pfizer vaccine as a first dose, the second dose should also be the Pfizer vaccine. If you have the AstraZeneca vaccine as a first dose, the second dose should also be the AstraZeneca vaccine.”

That said, Dr Ramsay suggested there may be “extremely rare occasions” where either the same vaccine is not available (supply and demand is an issue right now), or where it is not known what vaccine the patient received for their first dose.

In these very rare circumstances, different vaccines may be given. “Every effort should be made to give them 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 said.

There isn’t any data to suggest mixing the two vaccines works and is safe. Trials are underway where people will receive different vaccines at different time points to see how effective it is. “That might provide us with more data as to whether you can mix the vaccines,” said Prof Pirmohamed.

Some experts believe a Covid-19 vaccine combo could work in theory, but they stress that more research is needed to explore this.

Professor Helen Fletcher, an immunologist at the London School of Hygiene & Tropical Medicine, said diseases such as HIV, TB and malaria, the immune response to one vaccine is not sufficient to provide a high level of protection, so the next logical step is to combine two different vaccines delivering the same antigen in what she describes as “a heterologous prime-boost regimen”.

Both the UK vaccines in circulation are based on the SARS-CoV-2 virus’ spike protein. A “heterologous prime-boost regimen” in this instance would mean presenting the body with the spike protein using one vaccine and then presenting the same spike protein using a different vaccine, which Prof Fletcher suggests “could give a much stronger immune response to the spike”.

This kind of strategy would be important if clinical trials of the vaccines had shown they were only weakly protective, “as combining vaccines could boost the level and duration of efficacy”. But with the vaccines’ efficacy being around 90%, there is “little advantage” to pursuing this strategy, she added.

“Based on previous studies which combine different vaccine types, a combination of the AstraZeneca and Pfizer vaccines is likely to be safe, but it’s important that this is tested in the context of a clinical vaccine trial.”

Experts are still learning about Covid-19. The information in this story is what was known or available at the time of publication, but guidance could change as scientists discover more about the virus. To keep up to date with health advice and cases in your area, visit gov.uk/coronavirus and nhs.uk.