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Public Health England (PHE) said it did not recommend mixing Covid-19 vaccines from different suppliers.
The warning comes as distribution of the Oxford / AstraZeneca vaccine is set to begin in the UK next week, the second rollout after the Pfizer / BioNTech vaccine launched in December, with both requiring two doses.
The UK government previously issued guidelines to NHS doctors saying that if a person who received their first coronavirus vaccine returns for their second but the same type is not available, or if the first type of vaccine is unknown, then it is “reasonable” to offer a dose of another vaccine.
“This option is best if the person is likely to be immediately at high risk or is considered unlikely to reoccur,” the guide adds.
After questions were raised about the risks, Dr Mary Ramsay, head of PHE vaccinations, said mixing is not recommended and should only happen on “rare occasions.”
“We don’t recommend mixing Covid-19 vaccines – if your first dose is the Pfizer vaccine, you shouldn’t get the AstraZeneca vaccine for your second dose and vice versa,” she says.
“There may be extremely rare occasions when the same vaccine is not available, or where it is not known which vaccine the patient has received.
“Every effort should be made to give them the same vaccine, but when that is not possible, it is better to give a second dose of another vaccine than not at all.”
Each of the approved vaccines uses different technology to induce an immune response to Covid-19.
The Pfizer vaccine uses mRNA technology – a first for a vaccine – that introduces a messenger sequence into the body that contains genetic instructions that allow the vaccinated person’s own cells to produce the antigens and generate an immune response.
The Oxford vaccine uses standard vaccine technology, introducing the coronavirus gene into human cells to make the unique Covid-19 spike protein to which the immune system builds a response if the real virus enters the body.
Dr Simon Clarke, associate professor of cell microbiology at the University of Reading, said mixing vaccines without any data is a “huge gamble”.
“The consequences of a mistake are huge, it’s the most important thing in the world right now.
“It could mean the failure of the whole vaccination program, just like we have this new variant. The chances of it not working are real, and no one knows it.
Dr Julian Tang, a consultant virologist at Leicester Royal Infirmary, said there was a plan to test mixing jabs, but didn’t think this was a major issue as vaccines are often mixed between doses.
Dr Tang said he understood why people would be concerned about Pfizer and Oxford vaccines using different technology.
“But the vaccines are actually not that different, they both induce an immune response for the spike protein.
“I think the risk of people getting the virus before they get vaccinated and die is higher than mixing vaccines,” he said.
He added that the United States could be more cautious of this approach as the medico-legal risks are much higher than in the United Kingdom.
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