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The long-term mixing of different Covid-19 vaccines is unlikely to be harmful – and could even provide better protection against the coronavirus, several top experts said.
Taking one type of vaccine today and a separate dose by another manufacturer a year down the line could become normal, particularly if the pandemic continues for years.
Several top scientists gave their expert view to The National as many people debate whether to take the first available shot or wait.
Although the six vaccines approved for use around the world were each tested on hundreds of thousands of people, inevitably some have a preference or have medical reasons to hold off for now.
“You shouldn’t need it, but you cannot be over-vaccinated, so any risk would be very low,” said Prof Ian Jones, professor of virology at the University of Reading in the UK.
The approved vaccines were extensively tested, but it remains unclear how long the protection they offer will last.
With the vaccines released so far, a booster injection is required within weeks of the first dose, but it may be that, beyond this, a further shot is needed to sustain protection.
Prof Jones said that if a person completed two doses of one vaccine now, and took a different coronavirus vaccine a year later, there were unlikely to be any risks.
But experts said they draw a sharp distinction between that practice and that of combining vaccines within a short space of time.
Prof Paul Hunter, professor of medicine and infectious diseases at the University of East Anglia, UK, said one drug could stimulate enzymes in the liver that break down another drug, making it less effective.
“Interactions are one of the things we hammer into medical students now,” he said.
“They have to be careful.”
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If a booster is needed even later on, scientists say there may be an upside to using a different coronavirus vaccine.
Known as heterologous prime boosting, this phenomenon has been seen with vaccinations against numerous diseases.
“This has been known to greatly increase both antibody and T-cell immunogenicity when performed using certain vector combinations, above repeated dosing with the same vaccine candidate,” the World Health Organisation said in a 2014 briefing document.
Using the same vaccine repeatedly can lead the immune system to target the vaccine itself, reducing its efficacy.
An example is adenoviral vector immunity. This may affect the Oxford-AstraZeneca vaccine, which is based on a harmless form of an adenovirus that normally infects chimpanzees.
The adenovirus has had DNA added so that, once it enters human cells, it causes them to produce coronavirus spike proteins.
It is the body’s immune response to these spike proteins that offers protection against the coronavirus.
But if there is an immune response against the adenoviral vector itself, spike protein production on subsequent doses is hampered, and immunity fails to improve as it should.
“This might be why the AstraZeneca vaccine is not as effective as the other vaccines,” Prof Hunter said.
“You give the second dose and ... if the body has already developed some immunity to the carrier virus, it gets destroyed before it’s had a chance to insert its payload.”
Trials are under way to find out whether protection is stronger when a person receives one dose of the Oxford-AstraZeneca vaccine and one dose of Russia’s Sputnik V vaccine, which is based on a different adenovirus.
Immunity to the vaccine is thought to be less of a problem with the two mRNA vaccines, made by Pfizer-BioNTech and Moderna.
“We may well, if there are issues around people developing immunity to the carrier and we have to give boosters in a year or so’s time, deliberately switch vaccines,” Prof Hunter said.
“But we need to do a lot of work between now and then before we decide.”