SCIENTISTS SAY VACCINE MIXES ARE NOT LIKELY TO BE HARMFUL
▶ As inoculation programmes begin, combined treatments cannot be ruled out, writes Daniel Bardsley
The long-term mixing of coronavirus vaccines is unlikely to be harmful and could improve protection, academics said. Taking one type of vaccine today and a separate injection from another manufacturer a year later could become the norm, especially if the pandemic continues.
Several scientists gave their views to The National as 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 patients will have a preference or 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 Ian Jones, professor of virology at the University of Reading in England.
The approved vaccines were extensively tested, but it is not clear 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 vaccine a year later, there were unlikely to be risks.
But scientists said they drew a distinction between that practice and combining vaccines within a short space of time.
Paul Hunter, professor of medicine and infectious diseases at the University of East Anglia in England, 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. They have to be careful,” he said.
If a booster is needed later on, scientists said there may be an advantage in using another vaccine.
This phenomenon, known as heterologous prime boosting, has been reported with vaccinations against many 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 document.
Using the same vaccine repeatedly can turn the immune system against the vaccine.
An example is adenoviral vector immunity. This may affect the Oxford-AstraZeneca vaccine, which is based on a harmless form of an adenovirus that usually infects chimpanzees.
The adenovirus has 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 proteins that offers protection.
But if there is an immune response against the adenoviral vector itself, spike protein production on subsequent doses is impeded.
“This might be why the AstraZeneca vaccine is not as effective as the others,” 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 a 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, deliberately switch vaccines,” Prof Hunter said.
“But we need to do a lot of work between now and then before we decide.”