MIXING doses of AstraZeneca and Pfizer vaccines seems safe, but may cause more side effects.
According to the first study of its kind, taking a dose of both vaccines can make people sicker than if they had just received one type.
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But the effects are short-lived and last no more than 48 hours, scientists at Oxford University reassured.
Most Covid injections require two doses at least 21 days apart, the first being called a “prime” and the second “booster”.
The Com-COV study aimed to see if two vaccines that work very differently can work in tandem.
It recruited 830 volunteers aged 50 and over across England to review four different dosing regimens:
- a first dose of Oxford-AstraZeneca vaccine followed by a booster with Pfizer vaccine or another dose of Oxford-AstraZeneca vaccine
- or a first dose of Pfizer vaccine followed by a booster with Oxford-AstraZeneca vaccine or another dose of Pfizer vaccine.
Participants were randomly assigned to one of the dosing groups and then reported how they felt within seven days of their injection.
Some 41 percent of people who received a dose of Pfizer followed by a dose of AZ reported feeling feverish.
This was double the 21 percent who had two doses of Pfizer.
Of those who had the AZ jab followed by Pfizer’s, 34 percent reported a fever compared to 10 percent of those whose doses were both the AZ jab.
Similar increases were seen for chills, fatigue, headache, joint pain, muscle pain, and a general feeling of being unwell.
Since people who received a cocktail of doses had more side effects, they were also more likely to take paracetamol, according to the study.
How do Covid vaccines work?
The AstraZeneca vaccine (as well as the Johnson & Johnson vaccine) uses a genetically modified version of a chimpanzee cold.
Called an adenoviral vector, it carries coronavirus DNA into human cells.
It then becomes part of the host cell and triggers an immune response in the body.
The jab made by Pfizer (as well as Moderna) uses mRNA technology, through which a bit of genetic code for the coronavirus is introduced into the body where it tells cells to make a protein on the virus called a spike protein.
The immune system responds by producing antibodies.
Both create the same effect but the route is slightly different.
About 60% switched to the pain reliever drug, compared with 36 to 41% of those who had the same vaccine for each dose.
Preliminary data has not yet been released, but will be available in June of this year.
In a related commentary published in The Lancet, the researchers noted that all of the participants were over the age of 50.
They predict that young people will be even more likely to see an increase in side effects.
Younger people already report more symptoms in the days following a stroke, which scientists say is the result of a stronger immune response.
Professor Matthew Snape, chief investigator of the trial, said: “The results of this study suggest that mixed dosing regimens may lead to increased absences from work the day after vaccination.”
He said this was important when planning the immunization of healthcare workers, of whom there are already shortages.
Professor Snape added, “Importantly, there are no safety issues or signals, and that doesn’t tell us if the immune response will be affected.
“We hope to publish this data in the coming months. In the meantime, we have adapted the current study to assess whether early and regular use of paracetamol reduces the frequency of these reactions.
Links of blood clots
The Com-COV study was launched earlier this year after extremely rare blood clots were linked to the AstraZeneca vaccine.
Although AstraZeneca jab is still considered extremely beneficial, some countries limit second doses.
Germany, France, Sweden, Norway and Denmark are among those pushing for mixed vaccines among those who have already received a dose of AstraZeneca.
In the UK, everyone is welcome to receive their second dose of AstraZeneca vaccine.
But it will no longer be given to people under 40 who seem to be at greatest risk for blood clots.
The combination of vaccines would also be beneficial if stocks become limited in the future and thus slow down programs.
There is also the potential for it to create even better protection against Covid, as Professor Snape has already said, there are warning signs that this could be the case.
He told Times Radio in February: “Animal studies in mice suggest that you might get a better immune response if, for example, you combine AstraZeneca-like vaccines with an RNA-like vaccine, and that seems to generate in some people. aspects, better immune response.
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“So it will be interesting to see if we see that in humans as well.”
The Com-COV study was expanded in April to also examine the mix of Moderna and Novavax vaccines.
Professor Snape said: “The main driver of this study is to increase the UK’s schedule flexibility and resilience in the event of supply issues or the availability of any of the vaccines.”