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Injecting the second dose with a different vaccine increases mild side effects.

05/13/2021 at 1:10 PM CEST

A study carried out at the University of Oxford analyzes the safety and efficacy that mixing two different doses of vaccines could have, as some countries (including Spain) intend to do to complete the regimen after the first dose of AstraZeneca.

The first results are very unfavorable to this possibility because researchers have found that one dose of the Pfizer / BioNTech vaccine followed by one of the Oxford / AstraZeneca vaccine (or vice versa) induces a higher frequency of mild to moderate side effects than when both doses of the same vaccine are administered, in either of the two.

It is true that it is still the initial data obtained in this study, but it is also true that it is collected in an article published in The Lancet, which usually publishes only verified research.

The Com-Cov study involves 830 participants aged 50 and over, and is testing four combinations:

– Oxford / AstraZeneca + Oxford / AstraZeneca.

– Oxford / AstraZeneca + Pfizer / BioNTech.

– Pfizer / BioNTech + Pfizer / BioNTech.

– Pfizer / BioNTech + Oxford / AstraZeneca.

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First data with two different vaccines

The first section was obtained with a group of 463 participants who received their first and second doses (of any of the 4 possible combinations) with an interval of four weeks.

Participants reported their symptoms for seven days after the second dose. And the first conclusion that could be drawn is that with both the Pfizer and AstraZeneca vaccines, the mixed doses caused more side effects.

Fever: In this very predictable side effect the researchers found that:

– Only 10% of people vaccinated with the two doses of Oxford / AstraZeneca reported feeling feverish.

– In the case of those who received the full Pfizer / BioNTech regimen, those who had a fever was 21%.

– 34% of patients receiving Oxford / AstraZeneca mixed sequence plus Pfizer / BioNTech reported fever.

– The highest percentage of febrile reaction (41%) was produced by the combination that many countries intend to carry out, which is none other than completing the first dose of Oxford / AstraZeneca with a second dose of Pfizer / BioNTech.

Very similar percentages were found for the side effects of chills, fatigue, headache, joint pain, malaise, and muscle pain.

But it is important to note that we are talking about short-term adverse reactions. And that in the entire study, so far, no security problems that deserve to be highlighted have been found.

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same cases, but more often

The principal investigator of this study, and associate professor of pediatrics and vaccines at the University of Oxford, Matthew Snape, told The Guardian:

– «This is the type of reaction that anyone expects a vaccine to produce, and it is broadly what we are finding in the vaccination programs carried out in different countries. The only difference is that in these cases they occur more frequently.

And we are talking about groups of volunteers whose age was equal to or greater than 50 years. Because if we transferred this data to the real world, in which there are younger age groups, it would be normal that we would find stronger reactions to vaccines, he noted.

– «It would be logical to hope, says the main researcher, that the data that we are finding now and that show stronger reactions in mixed vaccinations, will be maintained in the younger age groups. We may even see more reactions.

At the moment we are not talking about dangerous reactions. The consequences found so far, and the most likely with such a small sample compared to reality, should only concern us in terms of absenteeism.

Because a 41% rate of fever or malaise could prevent an equal percentage of people from going to work the next day. And it can be many people.

Now the question is to see if these combinations cause some kind of serious reaction in the form of clots or thrombosis. But the hopes of finding evidence in this regard are few, since these are events so rare that with the percentage of the population studied they are not likely to appear.

What is expected now is that after a few more days we will be able to know data on the efficacy of the two doses, in the 4 combinations of vaccines studied.

Because the immune response achieved by mixed vaccination regimens will be very important to guide future decisions.

The trial investigators are also evaluating the impact of dosing participants 12 weeks apart.

And another study is already underway that includes the Moderna and Novavax vaccines as a second dose in the combinations.

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