We do not know what effects mixing vaccines has

France and Germany have announced that they will mix different vaccines. It is a political decision, not a scientific one.

In recent days, the governments of some European countries, such as France or Germany, have made public their intention to administer a different vaccine to those who received the first dose of AstraZeneca. In a risky decision that, again, is positioned against the recommendations of the European Medicines Agency, the WHO, the CDC of the United States or even that of the manufacturers that have developed the vaccines. Social pressure, political expediency or delays and stoppages in supply are becoming important factors in making decisions, in many cases contrary to what science, data or available studies dictate.

The controversial decision of these countries has raised opposing voices that fear that our own Ministry of Health, so easily influenced by any blade of air that rises, may also opt for this idea. The mistrust is understandable considering the countless hesitations, stoppages and communication errors of the Ministry of Carolina Darias in recent weeks.

On the website of the Ministry of Health we can find the document with the technical guide for the use of the AstraZeneca vaccine, where it is clearly indicated:

No evidence of interchangeability between the different vaccines against COVID-19, including vaccines with the same mechanism of action, so the guidelines must always be completed with the same vaccine applied in the first dose.

Although, in the following paragraph, it is also indicated that:

In exceptional circumstances where a person has started vaccination with a vaccine that is no longer available at the vaccination site for an adequate period of time, or if it is not known which particular product has been used in the first dose, it is considered reasonable to complete vaccination with the product available at the point of vaccination at that time, especially if it is likely that the person could be exposed to a high risk of infection or if it is unlikely that it can be retaken for another occasion.

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At the moment, and after the strategy changes of the last week, in our country the AZ vaccine is only being administered to people between 60 and 65 years old, while it is still up in the air to know what they plan to do with all those who They have already received their first dose and are waiting for the second. The Ministry of Health announced that it is “considering all the options”And Fernando Simón himself stated at a press conference that“ a second dose of Astrazeneca can be given, another type of vaccine can be administered or even a second dose can not be given. We have time to study ”.

In short, Spain has not yet decided if it is going to mix doses of different vaccines or what its position will be with people who have already received a first dose of AstraZeneca. It may sound like a new hesitation from a Ministry of Health, which lately has us too accustomed to doubts and mistakes, however the option of waiting for the publication of scientific studies is probably the most prudent.

Are there no studies on the effects of mixing vaccines?

That is the main problem: we still do not have studies in this regard, at least not with humans. A first study was published at the end of March, but it was carried out in mice and is still in a pre-print state and without peer review. The results were encouraging since the group of rodents that received the two different vaccines (a self-amplifying RNA vaccine and an adenoviral vectorized vaccine) offered a higher antibody response than those mice that received only one type of vaccine.

Mixing vaccines could prove positive

It would achieve more flexibility and agility in vaccination campaigns, it would help to overcome delays and stoppages in the supply, it would increase the rate of administration and it could even increase the immune response (as has happened in the mouse study). It must be understood that it would not be something unreasonable, in fact, the combination of different formulations is being tested on other viruses, such as HIV or Ebola, and even on bacteria such as tuberculosis. The fundamental question is that we do not know … we do not know yet.

Mixing vaccines could be negative

It could increase the risk of side effects or even cause new ones to appear. Some vaccines may work well and others may work worse. It could also limit how highly effective existing vaccines have been, or it might just not change anything. The truth, and it is important to emphasize it, is that we do not know … we do not know yet.

COM-COV (Comparing COVID-19 Vaccine Schedule Combinations), the main study that analyzes the risks and benefits of mixing different vaccinesCOM-COV (Comparing COVID-19 Vaccine Schedule Combinations), the main study that analyzes the risks and benefits of mixing different vaccines

COM-COV (Comparing COVID-19 Vaccine Schedule Combinations), the main study that analyzes the risks and benefits of mixing different vaccines

The governments of France and Germany may be right or they could be making a colossal mistake. I do not know to what extent we want to play Russian roulette with these decisions, but while we do not have studies and analysis on the subject, the most appropriate recommendation seems to be to pay attention to international institutions and the manufacturers themselves: do not combine vaccines until we know more.

The great study that we are all waiting for is called COM-COV (Comparing COVID-19 Vaccine Schedule combinatios) and is being carried out by the University of Oxford, one of the institutions most interested in having this issue resolved as soon as possible. COM-COV is studying the use of different combinations of approved COVID-19 vaccines for the first and second immunization doses.

Oxford University itself advises that “our researchers are working hard to make the results available to inform UK immunization policy as quickly as possible”, and even ask for a favor: let us work in peace and have a little patience, we will have the results soon.

In short: we still do not know if mixing vaccines will enhance them, weaken their effectiveness, deploy greater risks or be the right decision. We still do not have enough studies to know for sure. Therefore, and while that knowledge arrives, the recommendation should be clear: do not mix vaccines. If France or Germany announce that they are going to do it, they are announcing a political decision, not a scientific one… there they are.

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Scientific references and more information:

Spencer, Alexandra J., et al. “Heterologous Vaccination Regimens with Self-Amplifying RNA and Adenoviral COVID Vaccines Induce Robust Immune Responses in Mice”. BioRxiv, March 2021, DOI: 10.1101 / 2021.01.28.428665.

University of Oxford: Official website of the COM-COV study

Ministry of Health: Technical Guide for the use of the AstraZeneca vaccine