The Spanish Agency for Medicines and Health Products (AEMPS) has authorized the first clinical trial combining different vaccines against COVID-19 in Spain, specifically to evaluate both the safety and efficacy of injecting a second dose of Pfizer vaccine in people who have already received a first dose of AstraZeneca.
There is evidence that mixing vaccines can be a good strategy to avoid supply problems for some of them or stoppage in their administration due to the detection of adverse effects, but so far there are no clinical data.
Vaccines that use two different technologies
One of the basic rules to consider when mixing vaccines is that they contain the same type and comparable amount of vaccine antigen. In this specific case, mixing AstraZeneca vaccine with Pfizer vaccine violates this rule, because both vaccines use different immunization strategies (viral vector and mRNA, respectively) and therefore do not contain the same type of antigen or the same amount.
It can be said that, in both cases, the final vaccine antigen is protein S., but the way in which our immune system will receive the signal is different.
Based on previous combination studies of other vaccines, we can expect the effectiveness results to be equal to or even better than using the same vaccine. The theory is that, as soon as the mRNA in the Pfizer vaccine triggers the formation of protein S, it will be seen if there is a fast and strong immune response, already learned thanks to AstraZeneca. But in science, to be sure of something, we must carry out clinical trials like the one now being launched in Spain.
This trial will allow us to find out what can happen to those people under 60 years of age who have already been vaccinated with AstraZeneca and who will not receive the second planned dose, following the conclusions of the European Pharmacovigilance Risk Assessment Committee (PRAC) of the European Medicines Agency (EMA) on the administration of AstraZeneca and its association with rare thrombi. That is, with these results it will be possible to evaluate the possibility that no more AstraZeneca doses are to be administered in Europe.
Specifically, the protective effect and safety of administering a dose of Pfizer will be evaluated in patients who have already received a dose of AstraZeneca at least 8 weeks after that dose. A total of 600 randomly selected volunteers participate in this phase 2 clinical trial that will be carried out at the Cruces University Hospital (Bilbao), at the Clinic and Vall d’Hebron (Barcelona), and in La Paz and San Carlos (Madrid) –
400 people will receive a dose of Pfizer, followed by 28 days of observation. The rest of the participants will not receive any new dose and will be closely monitored for another 28 days to observe the effect of the vaccine received on their day.
The Centers for Disease Control and Prevention maintain that the safety and efficacy of mixing these products has not yet been fully evaluated. Some studies in animals have suggested that this alternative could offer a reinforced immune response, but only in animals.
To be completely sure that these mixtures are effective and safe we will have to wait at least until the summer, when the results of a trial started in the United Kingdom that also specifically seeks to mix the
Recently, in addition, it has been announced that this study will be expanded to include Moderna and Novavax vaccines, the new vaccine that is close to being approved and whose efficacy reaches an efficacy of 96.4% against the original strain of the virus and 89.7% in general.