Given the flood of news that indicated that, in response to the recommendation of Health, AstraZeneca was being overwhelmingly chosen by those who could choose their second dose, sources from the Ministry said yesterday in El País that communities had been called “to order not to recommend the second dose of AstraZeneca” and they justified their decision in that “20 thrombi had been registered in Spain, four fatal”.
Furthermore, El País expanded the news with the confirmation of the Spanish Medicines Agency of those 20 cases and added that “the effect that the latter may have is not known at the moment.” But Are we really blind to the effect that second doses can have on those vaccinated?
What do we know about the second dose?
Over 10 million second doses in the UK: There are things we do know about the second dose of AstraZeneca, we not only have data from clinical trials (which support the recommendations of the European Medicines Agency, EMA), we have data from the UK. As of May 19, British authorities had identified 332 cases of severe thromboembolism. Only 17 of them had been after the second dose. And we are not talking about a study of 676 people, until that same May 19, 24.2 million first doses and 10.7 million seconds had been given.
Based on these data, the MHRA (the British agency) has clear recommendations: “Anyone who has experienced cerebral or other blood clots with low platelet levels after the first dose of the COVID-19 AstraZeneca vaccine should not receive the second dose. Anyone who has not had these side effects should show up for their second dose when summoned“.
There are also countries that are using Pfizer: Countries such as France and Germany changed the vaccination schedule for AstraZeneca and have been inoculating the second dose with the Pfizer injectable for weeks without noticeable complications having yet been detected. We must not forget that the strategy of mixing different vaccines is carried out regularly and without problems in vaccination campaigns against other diseases.
Pending the results of the British Com-Cov clinical trial on what immunity is generated by vaccination with different injectables, we only know for sure that these schedules induce more mild side effects (low-grade fever, chills, tiredness, headaches, arthralgias, myalgias and discomfort) after the booster dose than when using the same vaccine.
Why have some countries chosen this pattern? Regardless of supply or other reasons, it is true that all studies on thrombotic events link them to the “adenoviral vector” that AstraZeneca uses. A vector that also uses the Johnson and Johnson vaccine. Therefore, although the differential is small, there are reasons to think that eliminating this vector (that is, using mRNA vaccines) would improve the safety of the regimen against thrombi.
AstraZeneca or Pfizer? To this day, the opinion of the European Medicines Agency has not changed and continues to recommend completing the AstraZeneca vaccination schedule with the same vaccine. The reason is simple: the Agency only has data to support this guideline. That is not to say that the switch to Pfizer is arbitrary or unreasonable. On the contrary, there are technical reasons to think that using mRNA vaccines in the second dose may be safer; what there are not are solid clinical trials to prove it.
In this matter Pfizer’s biological plausibility is pitted against AstraZeneca’s statistical evidence. However, we are talking about really very small percentages. That is, if we use the usual standards in the pharmaceutical industry, there is no substantial difference between opting for one alternative or the other. Both are safe, effective and offer enough reliability to be robust alternatives.