The Spanish Combivacs study attempted to provide evidence that offering a Pfizer-BioNTech vaccine as a second dose might be a good option for people who received the first dose of AstraZeneca.
I did this with a sample of 600 volunteers who had already received the AstraZeneca vaccine. 400 of them were vaccinated with the Pfizer vaccine at least eight weeks later. The other 200 did not receive any vaccine as a second dose.
With a sample of this size, one can detect frequent or very frequent side effects – defined as those affecting more than one in every hundred people. But less frequent but also important side effects cannot be detected, so it cannot be concluded that combining the first dose of AstraZeneca with the second dose of Pfizer is safe.
It only allows us to conclude that vaccination with Pfizer leads to a stronger immune response than vaccination with none.
It cannot be concluded that administering Pfizer as a second dose is more effective than giving a second dose of AstraZeneca, because this study did not investigate. It only allows us to conclude that vaccination with Pfizer induces a stronger immune response than vaccination with nothing, which was exactly what was expected.
In the end, the Combivacs study doesn’t answer a really relevant question: What is the best option for people who have received the first dose of AstraZeneca and are waiting for the second? The answer will be presented from June through the Com-Cov and Com-Cov2 studies coordinated by the University of Oxford.