What happens if two different corona vaccines are administered? So the first time with Pfizer and the second time with Moderna, for example.

Answer

There is currently one study, called CombivacS, whose results are known for 600 people who received AstraZeneca vaccine as the first injection and Pfizer for their second injection. The results were very positive because the immune response against the virus appeared to be highly stimulated in these subjects. They made a lot of antibodies that recognized and inactivated the virus in laboratory conditions. The response was better than in people who received two times the AstraZeneca vaccine, but it is not clear from this study whether it would also be better than two times the Pfizer vaccine.

More studies of this nature are underway, but in general immunologists expect such “mix-and-match” vaccinations (combinations of two different vaccines) to give very good results. They think they can complement or reinforce each other. And that is good news for countries that would have a shortage of one of the COVID vaccines. This can also be important if one wishes to repeat the vaccination even more, if new variants (mutants) appear or to extend protection over a longer period of time. Vaccines such as those from AstraZeneca, Johnson&Johnson and Sputnik make use of harmless adenoviruses, but an immune response is also built up against them so that they become less effective with repeated administration. Repeated vaccination (more than 2x) with the RNA vaccines, such as Pfizer and Moderna, could cause more side effects. As is often the case with COVID, the future, the advancing understanding of science, will teach us how best to deal with this.