Heterologous SARS-CoV-2 Vaccination With ChAdOx-1 and BNT162b2

The immune escape variants are spreading worldwide. Local transmission of the B.1.351 is observed in several European regions while P1 cases are mostly still travel associated. In Austria, there is local transmission of the Czech variant B.1.258 in several regions, while the P1 variant from Brazil plays no role yet. However, B.1.351 local transmission is observed in Tyrol, where 20-30% of SARS-CoV-2 infections were caused by this immune escape variant in January/February 2021, but then controlled but not eliminated by intensive testing. In March and April the largest world-wide outbreak of B.1.1.7 + E484K, also an immune escape variant, was observed in Tyrol. Although, some of the diagnosed cases seemed to be false positive for the E484K mutation, still several hundred have currently already been confirmed, a number that is internationally still unprecedented. It is expected that until the winter infection season, that is likely to hit Europe after a phase of low-level infections in summer, the immune escape variants will make up a substantial portion of SARS-CoV-2 infections in the countries with high seroprevalence after the vaccine campaigns such as Austria. Therefore, the vaccine programs must build up an effective immunity against the wild-type virus as well as the immune escape variants. ChAdOx1-S contributes considerably to the arsenal of vaccines purchased by the European Community. The investigators propose to test whether this vaccine could still be used to build up a cross-protective immunity in the population, when combined with a heterologous boost with the mRNA vaccine BNT162b2. Heterologous prime - boost vaccine regimen including vector vaccines have been broadly used and are generally more effective than homologous vector prime-boost regimen. If ChAdOx1-S followed by BNT162b2 builds up the same or even higher level of cross-neutralizing antibodies as BNT162b2 prime-boost, which is highly likely, the vector vaccine could potentially be included in the program for building a substantial level of cross-immunity against the immune escape variants in the population. Therefore, this study compares the level of cross-neutralizing antibodies induced by classical BNT162b2 and ChAdOx1-S vaccinations with the heterologous prime and boost with ChAdOx1-S followed by BNT162b2, respectively