  This work proposes a scalable, blockchain-based platform for the secure
sharing of COVID-19 or other disease vaccination certificates. As an indicative
use case, we simulate a large-scale deployment by considering the countries of
the European Union. The proposed platform is evaluated through extensive
simulations in terms of computing resource usage, network response time and
bandwidth. Based on the results, the proposed scheme shows satisfactory
performance across all major evaluation criteria, suggesting that it can set
the pace for real implementations. Vis-\`a-vis the related work, the proposed
platform is novel, especially through the prism of a large-scale, full-fledged
implementation and its assessment.
