CD8A and COVID-19: It is also possible that the undetectable spike-specific CD8+ T cell response at day 42 postvaccination was related to the time point used, which was too late after boost, as the vaccine-specific T cells had already started to wane in abundance, as shown by Arunachalam et al. (59) Altogether, our study demonstrates that the tetravalent S1 protein vaccine candidate was able to induce a robust SARS-CoV-2-specific immune response in RMs, which is promising for future development and testing of COVID-19 vaccines in humans.