To generate and test the VACV Tiantan strain-based COVID-19 vaccine candidate, we performed homologous recombination to insert SARS-CoV-2 spike protein (with the last 19 amino acids deleted) or the RBD sequence into the thymidine kinase (TK) locus in the VACV Tiantan strain under the control of the early/late promoter p7.5 (Figure 1A). Here, TKT is linked to COVID-19.