PROS1 and infection: Hamster sera in the convalescent phase of the infection of WK-521 WT or S gene mutants showed neutralizing activity against both TY7-501 and QK002 variants (Fig. 5A and Fig. S2A), whereas the cross-reactivity observed with TY7-501 was lower than that with QK002, presumably owing to the K417T, E484K, and N501Y substitutions in the S protein of TY7-501 (Fig. S3) (32, –, 36).