Fig. 2. Stepwise expression of the four structural SARS-CoV-2 proteins and dsRNA correlates with staged release of viral transcripts (5 hpi) and infectious virus (7.5 to 10 hpi), respectively.

(A) The fraction of cells positive for each of the structural proteins of SARS-CoV-2 or viral dsRNA was determined by immunostaining of infected Vero cells. For each time point, 30 to 35 wide-field microscope images, corresponding to 1000 to 1500 cells per sample, were analyzed. For the control time point of 0 hpi, around 250 cells were analyzed per sample. For the count of N- and S-positive cells, three samples per time point were analyzed. (B) The copy number of the viral transcripts in the cell supernatant was measured by RT-qPCR. Release of viral RNA was observed from 5 hpi onward when first cells started expressing N protein. (C) The infectious titer of the cell supernatant was determined by the plaque assay. Newly formed infective SARS-CoV-2 virions were released from cells from 10 hpi onward. For both assays, two replicates were carried out. Significance was tested with an unpaired t test. (D) Stage-dependent replication kinetics. ns, not significant.