FIG. 5.

Ratio of cell-associated virus to secreted virus. SK6 cells grown in 24-well plates were infected with virus variants CoBrB ⁴⁷⁶S, CoBrB ⁴⁷⁶S/C1.1.1 (E2-p7-pNS2-3′), C1.1.1/CoBrB, and CoBrB/C1.1.1 (N^pro′-C-E^rns-E1) (produced on SK6 cells) at an MOI of 1. After infection for 30 min at 37°C, virus was removed, and cells were washed. At 2 days p.i., 1 ml of supernatant was collected and stored at −70°C. One milliliter of culture medium was added to the cells before freeze-thawing twice at −70°C. Samples were titrated on SK6 cells in 24-well plates. The ratio of secreted virus to cell-associated virus was determined in a plaque assay and calculated by the following formula: ratio = log₁₀ PFU of cell-associated virus − log₁₀ PFU of secreted virus. An average ratio value for all tested viruses was determined from two experiments.