SARS-CoV-2/Finland/1/2020 virus isolation
The SARS-CoV-2 virus SARS-CoV-2/Finland/1/2020 was isolated in a biosafety level 3 (BSL-3) laboratory in Vero E6 cells from the Day 4 nasopharyngeal swab (NPS) and nasopharyngeal aspirate (NPA) specimens (Table). The samples were inoculated into the cells for 1 h at 37 °C and 5% CO2 and fresh culture medium (Eagle's minimum essential medium (EMEM) supplemented with 2% fetal bovine serum (FBS), 0.6 μg/mL penicillin, 60 μg/mL streptomycin, 2 mM L-glutamine, 20 mM HEPES) were added for incubation. On the 4th day of incubation, half of the cultures were blind-passaged onto fresh Vero E6 cells and the rest of original passages were incubated further. After 4 days incubation a clear CPE was detected in the NPA-originated passage 2. The propagation of stock virus was done by passaging a low virus dose once again in Vero E6 cells, and virus culture was harvested on the 3rd day. Virus concentration was followed by RT-PCR. The Ct value for virus passage 1 on the 6th day of incubation was 17.65 and for passage 2 on the 2nd day, before any CPE was 20.63, whereas those of the NPS specimen remained at Ct values between 35 and 36.