SARS-CoV-2/Finland/1/2020 whole-genome sequencing
Nearly the complete coding region of SARS-CoV-2 (GenBank accession number: MT020781) was sequenced from the NPS collected on Day 4 (Table) and the complete coding region was sequenced from the virus isolate obtained after three passages in Vero E6 cells. The virus had 1 nt substitution C21707T compared with the reference strain Wuhan-Hu-1 collected in Wuhan China, December 2019 (NC_045512) [3] which had led to a histidine to tyrosine (H49Y) substitution in the N-terminal domain of the spike glycoprotein.

Antibody response during the SARS-CoV-2 infection
Serum samples were collected from the index case on Days 4, 9, 10 and 20 from onset of the first symptoms (Figure 1). Presence of serum IgM and IgG antibodies against SARS-CoV-2 was analysed by immunofluorescence assays (IFA) based on Vero E6 cells infected with passage 4 of the patient’s isolate SARS-CoV-2/Finland/1/2020 virus and transferred onto microscope slides and fixed with acetone (Figure 2). Serum samples from the index case were serially diluted and incubated for 2 h for IgM and 30 min for IgG. Antibodies were visualised with fluorescein isothiocyanate (FITC)-conjugated anti-human IgM or IgG antibodies. While the antibodies were undetectable on Day 4 after onset of symptoms, IgG titres rose to 80 and 1,280 and IgM titres to 80 and 320 on Days 9 and 20, respectively (Table). Random serum samples from staff members of the University of Helsinki (n = 19) did not show specific binding at dilutions greater than 20 (Figure 2).
Figure 2  Immunofluorescence assay of serum samples, COVID-19 index case, Finland, January–February 2020
COVID-19: coronavirus disease 2019.
Anti-SARS-CoV-2 IgM and IgG antibodies were detectable by immunofluorescence assay in samples from Days 9, 10 and 20 after onset of illness. Both IgM and IgG were found at a titre of 80 on Day 9, titres on Day 20 were 320 and 1,280. As an example, dilutions 1:20 and 1:160 from the Day 20 sample are shown for, respectively, IgM and IgG of the index case. Dilution 20 shown for the control serum. Mock- and SARS-CoV2-infected Vero E6 cells collected on Day 6 post infection were lysed in Laemmli sample buffer, and Western blotting (WB) of lysates was performed as described previously [4]. At 1:200 dilution, the convalescent serum on Day 20 identified SARS-CoV2 N, S and E protein bands (Figure 3). At higher exposure, all bands were detectable even at 1:1,600 serum dilution (Figure 3).
Figure 3  Western blot of mock- and SARS-CoV-2 infected Vero E6 cells using patient serum collected 20 days after onset of symptoms, Finland, January–February 2020
Top left panel: total protein staining (Ponceau S) of the nitrocellulose membrane before probing. Top right panel: strips probed with different dilutions of the patient serum at low exposure. Bottom panel: the same membranes individually contrasted for higher band intensity. The arrows indicate SARS-CoV-2 proteins, the labelling assumes that the migration of SARS-CoV-2 proteins was similar to that of Vero E6-expressed SARS-CoV proteins [23]. The bands migrating at ca 110 and 90 kDa probably represent S1 and S2, respectively. Marker M: Precision Plus Dual Colour Standards (Bio-Rad). The detection was done using Odyssey Infrared Imaging System (LI-COR) using goat anti-human IR800 conjugate at 1:10,000 dilution. SARS-CoV-2-specific neutralising antibody levels were measured in duplicate with the MN test in a BSL-3 laboratory. The serum samples were heat-inactivated at 56 °C for 30 min and 2-fold serially diluted starting from 1:4 in EMEM supplemented with 2% of heat-inactivated FBS and antibiotics. Fifty plaque‐forming units (PFU) of the SARS-CoV-2/Finland/1/2020 strain were added to the serum dilutions and incubated for 1 h at 37 °C. Vero E6 cells (5 × 104/well) were added to the virus–serum mix, and the mixture was incubated in 96-well plates for 4 days at 37 °C with 5% CO2. Neutralisation was assessed by CPE. The neutralisation endpoint was determined as the 50% endpoint of the serum that inhibited the SARS-CoV-2 infection observed by CPE of inoculated cells.
Diagnostic serum samples from the index case and her three asymptomatic close contacts were studied with the MN test. During the acute phase of infection, no neutralising antibodies were detected. The patient seroconverted for neutralising antibodies between Day 4 and 9, with the titre increasing to 160 on Day 20 (Table). The serum specimens were confirmed not to be toxic or infective to the cells as such.
Serum samples taken from the three close contacts tested negative in MN test. We also tested serum samples collected in 2019 from 83 Finnish subjects aged 4 to 89 years and all tested negative. Sera known to be positive for IgG against human coronavirus OC43 and 229E [5] and rabbit or guinea pig antibody against SARS-CoV N protein [6] could not neutralise the virus.

Ethical statement
The investigations were carried out in accordance with the General Data Protection Regulation (Regulation (EU) 2016/679 and Directive 95/46/EC) and the Finnish Personal Data Act (Finlex 523/1999) The Finnish Communicable Diseases Act (Finlex 1227/2016) allows sampling for diagnostic and surveillance purposes.
The convalescent serum sample was obtained on 14 February through informed consent of the patient and research permits (TYH2018322, TYH2019263) from the Helsinki University Hospital Laboratory.
Finnish population serum samples were collected during 2019. The study protocol was approved by the Ethics Committee of the Department of Medicine, Helsinki University Hospital (Permission 433/13/03/00/15).
Serum samples of University of Helsinki staff members were used under informed consent.