Discussion
In the early phase of the COVID-19 outbreak, confirmed cases outside China were mostly imported among travellers from Wuhan [7]. The first case in Finland was detected on 29 January among the first imported cases in Europe. The case presented mild symptoms without pneumonia: runny nose, nausea, high fever, cough, muscular weakness and fatigue. No secondary transmission events were detected despite active follow-up by the Lapland Hospital district and THL.
As at 17 March 2020, 358 additional laboratory-confirmed cases of COVID-19 have been detected in Finland. Many of them are travel-related (mostly from northern Italy and Austria) but there is also local transmission from the travel-related cases. The risk of widespread national community transmission of COVID-19 infection in the European Union, European Economic Area and the United Kingdom in the coming weeks is considered high by the European Centre for Disease Prevention and Control [8].
The sequence of the viral genome of the patient was nearly identical to the reference strain from Wuhan, reflecting an early importation from China. Later sequence information in Finland (up to 2 March) showed clustering with strains circulating in Italy (see nextstrain.org/ncov) [9].
Current guidelines from the World Health Organization for testing COVID-19 recommend collection of both acute and convalescent serum samples from patients for serological testing, which can support the identification of the immune response to a specific viral pathogen [10]. The SARS-CoV-2 nucleic acid has been found also in anal swabs and blood [11], however we did not detect it in serum samples in this case. As yet, only limited data are available on antibody responses during SARS-CoV-2 infection [11,12]. Further studies are needed to better understand the seroprevalence of antibodies to different corona viruses in populations and the role of these antibodies in the risk of disease.
In accordance with earlier findings [11], we found that both IgM and IgG titres were low or undetectable at on Day 4 (the second day after admission to hospital) yet increasing on Day 9–10, i.e. 5–6 days after the first sampling. Using other detection methods beyond IFA as well as recombinant antigens and analysing samples from a larger number of patients will shed more light on this. The time of first appearance of anti-SARS-CoV antibodies has ranged from Day 3 to 42 and Day 5 to 47 for IgM and IgG antibodies, respectively [13].
The WB of the serum sample collected at convalescence showed a prominent response against the N and S protein, confirming their role as main candidate diagnostic targets for antibody tests. However, the patient serum appeared to recognise also the E protein and the processed S1 and S2 proteins. Although WB detects mainly linear epitopes, the strong antibody response against the S protein correlated well with the results of the MN assay.
Monitoring of the binding antibodies is suggested to be a more sensitive method than measuring functional neutralising antibodies for serological detection of human coronavirus (hCoV) infections [14]. However, hCoV OC43 and 229E samples can also cross-react with SARS-CoV ELISA testing [15]. The SARS-CoV-2 CPE-based MN test using live virus appeared to be very specific, while laborious to conduct requiring a BSL-3 laboratory. An increase of at least 4-fold in the neutralising antibodies indicating a positive response was detected at Day 9–10 after the first symptoms and at Day 20, the antibody levels were still increasing. Our findings indicate that the MN assay is specific for functional SARS-CoV-2 antibodies and could be applied in surveillance of population immunity for this virus. The assay can be used as confirmatory tool for SARS-CoV-2 specificity in the development of more accessible diagnostic tools such as assays based on detecting binding antibodies. Previous studies on patients with SARS-CoV infection indicated that the median time for seroconversion was 20 days, by which time 60–75% of patients had IgG against the virus [13,16]. That IgM and IgG antibodies were present within 2 weeks from the onset of symptoms in our study suggests that early convalescent patients may be suitable sources of therapeutic antibodies [17]. In accordance with our finding, a recent preprint report on patients admitted to hospital with confirmed SARS-CoV-2 infection in China indicated that the median time to seroconversion was 11–14 days, depending on the immunological assay used [18].
No neutralising SARS-CoV-2 antibodies were detected in the close contacts nor in the control population samples collected during 2019 in Finland. A low prevalence (0.21%) of antibodies against Middle East respiratory syndrome coronavirus was reported in the general population of Qatar [19]. A meta-analysis of seroprevalence to SARS-CoV among different human populations yielded an overall low seroprevalence (0.10%), although it was slightly higher (0.23%) among healthcare workers and others who had close contact with SARS patients [20]. Binding and neutralising HCoV antibodies were found to be higher in older adults [14]. In total 97% and 99% of serum samples from healthy adults had antibodies to HCoV-229E and HCoV-OC43, respectively [21], and 75% and 65% of the children in the age group 2.5–3.5 years were found to be seropositive for, respectively, HCoV-NL63 and HCoV-229E [22].
While it has been suggested that the late seroconversion in most SARS patients reduces the value of serological assays during the incubation and initial phases of SARS [13], serological testing is suggested for the confirmation of a SARS CoV-2 infection [11]. After understanding better the kinetics, specificity and sensitivity of the assays in development, the serological testing may help contact tracing of clusters and have a role in diagnosing acute and past SARS-CoV-2 infections.