E: envelope protein gene; HUS: Helsinki University Hospital Laboratory; IgG: immunoglobulin G; IgM: immunoglobulin M; MN: microneutralisation test; N: nucleocapsid protein gene; NA: not applicable; ND: not done; Neg: negative; NPA: nasopharyngeal aspirate; NPS: nasopharyngeal swab; RdRp: RNA-dependent RNA polymerase gene; RT-PCR: reverse-transcription PCR; THL: Finnish Institute for Health and Welfare; UH: University of Helsinki.