The cDNAs of specimen’s RNA were generated using the SuperScript™ IV First-Strand Synthesis System (Invitrogen, Carlsbad, USA) according to the manufacturer's instructions. Subsequently, the amplification of PCR multiplex reactions was performed in the steps according to the manufacturer's protocol (Agena Bioscience, San Diego, CA, USA) using a proprietary set of primers in 36 positions designed for SARS-CoV-2 clade classification. The PCR products were then prepared as a matrix for the MassARRAY system (Agena Bioscience). Mass signals for the different alleles were captured with high accuracy by MALDI-TOF mass spectrometry. MassARRAY Typer v4.0 (Agena Bioscience, San Diego, CA, USA) was used to generate the genotyping data of the specimens.