The neutralization assay for HCoV-229E antibodies is essentially identical to the SARS-CoV-2, where samples were used undiluted or prediluted, then serially diluted in 2-fold steps and mixed 1:2 with 103.0 TCID50/mL HCoV-229E (Cat. no. VR-740, American Type Culture Collection [ATCC], Rockville, MD), incubated, and titrated on MRC-5 cells (Cat. no. CCL-171, ATCC, Rockville, MD). The virus-induced cytopathic effect was determined after 7–9 days of incubation.