Serological analyses against endemic human coronaviruses and SARS-CoV-2 in children and adults using samples collected before the COVID-19 pandemic.

Objectives: Four endemic human coronaviruses (HCoVs), HCoV-229E, HCoV-NL63, HCoV-HKU1, and HCoV-OC43, infect humans during childhood and cause the common cold. COVID-19 caused by SARS-CoV-2 leads to mild symptoms in children, possibly owing to the protection conferred by immunity developed during a previous HCoV infection. This study analyzed the seroreactivity of four endemic HCoVs and SARS-CoV-2 in children and adults.

Methods: A total of 747 serum samples (from individuals aged 6 months to 69 years) were collected from 2015 to 2019 before the COVID-19 pandemic in Japan. The samples were tested for immunoglobulin G antibodies against the four endemic HCoVs and SARS-CoV-2 wild-type spike ectodomain proteins using enzyme-linked immunosorbent assay.

Results: The seroprevalence of endemic HCoVs (except HCoV-229E) showed 90% positivity by 3-4 years old, whereas HCoV-229E seroprevalence was observed at 8 years old. Approximately 35% of the samples showed reactivity to SARS-CoV-2 and did not change with age. However, the children's group presented higher antibody levels than the adult group. The sample reactivity against SARS-CoV-2 did not confirm neutralization capability.

Conclusions: The reactive samples against SARS-CoV-2 showed varying antibody levels among different age groups. These findings may contribute to a deeper understanding of the clinical symptoms of COVID-19 and coronavirus diseases.