SARS-CoV-2 antibody responses in COVID-19 confirmed cases in a university hospital

COVID-19 diagnosis relies on the detection of Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) genome or antigen. Antibodies have limited use for diagnosis but contribute to serological studies and epidemiologic analysis. We aimed to elucidate anti-N and anti-S antibody responses among patients with PCR-positive and indeterminate results, and/or those with rapid molecular test-positive results.

267 serum samples from 199 patients were collected from April 2020 until November 2021. The timing of serum sampling was determined from the diagnostic testing date. Anti-N and anti-S were tested using the ECLIA platform. The patients' COVID-19 clinical categories were retrieved from an online system, and data were analysed using SPSS, version 28.

82.4% of patients' PCR were positive, while 17.6% were indeterminate. Overall sero-reactivity rate for anti-N and anti-S is 54.3% and 65.3%, respectively. The sero-reactivity rate and level of anti-S antibodies peaked at day 9 to 14 post-PCR but declined after day 15. Anti-N has significantly higher sero-reactivity rate (p<0.001) in the PCR-positive group. Anti-S sero-reactivity rate was significantly higher in mild COVID-19 infections (p=0.027).

In a cohort with the majority belonging to the pre-vaccination period during the COVID-19 pandemic, sero-reactivity of the SARS-CoV-2 antibodies were highest from week two following laboratory diagnosis. Currently, the clinical utility of SARS-CoV-2 antibody testing is limited due to the current endemicity of the virus as well as the vaccinated population. Further research could explore how viral evolution and immunisation impact antibody responses and the detection of the various antibodies.