Selection, Characterization, Calibration, and Distribution of the U.S. Serology Standard for Anti-SARS-CoV-2 Antibody Detection

The SARS-CoV-2 pandemic resulted in a demand for highly specific and sensitive serological testing to evaluate seroprevalence and antiviral immune responses to infection and vaccines. Hence, there was an urgent need for a serology standard to harmonize results across different natural history and vaccine studies. The Frederick National Laboratory for Cancer Research (FNLCR) generated a U.S. serology standard for SARS-CoV-2 serology assays and subsequently calibrated it to the WHO international standard (National Institute for Biological Standards and Control [NIBSC] code 20/136) (WHO IS). The development included a collaborative study to evaluate the suitability of the U.S. serology standard as a calibrator for SARS-CoV-2 serology assays. The eight laboratories participating in the study tested a total of 17 assays, which included commercial and in-house-derived binding antibody assays, as well as neutralization assays. Notably, the use of the U.S. serology standard to normalize results led to a reduction in the inter-assay coefficient of variation (CV) for IgM levels (pre-normalization range, 370.6% to 1,026.7%, and post-normalization range, 52.8% to 242.3%) and a reduction in the inter-assay CV for IgG levels (pre-normalization range, 3,416.3% to 6,160.8%, and post-normalization range, 41.6% to 134.6%). The following results were assigned to the U.S. serology standard following calibration against the WHO IS: 246 binding antibody units (BAU)/mL for Spike IgM, 764 BAU/mL for Spike IgG, 1,037 BAU/mL for Nucleocapsid IgM, 681 BAU/mL for Nucleocapsid IgG assays, and 813 neutralizing international units (IU)/mL for neutralization assays. The U.S. serology standard has been made publicly available as a resource to the scientific community around the globe to help harmonize results between laboratories.