Identification of natural SARS-CoV-2 infection in seroprevalence studies among vaccinated populations

Abstract

Importance: Identification of SARS-CoV-2 infection via antibody assays is important for monitoring natural infection rates. Most antibody assays cannot distinguish natural infection from vaccination. Objective: To assess the accuracy of a nucleocapsid-containing assay in identifying natural infection among vaccinated individuals. Design: A longitudinal cohort comprised of healthcare workers (HCW) in the Minneapolis/St. Paul metropolitan area was enrolled. Two rounds of seroprevalence studies separated by one month were conducted from 11/2020-1/2021. Capillary blood from round 1 and 2 was tested for IgG antibodies against SARS-CoV-2 spike proteins with a qualitative chemiluminescent ELISA (spike-only assay). In a subsample of participants (n=82) at round 2, a second assay was performed that measured IgGs reactive to SARS-CoV-2 nucleocapsid protein (nucleocapsid-containing assay). Round 1 biospecimen collections occurred prior to vaccination in all participants. Vaccination status at round 2 was determined via self-report. Setting: The Minneapolis/St. Paul, Minnesota metropolitan area. Participants: HCW age 18-80 years. Exposures: Round 1 recent SARS-CoV-2 infection assessed via a spike-only assay and participant self-report. Outcomes: Round 2 SARS-CoV-2 infection assessed via the nucleocapsid-containing assay. Area under the curve (AUC) was computed to determine the discriminatory ability of round 2 IgG reactivity to nucleocapsid for identification of recent infection determined during round 1. Results: Participants had a mean age of 40 (range=23-66) years, 83% were female, 46% reported vaccination prior to the round 2 testing. Round 1 seroprevalence was 9.5%. Among those not recently infected, when comparing vaccinated vs. unvaccinated individuals, elevated levels of spike 1 (p<0.001) and spike 2 (p=0.01) were observed while nucleocapsid levels were not statistically significantly different (p=0.90). Among all participants, nucleocapsid response predicted recent infection with an AUC(95%CI) of 0.93(0.88,0.99). Among individuals vaccinated >10 days prior to antibody testing, the specificity of the nucleocapsid-containing assay was 92% and while the specificity of the spike-only assay was 0%. Conclusions and Relevance: An IgG assay identifying reactivity to nucleocapsid protein is an accurate predictor of natural infection among vaccinated individuals while a spike-only assay performed poorly. In the era of SARS-CoV-2 vaccination, seroprevalence studies monitoring natural infection will require assays that do not rely on spike-protein response alone.

Silent SARS-CoV-2 infection: seroprevalence study of SARS-CoV-2 anti- nucleocapsid IgG antibodies in Kragujevac, Serbia

“…Studies have demonstrated this protein is highly immunogenic and plentiful, with antibodies directed against it being more sensitive to early infection detection than antibodies targeting the spike protein (Burbelo et al, 2020). In addition, detecting anti-N antibodies allows differentiating individuals vaccinated with S-directed vaccines based on the history of infection (Demmer et al, 2021). IgG directed against the N protein can indicate both symptomatic and asymptomatic SARS-CoV-2 infection and vaccination with the BBIBP-CorV vaccine.…”

Performance of Immunoglobulin G Serology on Finger Prick Capillary Dried Blood Spot Samples to Detect a SARS-CoV-2 Antibody Response

“…Therefore, DBS collection and anti-IgG serology show promise as a tool to measure SARS-CoV-2 seropositivity at the population level. The benefits include comparable diagnostic accuracy to that of serum samples, with improved population reach and the potential to discriminate natural from vaccine-elicited seroconversion by measuring anti-S and anti-N IgG reactivity in a single reaction ( 20 ). In a theoretical population with 75% COVID-19 seroprevalence, a positive DBS-MSD result reliably indicates seropositivity with a high PPV and does not require additional testing.…”

Performance of Immunoglobulin G Serology on Finger Prick Capillary Dried Blood Spot Samples to Measure SARS-CoV-2 Humoral Immunogenicity

“…Therefore, DBS collection and anti-IgG serology shows promise as a tool to measure SARS-CoV-2 seropositivity at the population level. The benefits include comparable diagnostic accuracy to that of serum samples, with improved population reach, and the potential to discriminate natural from vaccine elicited seroconversion by measuring anti-S and anti-N IgG reactivity 14 . In a theoretical population with seventy-five percent COVID-19 vaccine coverage, a positive DBS-MSD result reliably indicates seropositivity with a high PPV and does not require additional testing.…”