Longitudinal Follow Up of Immune Responses to SARS-CoV-2 in Health Care Workers in Sweden With Several Different Commercial IgG-Assays, Measurement of Neutralizing Antibodies and CD4+ T-Cell Responses

Abstract

Background: The risk of SARS-CoV-2 infection among health care workers (HCWs) is a concern, but studies that conclusively determine whether HCWs are over-represented remain limited. Furthermore, methods used to confirm past infection vary and the immunological response after mild COVID-19 is still not well defined.

Method: 314 HCWs were recruited from a Swedish Infectious Diseases clinic caring for COVID-19 patients. IgG antibodies were measured using two commercial assays (Abbot Architect nucleocapsid (N)-assay and YHLO iFlash-1800 N and spike (S)-assays) at five time-points, from March 2020 to January 2021, covering two pandemic waves. Seroprevalence was assessed in matched blood donors at three time-points. More extensive analyses were performed in 190 HCWs in September/October 2020, including two additional IgG-assays (DiaSorin LiaisonXL S1/S2 and Abbot Architect receptor-binding domain (RBD)-assays), neutralizing antibodies (NAbs), and CD4⁺ T-cell reactivity using an in-house developed in vitro whole-blood assay based on flow cytometric detection of activated cells after stimulation with Spike S1-subunit or Spike, Membrane and Nucleocapsid (SMN) overlapping peptide pools.

Findings: Seroprevalence was higher among HCWs compared to sex and age-matched blood donors at all time-points. Seropositivity increased from 6.4% to 16.3% among HCWs between May 2020 and January 2021, compared to 3.6% to 11.9% among blood donors. We found significant correlations and high levels of agreement between NAbs and all four commercial IgG-assays. At 200-300 days post PCR-verified infection, there was a wide variation in sensitivity between the commercial IgG-assays, ranging from <30% in the N-assay to >90% in the RBD-assay. There was only moderate agreement between NAbs and CD4⁺ T-cell reactivity to S1 or SMN. Pre-existing CD4⁺ T-cell reactivity was present in similar proportions among HCW who subsequently became infected and those that did not.

Conclusions: HCWs in COVID-19 patient care in Sweden have been infected with SARS-CoV-2 at a higher rate compared to blood donors. We demonstrate substantial variation between different IgG-assays and propose that multiple serological targets should be used to verify past infection. Our data suggest that CD4⁺ T-cell reactivity is not a suitable measure of past infection and does not reliably indicate protection from infection in naive individuals.

Keywords: CD4+ T cells; SARS-CoV-2; antibodies; health care workers; neutralizing antibodies.

Figure 1

IgG positivity in the N+S assay in health care workers and blood donors over time. Proportion of IgG-positive health care workers (white circles) and blood donors (red triangles), as measured with the N+S-assay, at each sampling time-point (TP) from March 2020 to January 2021.

Figure 2

SARS-CoV-2 specific antibodies measured by different assays in health care workers with and without verified COVID-19. Health care workers with (pink circles) and without (blue squares) verified COVID-19. Concentrations of IgG measured by different commercial IgG-assays and by virus neutralization at time-point 4. Medians indicated by horizontal lines. The upper dashed lines indicate the cut-off for positivity, and the lower dotted lines indicate the lowest detectable concentration/index.

Figure 3

ROC curves presenting results from different commercial IgG-assays and neutralizing antibody titers (NT) in health care workers with and without verified infection at time-point 4. Identity line (diagonal).

Figure 4

Correlations between levels of neutralizing antibodies (NAbs) and levels of SARS-CoV-2 specific IgG measured at time-point 4 by commercial assays among health care workers with verified infection. Dotted lines indicate the cut-off for positivity in each commercial IgG-assay.

Figure 5

ROC curves presenting results from CD4⁺ T cell assays in health care workers with and without verified infection. Identity line (diagonal).

Figure 6

Correlation between CD4⁺ T-cell reactivity against S1 and SMN peptide pools. Correlations between proportions of OX40⁺CD25⁺ cells among all CD4⁺ T cells after stimulation with the two different peptide pools at time-point 4 in health care workers with verified COVID-19.

Figure 7

Correlations between CD4⁺ T-cell reactivity against S1 and SMN peptide pools and neutralizing antibody titers in health care workers with verified COVID-19.