SARS-CoV-2 Seroprevalence and Antibody Kinetics Among Health Care Workers in a Spanish Hospital After 3 Months of Follow-up

Abstract

Background: At the COVID-19 spring 2020 pandemic peak in Spain, prevalence of SARS-CoV-2 infection in a cohort of 578 randomly selected health care workers (HCWs) from Hospital Clínic de Barcelona was 11.2%.

Methods: A follow-up survey 1 month later (April-May 2020) measured infection by rRT-PCR and IgM, IgA, and IgG to the receptor-binding domain of the spike protein by Luminex. Antibody kinetics, including IgG subclasses, was assessed until month 3.

Results: At month 1, the prevalence of infection measured by rRT-PCR and serology was 14.9% (84/565) and seroprevalence 14.5% (82/565). We found 25 (5%) new infections in 501 participants without previous evidence of infection. IgM, IgG, and IgA levels declined in 3 months (antibody decay rates 0.15 [95% CI, .11-.19], 0.66 [95% CI, .54-.82], and 0.12 [95% CI, .09-.16], respectively), and 68.33% of HCWs had seroreverted for IgM, 3.08% for IgG, and 24.29% for IgA. The most frequent subclass responses were IgG1 (highest levels) and IgG2, followed by IgG3, and only IgA1 but no IgA2 was detected.

Conclusions: Continuous and improved surveillance of SARS-CoV-2 infections in HCWs remains critical, particularly in high-risk groups. The observed fast decay of IgA and IgM levels has implications for seroprevalence studies using these isotypes.

Keywords: COVID-19; SARS-CoV-2; antibodies; health care workers; kinetics; longitudinal cohort; seroprevalence.

Figure 1.

Kinetics of SARS-CoV-2 antibodies in seropositive individuals during the 3 months of follow-up. Levels (MFI) of IgM, IgG, and IgA against receptor-binding domain of the SARS-CoV-2 spike glycoprotein stratified by asymptomatic participants and participants who reported COVID-19–compatible symptoms for the first time at recruitment (M 0) or at M 1. No participants reported symptoms for the first time at M 3. Lines indicate paired samples. Yellow dots depict individuals (IgM, n = 12; IgG, n = 41; IgA, n = 35) who had detectable antibody levels at all study visits when antibody levels where measured; burgundy dots, individuals who seroconverted for a particular isotype at M 1 (IgM, n = 7; IgG, n = 21; IgA, n = 17); green dots, individuals who seroreverted between M 0 and M 1 (IgM, n = 3; IgG, n = 0; IgA, n = 4) or M 1 and M 3 (IgM, n = 21; IgG, n = 1; IgA, n = 6); pink dots, individuals who seroconverted from M 0 to M 1 and then seroreverted (IgM, n = 17; IgG, n = 1; IgA, n = 7); and blue dots, individuals who seroreverted and seroconverted again (IgM, n = 0; IgG, n = 1; IgA, n = 1). Abbreviations: COVID-19, coronavirus disease 2019; IgA, immunoglobulin A; IgG, immunoglobulin G; IgM, immunoglobulin M; IQR, interquartile range; M, month; MFI, median fluorescence intensity; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2. See online version for color figures.

Figure 2.

SARS-CoV-2 antibody levels by demographic and clinical variables. Levels (MFI) of IgM (n = 60), IgG (n = 66), and IgA (n = 71) against receptor-binding domain of the SARS-CoV-2 spike glycoprotein stratified by (A) age, (B) presence of symptoms, and (C) duration of symptoms. Graphs show data from accumulative month 0 and month 1 seropositive individuals: month 0 antibody levels from seropositive individuals at month 0 plus month 1 antibody levels from individuals who seroconverted from month 0 to month 1. Percentages indicate the sum of proportions of seropositive subjects from recruitment and month 1 within each category of the x-axis with respect to the total number of samples from each visit (A and B) or the proportion of individuals within each category of the x-axis with respect to the total number seropositive symptomatic (C). The center line of boxes depicts the median MFI; the lower and upper hinges correspond to the first and third quartiles; the distance between the first and third quartiles corresponds to the IQR; whiskers extend from the hinge to the highest or lowest value within 1.5 × IQR of the respective hinge. Wilcoxon rank test was used to assess statistically significant differences in antibody levels between groups. Abbreviations: IgA, immunoglobulin A; IgG, immunoglobulin G; IgM, immunoglobulin M; IQR, interquartile range; MFI, median fluorescence intensity; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 3.

SARS-CoV-2 antibody levels by time since first rRT-PCR and onset of symptoms. Levels (MFI) of IgM, IgG, and IgA against receptor-binding domain of the SARS-CoV-2 spike glycoprotein by (A) days since the first positive rRT-PCR, and (B) days since onset of any symptom. Graphs show pooled month 0, month 1, and month 3 data. Data in (A) are only for individuals with any rRT-PCR positive (n = 142). Data in (B) are only for seropositive individuals at any visit since onset of any symptom compatible with COVID-19 (n = 121 for IgM, n = 145 for IgG, and n = 149 for IgA). The fitted curves were calculated using the LOESS method. Shaded areas represent 95% confidence intervals. Abbreviations: IgA, immunoglobulin A; IgG, immunoglobulin G; IgM, immunoglobulin M; IQR, interquartile range; LOESS, locally estimated scatterplot smoothing; MFI, median fluorescence intensity; rRT-PCR, real time reverse-transcriptase polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Figure 4.

SARS-CoV-2 IgG subclass responses in IgG-seropositive individuals. All panels show levels (MFI) of IgG1, IgG2, IgG3, and IgG4 against receptor-binding domain of the SARS-CoV-2 spike glycoprotein. A, Boxplots. B, Heatmaps showing the MFI of IgG and IgG subclasses for all IgG-seropositive individuals at recruitment, month 1, and month 3 separately. C, Kinetics of IgG subclass levels in seropositive individuals from month 0 to month 3. Lines indicate paired samples; yellow dots depict individuals who had detectable antibody levels at all study visits when antibody levels where measured; burgundy and blue dots show individuals who seroconverted and seroreverted, respectively, for the represented subclass between visits. D, IgG subclass levels stratified by symptoms. E, IgG subclass levels stratified by days of symptoms. A, D, and E, Data from accumulative month 0 and month 1 seropositive individuals: month 0 antibody levels from seropositive individuals at month 0 plus month 1 antibody levels from individuals who seroconverted from month 0 to month 1. Percentages indicate the proportion of seropositive subjects within each category of the x-axis. The center line of boxes depicts the median of MFIs; the lower and upper hinges correspond to the first and third quartiles; the distance between the first and third quartiles corresponds to the IQR; whiskers extend from the hinge to the highest or lowest value within 1.5 × IQR of the respective hinge. Wilcoxon rank test was used to assess statistically significant differences in antibody levels between groups. Abbreviations: IgG, immunoglobulin G; IQR, interquartile range; MFI, median fluorescence intensity; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2. See online version for color figures.