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During the study period, a total of 579 paired serum and OF samples were collected, with 364 and 215 from the vaccine and control group, respectively. The median age of the participants in the vaccine group was 45 years (interquartile range [IQR], 35–53 years). In the control group, the median age was 34 years (IQR, 29–39 years). Further, 172 OF samples were collected from preschool children with a median age of 4 years (IQR, 3–5 years).
To evaluate the quality of OF sampling, the total HIgG concentrations in 751 OF samples (579 adults and 172 children) were determined. Overall, the mean concentration of HIgG in OF samples was 1.85±0.83 µg/mL. The mean concentration of HIgG in OF samples from children was 2.02±1.02 µg/mL, which was significantly higher than that of the adults at 1.85±0.77 µg/mL (t=2.223, P<0.05). Further, the mean HIgG concentration in the OF samples from the vaccine group was 2.01±0.72 µg/mL, which was significantly higher than that of the control group at 1.59±0.77 µg/mL (t=6.616, P<0.001) (Figure 1). However, there was no significant correlation between the HIgG concentration in the OF and the titer of IgG antibody against SARS-CoV-2 in the serum (R2=0.062). The threshold for HIgG concentration in qualified OF samples could be set to 0.3 µg/mL, as more than 95% of OF samples contained higher concentrations of HIgG. Based on this cutoff, 8 and 12 OF samples from the vaccine and control groups, respectively, were excluded from the study.
Figure 1.The distribution of HIgG titers in OF samples based on age and immune state. (A) The different levels of HIgG antibodies between adults group (over 18 years old) and children group (younger than 14 years); (B) The different levels of HIgG antibodies between vaccine group (received booster dose of COVID-19 vaccine) and control group (without COVID-19 infection or vaccination history).
Abbreviation: HIgG=human immunoglobulin G; OF=oral fluid; SARS-CoV-2=severe acute respiratory syndrome coronavirus 2; COVID-19=coronavirus disease 2019.The SARS-CoV-2 IgG antibody was detected in 397 paired OF and serum samples (194 from the vaccine group and 203 from the control group) using both the adapted in-house immunoassay and commercial magnetic particle-based CLIA. Compared to the titer of SARS-CoV-2 IgG in serum, a ROC curve was plotted for detecting SARS-CoV-2 IgG antibody in OF samples. The area under the ROC curve for SARS-CoV-2 IgG antibody in OF samples was 0.988, and the chemiluminescence cutoff value was set as 105,566.5 (Figure 2).
Figure 2.The ROC curve for SARS-CoV-2 IgG antibody detection in OF samples.
Note: The true positive rate (sensitivity) of SARS-CoV-2 IgG antibodies in OF is plotted in function of the false positive (1-specificity) for different cut-off points.
Abbreviation: ROC=receiver operating characteristic; OF=oral fluid; AUC=area under curve; SARS-CoV-2=severe acute respiratory syndrome coronavirus 2.
Based on the cutoff value of SARS-CoV-2 IgG antibodies in the OF, among 162 paired OF and serum samples in the vaccine group, 161 (99.38%) serum samples and 153 OF samples (94.44%) tested positive for SARS-CoV-2 IgG (Table 1). Compared with the serum test, the in-house OF assay had 95.06% accuracy, 95.03% sensitivity, and 100% specificity. Of the eight false-negative OF samples, the range of S/CO values in the OF samples was 0.187–0.881, whereas that in the matched serum samples was 1.574–10.776. The concentration of SARS-CoV-2 IgG in OF was significantly correlated with that of the serum (R2=0.62, P<0.001) (Figure 3).
Detection of
SARS-CoV-2 IgGSARS-CoV-2 IgG in serum Total Positive Negative SARS-CoV-2 IgG in OF Positive 153 0 153 Negative 8 1 9 Total 161 1 162 Abbreviation: SARS-CoV-2=severe acute respiratory syndrome coronavirus 2; OF=oral fluid. Table 1. Comparison of SARS-CoV-2 IgG antibody levels between the OF and serum samples in the vaccine group.
Figure 3.The correlation analysis of SARS-CoV-2 IgG between OF and serum samples.
Note: Scatter plots of S/CO value of SARS-CoV-2 IgG concentration in OF (y-axis) and S/CO value of SARS-CoV-2 IgG concentration in serum (x-axis).
Abbreviation: SARS-CoV-2=severe acute respiratory syndrome coronavirus 2; S/CO=chemiluminescence value of sample/cutoff value; OF=oral fluid.
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