A total of 88 COVID-19 patients from eleven designated hospitals were included in this study, of whom 43 were male and 45 were female. Their mean age was 56.43 years old (range 17–83) and the median interval between initial symptom onset and sample collection was 11 days (range 1–37). Thirty-two patients (36.4%) had severe/critical illnesses and required oxygen supplementation or higher life support, while the other 56 patients had mild or moderate symptoms (Table 1).
Mild/moderate cases Severe/critical cases P Total 56 (63.64%) 32 (36.36%) Gender 0.136 Male 24 (42.86%) 19 (59.38%) Female 32 (57.14%) 13 (40.62%) Age (mean ± SD, years) 57.05 ± 13.94 55.34 ± 12.89 0.571 Sample collecting time (days)a 0.003 Median 12 9 Interquartile range 9–18 5–12 Nucleic acid test 0.748 Positive 42 (75.00%) 23 (71.88%) Negative 14 (25.00%) 9 (28.13%) Antibody tests IgM positive 24 (42.86%) 5 (15.63%) 0.009 IgM negative 32 (57.14%) 27 (84.37%) IgG positive 44 (78.57%) 14 (43.75%) 0.001 IgG negative 12 (21.43%) 18 (56.25%) aSampling time: the time interval between symptom onset and sample collection
Table 1. Demographic information and test results of the studied subjects.
qPCR test confirmed 65 SARS-CoV-2 infected cases among 88 participants (73.86%). No significant difference was observed between the positive rates of two qPCR kits (37/53 versus 28/35, χ2 = 1.133, P = 0.287). On the other hand, the positive rates of serum IgM and IgG antibody against SARS-CoV-2 were 32.95% (29/88) and 65.91% (58/88), respectively (Table 2). Altogether, 84 COVID-19 cases (95.45%) were identified among all patients by the combination of NAT and antibody test, which was significantly more than single NAT (χ2 = 15.793, P < 0.001) or serologic test (χ2 = 24.643, P < 0.001). The consistency rate between results of antibody test and NAT was 48.86% [(39 + 4)/88].
NAT resultsa Antibody test resultsa Total IgM IgG/IgM + IgG Positive Negative Positive Negative Positive 20 (22.73%) 45 (51.14%) 39 (44.32%) 26 (29.55%) 65 (73.86%) Negative 9 (16.98%) 14 (15.91%) 19 (21.59%) 4 (4.54%) 23 (26.14%) Total 29 (32.95%) 59 (67.05%) 58 (65.91%) 30 (34.09%) 88 (100%) aCombination of NAT and antibody test had significantly higher detection rate than single NAT (χ2 = 15.793, P < 0.001) or serologic test (χ2 = 24.643, P < 0.001).
Table 2. Comparison of results of serum SARS-CoV-2 antibody tests and nucleic acid test (NAT).
Notably, all the patients that were positive for SARS-CoV-2 IgM were also positive for SARS-CoV-2 IgG. The earliest seroconversion of IgG antibody was observed 5 days after the disease onset, and that time interval of IgM antibody was 8 days (Fig. 1). For 51 patients with sample collected at 10 days or later after symptom onset, the seroconversion rate was 47.06% for IgM (24/51) and 82.35% for IgG (42/51). Both antibodies were detectable in samples collected over 30 days after onset.
Figure 1. The correlation between sample collecting time of COVID-19 patients and different test results combination. Six categories of samples with different test results were characterized on the left side of the figure. Each colored dot represented one patient sample and its time interval between symptom onset and sample collection was scaled on the lateral axis. The median time interval and interquartile range were reported for each category. PCR+: positive for SARS-CoV-2 RNA in nucleic acid test; PCR−: negative for SARS-CoV-2 RNA in nucleic acid test; IgM+/IgG+: positive for SARS-CoV-2 IgM/IgG antibody in serologic test; IgM−/IgG−: negative for SARS-CoV-2 IgM/IgG antibody in serologic test.
When comparing patients with mild/moderate symptoms and patients with severe/critical diseases, no obvious difference was found between their gender ratios (P = 0.136), age composition (P = 0.571) and NAT positive rates (P = 0.748), but the mild/moderate group had later sampling time and higher antibody positive rates than the severe/critical group (Table 1). When comparing to the severe/critical cases with the same sampling time, mild/moderate cases presented higher seroconversion rate and higher antibody titer for both IgM and IgG antibodies (Fig. 2). Similar analysis was performed on cases of different genders and in different age groups, but no significant difference was observed between their antibody levels and temporal profiles (Fig. 3).
Figure 2. Comparison of nucleic acid and serologic test results between COVID-19 patients with different disease severity. Study subjects were separated into mild/moderate cases (black dots) and severe/critical cases (red dots), and their nucleic acid and serologic test results were compared. The time interval between symptom onset and sample collection was scaled on the lateral axis in each panel. For nucleic acid test (left) and IgM (middle)/IgG (right) antibody tests, the vertical axes reported qPCR cycle thresholds (Ct) and S/Co values, respectively. The dash line represented the threshold of each test.
Figure 3. Comparison of serologic test results between COVID-19 patients of different genders and age groups. A The serologic test results of males (black dots) and females (grey dots) were compared. Every dot represented one patient and the vertical axis reported the S/Co values of SARS-CoV-2 IgM and IgG antibody tests. B The serologic test results of < 60 years group (black dots) and ≥ 60 years group (grey dots) were compared. Every dot represented one patient and the vertical axis reported the S/Co values of SARS-CoV-2 IgM and IgG antibody tests. The dash line represented the threshold of each test.