The demographic and clinical information
A total of 248 subjects were recruited from two cohorts, in which 174 (70.2%) were males. The subjects were aged from 5 to 80 years (mean 40.3). Based on their working environment, they were classified into HCW (176, 70.9%) and Non-HCW (72, 29.0%). HCW were enrolled from COVID-19 medicine units, COVID-19 ICUs, specialty consultants, and pharmacies. These HCW had different degree of workplace exposures, performing invasive bedside procedures, providing intraoperative and routine care to COVID-19 positive patients and working in pharmacies. The Non-HCW enrolled were the close contacts of HCW. The first cohort consisted of HCW (68.3%, 56/82) and Non-HCW (31.7%, 26/82) recovered from COVID-19 infection, and the second cohort consisted of HCW (72.3%, 120/166) and Non-HCW (27.7%, 46/166) undiagnosed for COVID-19 infection. The majority of the patients recalled having clinical symptoms consistent with COVID-19, including fever (122, 49.2%), fatigue (93, 37.5%), muscle pain (78, 31.5%), cough (58, 23.4%) and throat pain (58, 23.4%) (Table 1, and Figure 2). Comorbidities were reported in 106 (42.7%) patients, in the form of hypertension, diabetes, thyroid, asthma, cardiovascular disease, COPD etc (Table 1). Compared to undiagnosed for COVID-19 cases, the disease symptoms including fever (75.6% vs 36.1%) (P=<0.001), cough (40.2% vs 15.1%) (P=<0.001), fatigue (61.0% vs 25.9%) (P=<0.001), breathlessness (17.1% vs 5.4%) (P=0.002), muscle pain (42.7% vs 25.9%) (P=0.007), loss of appetite (26.8% vs 10.2%) (P=<0.001), altered smell (37.8% vs 4.8%) (P=<0.001), and loss of taste (41.5% vs 10.2%) (P=<0.001) were reported in higher frequency in COVID-19 positive cases (chi-square test) (Table 1, and Figure 2).
Characteristics of IgG antibodies in the study population
A total of 659 plasma samples were collected from the 248 study participants, In the first cohort, 82 patients recovered from COVID-19 infection provided 82 plasma samples at baseline visit and out of them 75 patients were followed up longitudinally for six months (three visits) and provided 307 plasma samples (the time interval between each visit is 45 days). In the second cohort, 166 undiagnosed subjects for COVID-19, with history of close contact with COVID-19 positive patients provided 166 plasma samples at baseline visit and out of them 62 patients with seropositive for anti-spike IgG assay were followed up longitudinally for six months (three visits) and provided 352 plasma samples, respectively. HCW and Non-HCW were followed for a median of 227 days (interquartile range, 166 to 202) after a positive antibody test.
The IgG antibodies against spike (S) protein were tested in 248 subjects at baseline visit, 137 samples at 1st, 2nd and 3rd visits, respectively. All the serum samples were collected between 1 and 238 days after onset of symptom (The number of days between visits was calculated from the date of symptom onset in the symptomatic COVID-19 positive individuals and the date of the first clinic attendance in the asymptomatic undiagnosed individuals with no PCR performed). IgG antibodies appeared from day 16 and remained stable during the course. In the first cohort, 70.7% (58/82) subjects were seropositive at their first anti-spike IgG assay at baseline visit, seroconversion occurred during the study period and 80.0% (60/75) by 1st visit, 90.6% (68/75) by 2nd visit and finally 82.6% (62/75) seropositive at 3rd visit have seroconverted. Similarly, in the second cohort, 37.3% (62/166) subjects were seropositive at their first anti-spike IgG assay at baseline visit. These 62 seropositive subjects were followed up and were looked for immune response; by the 1st visit 70.9% (44/62), 75.8% (47/62) by 2nd visit and finally 82.2% (51/62) were found to be seropositive. In the second cohort, 46.8% (29/62) subjects were asymptomatic, having no COVID-19 symptoms and were seropositive for anti-spike IgG at baseline visit. The positivity rates of IgG antibodies increased over time and remained stable across the study duration (Figure 1).
SARS-CoV-2 antibody titers in COVID-19 patients
The antibodies levels measured increased over the first three months and decreased slightly after that and remained at a plateau during the entire study duration. In COVID-19 positive cohort, we found a significant difference between IgG antibody levels at weeks 1-3 (2.14±1.98), weeks 3-7 (2.55±1.55), weeks 7-10 (2.21±1.30), weeks 10-20 (2.34±1.20) and weeks 20-30 (3.58±3.39) (P <0.001, respectively) (Figure 2 B). In the undiagnosed cohort, 76.3% (29/38) patients were asymptomatic at baseline visit (weeks 1-3) with IgG antibody levels (2.26±1.32). IgG antibody levels were also found to be significantly different in the second cohort at week 1-3 (2.41±1.28), weeks 3-7 (1.86±1.11), weeks 7-10 (1.59±1.13), weeks 10-20 (2.10±1.16) and weeks 20-30 (3.27±2.59) (P <0.001, respectively). The tabulated data of IgG antibody titers by their visits were shown in the Table 2 and Figure 1.
We then sought to evaluate the duration of appearance of IgG antibodies and their levels during the course of the disease. We observed the kinetics of the antibody levels remained mostly similar at all the four visits i.e., up to six months. The IgG antibodies started to appeared between weeks 1-3 and increased gradually and remained stable during the disease course. In the COVID-19 positive cohort, we found 70.7% (58/82) PCR positive and IgG antibody positive cases with significant difference in antibody titers 3.21±1.28 at baseline visit, 2.53±0.73 at visit 1, 2.82±1.41 at visit 2, and 4.32±3.75 at visit 3 (P <0.001, respectively) (Table 2). In the same cohort, 29.2% (24/82) cases were PCR positive and IgG antibody negative at the baseline, the IgG antibody levels were very low throughout the course of study with antibody titers 0.66±0.23 at baseline visit, 0.68±0.25 at visit 1, 0.53±0.33 at visit 2, and 0.84±0.25 at visit 3, (P=0.054, respectively) (Table 2). In the undiagnosed cohort, 37.3% (62/166) cases were found to be IgG positive at the baseline, and IgG antibody titers remained to be mostly similar in these cases throughout the course of study with antibody titers 2.24±1.28 at baseline visit, 2.10±0.77 at visit 1, 2.54±1.02 at visit 2, and 3.64±2.56 at visit 3, (P <0.001, respectively) (Table 2). In the same cohort, 46.7% (29/62) cases had developed IgG antibodies at the baseline, these cases were asymptomatic IgG antibody titers were initially low in these cases but increase gradually during the course of study with antibody titers 1.91±1.46 at baseline visit, 1.88±0.93 at visit 1, 2.38±1.35 at visit 2, and 3.24±2.68 at visit 3, (P =0.021, respectively) (Table 2). Further, 53.2% (33/62) cases had history of COVID-19 symptoms, similarly IgG antibody titers were initially low but increase gradually during the course of study with antibody titers 1.98±1.37 at baseline visit, 1.79±0.94 at visit 1, 2.09±1.08 at visit 2, and 3.48±0.46 at visit 3, (P <0.001, respectively) (Table 2).