Results
Patients diagnosed with COVID-19 from August 1st to September 1st, at The Third People’s Hospital of Shenzhen were enrolled in this study (n = 44). The characteristics including age, gender and disease severity were listed in Table 1 . Most patients were male and asymptomatic. The average age of the patients was 43 years (range, 22-62 years). Saliva and serum from patients were collected and the levels of IgA, IgG and IgM were measured. The highest COI values of each patient was used to present the immunoglobulin level in saliva or serum. As shown inFig. 1 and Table 2 , 14 patients presented positive IgA in saliva, while 7 and 4 patients presented positive IgG and IgM, respectively. Moreover, IgA presented higher COI value than IgG and IgM in saliva (p = 0.0128 and p = 0.0297, respectively). 24 negative-control patients were selected randomly from inpatient departments as negative control. IgA, IgG or IgM in saliva and serum specimens were all negative (Fig. 2 ).
Saliva and serum which were collected on the same day or two consecutive days were analyzed as matched samples (n=15) (Table 3) . As shown in Fig. 3A-C , 5 saliva specimens presented higher IgA level than matched serum. In general, IgA in saliva specimens showed a roughly same level with serum (saliva, 11 positive vs 4 negative; serum, 10 positive vs 5 negative). IgG and IgM levels in saliva specimens were lower than those in serum (p < 0.0001 and p = 0.0444, respectively). All saliva presented lower IgG level than serum (saliva, 5 positive vs 10 negative; serum, 15 positive vs 0 negative), and only one saliva specimen presented higher IgM level (saliva, 3 positive vs 12 negative; serum, 5 positive vs 10 negative). No clear correlation was observed among IgA, IgG and IgM positive samples (Fig. 3D ).
To investigate whether the test of saliva IgA could improve the diagnostic power of COVID-19 patients, the conversion rates of saliva IgA and the detection of viral nucleic acid were analyzed in the first and second week after hospitalization (n=39) (Table 4 ). Though all the patients were hospitalized with positive nucleic acid result at the beginning, the positive rate was as low as 35.90% in the first week, and then 12.82% in the second week. The positive rates were obviously increased with saliva IgA.