The transmission of 2019- novel coronavirus through eyes has recently attracted the researchers’ attention. There were sporadic cases of conjunctivitis reported in infected patients whose conjunctival swabs were positive for nucleic acid of coronavirus[11–13]. However, for all infected patients, the positive rate of conjunctival swabs was very low, lower than that of throat swabs[11–13], which was consistent with the results of our study. It seemed that the possibility of virus existence in the ocular surface is low. However, more detailed analysis of the eye sample collection process and the laboratory nucleic acid test results can help to obtain more information.
In previous studies, conjunctival swabs were used to collect eye secretions including the tears, which were usually insufficient for test[14]. It was reported that the amount of tears collected by Schirmer’s strip at one time was greater than that collected by other methods such as capillary tubes and surgical sponges [15].In the first nine patients (no.1–9) of this study, conjunctival swabs were used to collect samples and all the test results were negative. In the subsequent patients (no.10–29), Schirmer’s strips were used to collect sample, and 2 samples were detected positive. This result suggested that the positive rate of the virus detection may be higher when Schirmer’s strip was used. In addition, through the analysis of the volume of tear and the value of nucleic acid detection CT in 6 severe patients, it was found that there was indeed a quantity effect relationship between the two. This, to some extent, indicated that,the larger the amount of tear samples collected, the greater the probability of positive detection.
Another potential factor that affected the test results was the viral load in the sample. In theory, the more viral load, the higher the detection rate. The low positive rate of this study also indicated that the viral load in the ocular surface was relatively limited.
Due to the influence of multiple factors, the interpretation of the detection results should be more cautious—even throat swabs may have a high negative rate. In this study, throat swabs showed a positive detection rate of 54.3%, which was far lower than that in recently published online study. Different from ours, the sputum (non throat swab) samples of patients were taken for nucleic acid detection, so a high positive detection rate (91.7%) was obtained [13]. In practice, we found that most patients did not have sputum available. In addition to the way of sampling from bronchial secretion, the patients profile may also affect the nucleic acid detection results. For the patients with mild and moderate disease, we compared the baseline characteristics stratified by positive and negative throat swab Rt-PCR results (Table 2). Since the average age of the positive patients was greater than that of the negative patients, and the average interval between the sampled and diagnosis confirmed day for positive patients was shorter than that for negative patients,we suggested that the older the patients were, the earlier the detections were performed, the easier it was to detect nucleic acid fragments from throat swabs. However, for severe patients, because of their 100% positive rate, the possibility of the test results affected by the factors mentioned above should be small.
The fact that the consistency between the test results of throat swab and Schirmer’s strip was low and that only two of three patients with tear Rt-PCR positive result had throat swab
positive result, suggested that the tear test results were not related to throat swab test results. Moreover, the result that one patient (No. 13) showed positive result of tear but negative of throat swab reminded us that Schirmer’s strip detection may be used as an auxiliary means to identify patients with false negative throat swabs test result. Although the positive rate of tear test was very low, when facing this highly infectious disease, any additional screening for the control of disease transmission is of practical significance.
Limitation of this study was the relatively small sample size, especially the few severe patients included. In practice, in the face of the more infectious severe patients, sample collection performed in a single center had certain challenges. Another limitation also came from the small number of samples: no conjunctivitis patients could be included. Therefore, it was impossible to judge whether it was more likely to detect virus from the tears of conjunctivitis patients. Furthermore, we did not repeatedly collect and test samples,neither tears nor throat swab. Hence, we could not reduce the random errors occurred during the sample collection and testing process.
However, one of the advantages of this study was the simultaneous collection of throat swabs and tears from patients, which was helpful for us to compare the detection results of throat swab and tear. Another advantage came from the use of Schirmer’s strip, which could collect larger volume of tear samples. From this perspective, Schirmer’s strip outweighs the conjunctival swab. Most importantly, this study confirmed the practical value of Schirmer’s strip by demonstrating the positive tear test result from patient no.13 who had negative throat test result.
In the future, we should pay more attention to the virus detection from ocular surface in different stages of the disease and different degrees of infection. Isolating the virus and determining the virus load, if exists, from the ocular surface, and investigating the existence of eye-body infection pathway will help us to clarify the human-human transmission route via eyes.