Discussion
On 21 February 2020, a previously recovered COVID-19 patient in Chengdu (Sichuan province, China) was re-hospitalised after testing positive for the virus [8]. This aroused our great concern regarding potential infectivity of the recovered patients, especially for those asymptomatic cases with positive virus detection after discharge.
In our study, we conducted surveillance by regular virus testing and detected two positive cases (3.23%) among the 62 recovered medical staff. Surprisingly, Case 1 showed typical clinical and radiological manifestations on admission, while the manifestation in Case 2 was not typical. Moreover, both throat swab and stool tests turned negative in Case 1 before discharge. During home isolation, none of the two cases experienced discomfort, indicating that disease relapse was unlikely.
Currently, it is difficult to find a reasonable explanation for these observations. It was assumed that the virus can easily be detected in the upper respiratory tract at the stage of early infection. When the disease progressed, the virus was more likely to appear in the lower respiratory tract and other locations such as intestines and blood [9]. Thus, the virus may not be detected in throat swabs, especially for cases without expectoration. This may explain why some patients had negative RT-PCR for SARS-CoV-2 at initial presentation despite positive findings in chest CT, or positive RT-PCT and negative CT findings at initial presentation [10,11].
Moreover, our findings seem to indicate that after hospitalised treatment, there could be a possibility that a small proportion of clinically recovered patients may still carry a small amount of virus which is hard to detect. The current standard for diagnosing COVID19, the RT-PCR-based method, showed a high accuracy of 97% and the specific primers and probes guaranteed its diagnostic specificity, although 3% of cases may test false-negative because of potential sampling error [11]. Both cases had a positive detection (including weakly positive) three times during the follow-up, which decreased the possibility of false positives in these two cases.
After fulfilling the Chinese current criteria for discharge, it may still take a few days for the immune system to completely eliminate the residual viruses in the body [7]. During this period, the virus may rebound and test positive, but the patients were asymptomatic and chest CT showed no deterioration. If the patients’ immunity decreases, there is a risk of a relapse. On 6 February 2020, a recovered COVID-19 patient in Changde (Hunan province, China) had a fever and cough 2 days after discharge, and chest CT showed worsened status [12].