Abstract
Persistence of viral RNA in respiratory samples of COVID-19 patients and asymptomatically infected individuals is a common and important problem in the outbreak control. The main question is whether the persistence of viral RNA reflects viable virus shedding. And, if there is viable virus shedding, there is an interesting scientific question of how the virus survives in the presence of immune response, which should be developed soon after the infection. Possible explanations include a failure of developing immune response or an immune escape by viral mutations. To answer these questions, this project performed viral isolation in sequential respiratory samples and antibody detection in serum samples of 30 cases with mild-symptom or asymptomatic, who showed viral RNA positive by RT-PCR after 1 week from the diagnosis. Samples in the first week showed a large variation in the viral load levels which significantly declined in most samples from the second week forward together with positive for specific antibody in serum samples. For viral isolation, there were 6 positive samples with high viral load; 4 samples and 2 samples from the first and second week samples, respectively. The two culture positive samples from the second week, one of them had low level of neutralizing antibody in serum at the same time point and these neutralizing antibody level increased in the third and fourth week samples together with a drastic decline in viral load and negative viral culture. The other sample showed high viral load and positive viral culture together with negative neutralizing antibody in the samples from the second and third week. These data indicate that most patients and infected individuals with persistent positive RT-PCR have low level of viral load and this does not indicated shedding of viable virus. However, failure in mounting the immune response in some individuals may result in persistent shedding of viable virus. High viral load and negative neutralizing antibody may be used as markers to identify such individuals.
