Objective:To retrospectively analyze the test results of novel coronavirus (2019-nCoV) in different samples (throat swab, sputum and feces) collected from recovered COVID-19 patients in order to provide a more reliable basis for discharge and reduce the risk of recurrence after discharge.Methods:Throat swabs and sputum were sampled in pairs from 78 patients before discharge and sampled in pairs twice from 54 cases with an interval of 1-5 d. Real-time fluorescence quantitative RT-PCR was used to detect the virus in the two types of samples. Throat swab, sputum and fecal samples of six patients were tested for 2019-nCoV during follow-up.Results:The detection rate of viral nucleic acid was 46.15% in throat swabs and 50.00% in sputum samples. Test results of the second paired samples showed that the detection rate of viral nucleic acid was 25.93% in throat swabs and 46.30% in sputum samples, and the difference between the two types of samples was statistically significant ( P<0.05). During follow-up, 2019-nCoV nucleic acid could be detected in the fecal samples of the six patients, but not in their throat swab and sputum samples. Their fecal samples remained positive up to 52 d. Conclusions:In the late convalescence, the respiratory symptoms of COVID-19 patients gradually disappeared with the improvement of clinical symptoms. Moreover, the virus might enter the gastrointestinal tract from respiratory tract, and could long-term exist in recovered patients and be excreted in feces. In order to reduce the rate of missed detection and avoid false negative results, it was suggested to test the viral nucleic acid in different types of samples before a COVID-19 patient was discharged.

Objective:To analyze the whole genome traceability and variation analysis of SARS-CoV-2 in local COVID-19 outbreaks in Binhai New Area, Tianjin.Methods:The whole-genome high-throughput sequencing was performed on throat swab samples collected from one local asymptomatic infected person and five confirmed cases of COVID-19 in Binhai New Area of Tianjin from November 7 to December 5, 2020. The sequencing data were assembled by De novo. MAFFT v7.0 multiple sequence alignment program and MEGA X software were used to compare the above data and construct phylogenetic tree (Neighbor-joining method).Results:The genetic similarity between the sequences of 6 SARS-CoV-2 strains and Wuhan reference sequence (Wuhan-Hu-1) was greater than 99.9%. Two of six strains were genetically identical, conform to the L-Lineage European Branch Ⅱ.1(America Branch)/B.1; The other four strains had the same genes and were in line with the characteristics of L-Lineage European Branch Ⅰ/B.1.1.These six strains belonged to different evolutionary branches and two different transmission chains. There were 18 nucleotide mutation sites in sequences of six SARS-CoV-2 strains, eight of which were synonymous mutation sites, nine of which were missense mutation sites, resulting in nine amino acid mutation sites, and important mutation sites of RDRP-P323L and S-D614G were found in all of the six samples.Conclusions:In this study, there were two COVID-19 outbreaks in Binhai New Area of Tianjin, and the sequences of six SARS-CoV-2 strains belonged to different evolutionary branches and two different transmission chains. It might come from porters′ contact with imported cold chain items contaminated with SARS-CoV-2 from different sources. All the sequences of six SARS-CoV-2 strains had P323L and D614G mutations, which indicated that the virus mutation and transmission ability were stronger. The surveillance of important employees of the cold chain in Tianjin and local and imported cases should be continuously strengthened.