Objective To understand results of nucleic acid test of 2019-nCoV(SARS-CoV-2) and blood routine examination of persons who had close contact with coronavirus disease 2019 (COVID-19) cases. Methods 738 persons who were under medical observation in a designated isolation hotel from February 11 to 23, 2020 due to close contact with COVID-19 cases but without symptoms were selected, nasopharyngeal swabs were taken for 2019-nCoV nucleic acid test, and blood specimens were taken for blood routine detection, these persons were observed untill the end of isolation period. Results Among 738 persons who were under medical observation, 70 (9.49%) were positive for nucleic acid test of 2019-nCoV, 664 (89.97%) were negative, and 4 (0.54%) were suspected; in the initial nucleic acid test of 2019-nCoV, 52 persons were positive, 28 were suspected, and 658 were negative; 28 cases with suspected results were resampled the next day, 18 were positive, 6 were negative, and 4 were still suspected (After the second sampling, the observation period ended and these persons were transferred to the designated isolation observation place). Among all persons with positive nucleic acid test of 2019-nCoV and suspected persons under observation, white blood cell (WBC) count were all normal in blood routine, only 2 cases showed decreased lymphocyte count. During the first 14-day observation, among 738 medical observed persons, 40(5.42%) had COVID-19, 70 of whom were positive in nucleic acid test for 2019-nCoV, 42.86% (n=30) had fever, cough, shortness of breath and other clinical symptoms; among 644 persons with negative nucleic acid test, 1.51% (n=10) developed symptoms. All confirmed cases were transferred to designated hospitals for further treatment; at the end of the first observation period, those who did not turn negative for nucleic acid test of 2019-nCoV and those who still had suspected results were sent to the designated isolation place for continuous observation. During continuous observation period, 4.54% (2/44) developed symptoms and were confirmed with COVID-19, and were send to a designated hospital for treatment; the rest of the observed persons all turned negative in nucleic acid for 2019-nCoV detection. Incidence was 5.69% (42/738) during the whole isolation period. Conclusion Among persons with positive nucleic acid test of 2019-nCoV due to close contact with COVID-19 cases, proportion of latent infection is high, and there is a certain proportion of asymptomatic infected persons, which should be identified and diagnosed as early as possible.