Challenges Caused by Imported Cases Abroad for the Prevention and Control of COVID-19 in China

Background: In mainland China, no new domestic cases of COVID-19 had been reported for several weeks, it was current priority to prevent and control imported cases of COVID-19. We conducted this study to review the epidemiological characteristics of imported cases from abroad. Methods: From February 26th to April 4th,2020, the imported cases from abroad were enrolled in this study. The effect of prevention countermeasures in curbing the spread of COVID-19 was assessed in this study. Moreover, for these imported cases, most of them could not provide the definite exposure time,we supposed that the time of leaving epicenter could predict the progression of COVID-19, so we defined incubation period as from the date of leaving epicenter to date of symptom onset and the interval of symptom onset to final diagnosed time was defined as diagnostic time, we also defined confirmed time as from date of leaving epicenter to date of final diagnosed. Categorical variables were summarized as numbers and percentages, the difference among the variables were analyzed. Findings: For 670 cases imported from abroad, 555 were Chinese, 115 were foreigners. The confirmed cases had increased sharply after WHO announced COVID-19 as global pandemic, about 642 cases were confirmed after that day. On March 18th, all entry population were suggested to undergo nucleic acid testing, the daily new imported patients reached to peak with 50 cases one week later. To more powerfully curb the spread of COVID-19, in March 28th, China was compelled to temporarily suspend the entry of foreign passport holders with valid visas or residence permits. Apparently, confirmed cases had significant decreased, six days after implement of controlled measures, the daily new confirmed cases were reduced to 13 cases. Moreover, About 84.3% patients(166/197) presented symptoms one week after leaving the epicenter, notable, 7 patients (3.6%) had symptoms two weeks after leaving the epicenter, the median incubation period was 3.0 days (interquartile range,1.0 to 6.0), the 95th percentile was 11.6 days. Additionally, most of cases (92.9%) were detected positively of nucleic acid after symptom onset with four days, the median diagnostic time was 2.0 days, (interquartile range,1.0 to 3.0),the 95th percentile of the distribution was 5.0 days. Finally, about 5.8% patients were healthy carriers,the median confirmed time of asymptomatic patients was 4.0 days, (interquartile range ,2.0 to 9.0), there were still 37 patients (5.5%) who were final diagnosed two weeks after leaving the epicenter. The following variables might associated with confirmed time :symptom type(P=0.005), exported regions(P＜0.001) and symptom onset time(P＜0.001). Interpretation: The prevention countermeasures for imported cases implemented by the Chinese government played an indispensable role in curbing the spread of COVID-19, the time of departure from epicenter could provide to estimate the incubation period and confirmed time, 3.6% patients developed symptom two weeks after they leaving epicenter, it indicated that the two-week quarantine period might need to be prolonged, asymptomatic patients should be closely monitored. Funding Statement: This research was supported by grants from the Wu Jieping Medical Foundation (320.6750.17527) and Provincila Key R&D Program of Shaanxi(2017ZDCXL-SF-01- 04-01). Declaration of Interests: The authors declare that they have no conflicts of interest.