Abstract: Background: In March and July of 2021, there were two outbreaks of COVID-19 epidemic in Ruili City, Yunnan Province and there were continuous reports in China of imported infection cases from other countries or regions. Objective: To explore the epidemiological and clinical characteristics of local COVID-19 cases of adults and children and provide reference for the prevention and treatment of COVID-19. Design: Case series report. Methods: We retrospectively collected the local cases of adults and children infected by imported cases, admitted to Chinese Medicine and Dai Medical Hospital,a designated hospital for COVID-19 in Ruili City, from March 29 to April 30 (referred to as 3-29, SARS-CoV-2) and from July 4 to July 31 (referred to as 7-4, delta-CoV).Demographic, epidemiological history and clinical characteristics data of these cases were collected. Results: A total of 208 cases were included with 192 adults cases (3-29: 112 cases, 7-4: 80 cases) and 16 children (3-29: 5 cases, 7-4: 11 cases). None of the children were vaccinated. For adults, there were 15 cases (7-4, 18.8%) for 1 dose of vaccine, 19 cases for the first dose, and 14 cases (17.5%) for the second dose. According to epidemiological surveys, among 3-29 and 7-4 adults and children, close contacts accounted for 3.6% and 40.0%, and 20.2% and 54.5%, respectively. There were no critical or fatal cases in adults and children during these two outbreaks. Adults were mainly mild and common types, and children were mainly asymptomatic infections, mild and common types. 3-29 adults with underlying disease (17.0%) accounted for a higher proportion than those of 7-4 (13.8%), and there was 1 child(7-4) with the underlying disease of hepatitis B. Fever cases of 7-4 adults (41.2%) accounted for a higher proportion than those of 3-29 (15.2%), and children fever cases accounted for 45.4% in 7-4. There was a higher proportion of fatigue and pharyngeal discomfort in 7-4 adults than that of 3-29, and children mainly had cough and pharyngeal discomfort. The number of adult cases of lymphopenia in 7-4 adults was more than that of 3-29, and only one child case of lymphopenia was found in the group of 7-4. The proportion of adults with elevated LDH, ALT, AST, CRP and D-dimer was higher in the group of 7-4 than that of 3-29, and for children there were 2 cases (7-4) in elevated LDH, AST, CRP and D-dimer respectively, accounting for 18.2%. CT scan showed that children were mainly characterized by multiple ground-glass shadows and infiltration shadows in both lungs. In addition, adult cases also showed multiple small patch shadows and interstitial changes outside the lungs. No children received oxygen therapy. The oxygen therapy time for 7-4 and 3-29 adults was 150 h and 96 h, respectively. The time for nucleic acid test results turning negative was close (19 d vs 22 d) in the adults during the two outbreaks, while for children, the time for 7-4 (32 d) was longer that of 3-29 (15 d). Conclusion: The clinical manifestations of local adults and children infected by delta-CoV were more severe than those infected by SARS-CoV-2. The clinical manifestations of adult cases were more severe than that of children. The time for nucleic acid test results turning negative was shorter in children compared with adults. During the epidemic of delta-CoV, the proportion of vaccine breakthrough infections in 7-4 adults was up to 36.3%.

Key words: Children, SARS-CoV-2, Delta-CoV, Epidemiology, clinical features., Clinical characteristic