COVID-19 is an emerging infectious disease caused by 2019 novel coronavirus (2019-nCoV), with pulmonary inflammation as a prominent manifestation. It spread rapidly in the world. There were two cluster infections of COVID-19 in some areas of Beijing in January-March 2020 and June-July 2020, respectively. Similarities of clinical classification in two clusters were dominated moderate and mild type, but few severe types. The male to female ratio was 1.1:1. The most common symptoms were fever and cough. Twenty-seven percent patients had one or more of comorbidities. Ground glass opacity, lower lobe involvement and peripheral distribution were all the chest computed tomography features of COVID-19. Comparison of two groups, it was showed that patients in June-July group had milder symptoms and imaging than that in January-March group. In addition, family clusters of cases in the former were less than in the latter.
As a new infectious disease, COVID-19 needs to be recognized gradually. The influence of gender and age on the disease was inconsistent with the results reported in different studies. Some early research reports of gender and age showed that old people and men were more likely to be infected novel coronavirus, and old people with comorbidities were more likely to develop into severe cases with high mortality [2-5]. However，an analysis of 44,672 confirmed cases in Wuhan showed that 51.4 % of the patients were male, with a similar ratio of male to female. In this paper, 44% of the patients were aged between 20 and 50 years old, and 42% were aged between 50 and 70 years old. In our study, 69% of the patients were between 20 and 50 years old, and 26% were between 50 and 70 years old, which was younger than that reported in the literature. In addition, according to the official data of China Center for Diseases Prevention and Control (CDC), as of July 6, a total of 366 cases (335 confirmed cases and 31 asymptomatic infected cases) have been reported in Beijing. The male to female ratio is 1.2:1. The median age is 43 years old. There were moderate type（74.6%）, mild type (15.0%）, 2 severe type cases, 4 critical type cases. The proportion of severely and critically patients was significantly lower than those patients between January and March this year. These were consistent with our results. The most common symptoms of COVID-19 were fever and cough. However, the proportion of patients with fever and cough in our study was lower than other reports [3,5,7,8], especially in the June-July group. According to previous literature [3,5,7,8], there were fever (86%~100%) and cough (47%~77%). In our study, fever and cough were 52% and 38%, respectively. On the one hand, it was due to different from the research subjects. On the other hand, it was related to disease course and disease severity. Because severe cases and old people in our study were rare, we did not see the similar results.
Chest HRCT was a key way in diagnostic of COVID-19. The most common chest CT pattern was GGO in early stage of COVID-19. consolidation indicated disease progression [7,8]. According to guidelines, four phases were divided: 1) Early stage: ground-glass opacity (Fig. 2), GGO, which was mostly confined and scattered in the lower and middle lungs, mainly under the pleura. 2)Progressive stage (Fig. 3): GGO range expanded, density of lung opacities increasing, multiple lesions, multi-lung segment fusion accompanied by consolidation, especially in both peripheral lungs. 3) Severe stage: diffuse and extensive lung density was further increased, mainly with consolidation (white lung), which progressed rapidly. The density shadow of the original lesion was increased by more than 50% within 48 hours. 4) Absorption stage (Fig. 4): The inflammation will be absorbed and the extent will be minimized. The disease will improve, perhaps leaving the lung fiber cord focus. In our study, four phases were early stage (65%), progressive stage (27%), severe stage (3%), absorption stage (5%), respectively. Previous literature [8,10,11] evaluated the severity of pulmonary involvement by CT score, and the higher the score, the more severe the pulmonary lesion. The frequency of involvement of lower lobe of both lungs was higher than that of upper lobe. Distribution of lung lesions was mainly in peripheral lungs. These were in accordance with others studies. It was worth noting that some patients with no symptoms had abnormal chest imaging . A similar phenomenon was found in our study. This suggests that early imaging examination was important to suspected patients.
Compare the different characteristics of COVID-19 between January-March group and June-July group, the clinical symptoms of the latter were obviously milder than the former. In addition, the chest CT severity score and the number of lesion lung lobes in the latter were also less than in the former. But, family clusters of cases in the former were more than in the latter. There are several possible reasons. 1) People tend to gather at the beginning of this year. During the Spring Festival in January- February, as the biggest traditional Chinese festival, the people tended to be densely populated. This condition easily led to the spread of diseases among families. 2）In the early stages of a new infectious disease, there was a lack of awareness about this disease. From January to March, the first cluster infection of the disease began, patients did not come to the hospital until they developed significant symptoms. The number of patients in January-March was more symptoms than that in June-July. 3）Effective measures were taken early in the second cluster infection. Since June 11, 2020 Beijing has seen its second cluster infection. A series of measures to prevent transmission disease were taken immediately. For example, the market was immediately closed, contact people were quarantined, and nucleic acid testing was strengthened. As a result, the disease was effectively controlled in early stage. 4）The genetic sequence of the infected virus was also different in two cluster infections.
The present study has some sort of limitations. Firstly, the sample size was small. Larger prospective studies among different populations are needed to do. Secondly, there was lack of severe infection cases. Therefore, data on severely ill patients were insufficient. Thirdly, there was lack of information on patient treatment and outcomes. As confirmed patients were all transferred to designated hospitals for treatment. This leads to incomplete information about the treatment process and the final outcome of the patient, unable to fully grasp the entire development process of the disease.