Due to the epidemiological features of the infection, the case series described in our study were considered a familial cluster of COVID-19 infection. The cluster of COVID-19 infection, including two imported cases and four secondary cases, occurred within 12 days. In this cluster, our findings suggested that: 1) during the incubation period, COVID-19 patients could infect close contacts; 2) the early quarantine for close contacts of index patients remains a useful tool for controlling the epidemic; and 3) asymptomatic infection may be more common than previously thought. In fact, in the later stage of COVID-19 epidemic in Jilin, sporadic clusters of SARS-CoV-2 infection became a major source. Therefore, knowing the characteristics of a cluster of COVID-19 is useful to rapidly control the epidemic.
Respiratory and contact routes are proven transmission mechanisms for COVID-19 infection. However, in a recent study, SARS-CoV-2 RNA was detected in the fecal samples of COVID-19 patients, indicating the possibility of transmission via the fecal-oral route. (Jiang et al., 2020; Xu et al., 2020) In our study, respiratory and contact should be considered as the two main transmission mechanisms responsible for this cluster of infections. Moreover, an asymptomatic carrier was also suspected of being a source for transmission of COVID-19, (Bai et al., 2020b) and this key point was proved by Zhang et al. (Zhang et al., 2020) Although these new findings make the prevention and control of COVID-19 more complicated, they will lead to improved and more effective strategies. To avoid acquiring COVID-19, specific attention should be paid to factors such as environmental, direct contact, and social distance. (2020a; China)
Person-to-person transmission has been confirmed. It is unclear if during the incubation period, the virus is infective. A previous study by Yu et al. (Yu P, 2020) showed possible transmission of COVID-19 during the incubation period. Similarly, our data also supported that the virus remains infective during the incubation period. In this study, before the onset of symptoms of Case 1 (January 25), Cases 3 and 4 only had dinner with Case 1, as well as a dinner party, and Case 5 had another social contact with Case 1. Three cases were all infected with COVID-19. This special characteristic mentioned above is different from the transmission of SARS infection. In our study, the incubation period was relatively long and ranged from 9 to 17 days. Remarkably, although the incubation period of Case 1 was estimated at 9 days, the possibility remained of it being extended to longer than 9 days because of her residential location (Xiantao, Hubei). The incubation periods of Cases 5 and 6 were estimated from the first contact with Case 1, with the longest period being 17 days. Likewise, the incubation period of a secondary case has been reported over 14 days, (Guan et al., 2020) even reaching up to 20 days. (Yang et al., 2020b) Hence, a longer quarantine period, such as 2 or 3 weeks, may be required.
Usually, patients with COVID-19 present with fever, cough, or lung infiltrates. (Huang et al., 2020) In some cases, they may also have mild respiratory symptoms. More recently, asymptomatic infection has been reported. In Japan, the prevalence of asymptomatic infection is estimated at 41.6% (95% confidence interval: 16.7%, 66.7%) among COVID-19-infected individuals. (Nishiura et al., 2020) However, in China, a study of 72, 314 COVID-19 patients showed that asymptomatic carriers comprised 1.2% of all COVID-19 cases. (2020b) The difference in the prevalence of asymptomatic infection may be caused by sample size estimated and different subjects included between the two studies (high-risk vs. confirmed). In our study, due to positive SARS-CoV-2 RT-PCR results from fecal and pharyngeal samples, Case 6 was considered asymptomatic infection, the time of viral shedding was 8 days, longer than the time for Cases 4 and 5. We believe that asymptomatic infection is common. A similar result was reported in another familiar cluster of COVID-19 infection. Due to SARS-CoV-2 RT-PCR (+) and IgM assays (+), four cases without clinical symptoms were considered asymptomatic infection. (Bai et al., 2020a) Hence, on one hand, good personal hygiene is emphasized; on the other hand, due to no clinical symptoms and normal radiography, asymptomatic infection could be easily neglected. Hence, more attention should be given to improving the diagnostics and management of asymptomatic carriers, this would improve the prevention and control of the COVID-19 epidemic. (2020c)
For the diagnosis of COVID-19 patients, the sensitivities of IgM and IgG assays were 77.3%, and 83.3%, respectively, (Xiang et al., 2020) and RT-PCR for detection of COVID-19 has a lower sensitivity of 35%. (Mardani et al., 2020) It was then concluded that a significant proportion of COVID-19 would be diagnosed as suspected cases. Case 3, as a suspected COVID-19 case, presented with mild respiratory symptoms and abnormal radiography. Although RT-PCR for SARS-CoV-2 detection was performed several times, the case was not confirmed. However, due to contact history with the index case, Case 3 was identified, isolated, and treated as a suspected case. Fortunately, until now, several new assays, such as an automated chemiluminescent immunoassay, a reverse transcription loop-mediated isothermal amplification assay, and antibody detection assay for the diagnosis of COVID-19, have been evaluated and good performance has been confirmed. (Baek et al., 2020; Infantino et al., 2020; Xiang et al., 2020; Yan et al., 2020; Yang et al., 2020a) These new assays may improve the diagnostic dilemma of current assays for COVID-19.
Accumulated evidence suggests that SARS-CoV-2 is more infectious than SARS-CoV and MERS-CoV. (Bai et al., 2020a) However, this study of a familiar cluster of COVID-19 infection demonstrated a differential susceptibility. Although C1D and C5S lived with Cases 1 and 2, it remains to be found without COVID-19 infection. The result suggests a difference in the susceptibility to COVID-19 among individuals. The SARS-CoV-2 has preferential tropism to human airway epithelial cells through the same cellular receptor as that for SARS, angiotensin-converting enzyme 2 (ACE2), which is a central body receptor for the surface glycoprotein S of the virus. (Munster et al., 2020) Therefore, the down regulation of ACE2 expression is thought to explain the lower susceptibility to COVID-19. In addition, the differential susceptibility suggested by the study implicated that interventions, such as vaccine trials and preventive measures, for the COVD-19 control should be evaluated their feasibility.