Introduction
In December 2019, an outbreak of unidentified pneumonia characterized by fever, dry cough, and fatigue happened in the Huanan Seafood Wholesale Market, in Wuhan, Hubei, China(J. F.-W. Chan, Yuan, Kok, To, Chu, Yang, Xing, Liu, Yip, & Poon, 2020; Chen, W, G, & F, 2020; Hongzhou, W, & Yi-Wei, 2020). With the spread of the disease, the numbers of infected patients increased substantially which has become the most challenging health emergency in China and even all over the world. Sequence analysis of the coronavirus obtained from 5 patients in Wuhan has shown a structure typical to that of other coronaviruses such as SARS coronavirus and MERS coronavirus(N. Chen et al., 2020; Sohrabi et al., 2020). It also revealed that the new coronavirus has the smallest genetic distance from bat coronavirus, and about 80% similarity with SARS-CoV, and 50% similarity with MERS-CoV(P. Sun, Lu, Xu, Sun, & Pan, 2020). Thereafter, the coronavirus was designated as SARS-CoV-2 and the infected disease was named Coronavirus Disease 2019 (COVID-19) by the World Health Organization (WHO)(Li-Li et al., 2020; Sohrabi et al., 2020; Zhu et al., 2020). In addition, the nosocomial infection was found on January 20, 2020 which suggested that COVID-19 can be transmitted from human to human.
As of April 13, 2020 a total of 83696 cases have been confirmed in China and 1837079 cases reported in 209 countries outside of China. Due to the effective prevention and control measures in China, about 93.5% patients were cured and discharged and the existing confirmed cases decreased to 2083. However, the confirmed cases are still growing rapidly in foreign countries including United State, Italy, Spain, Germany and France, and United State has been particularly affected. With the further recognition of COVID-19 and experience in diagnosis and treatment cumulates, the National Health Commission (NHC) released a consensus about the Diagnosis and Treatment Scheme for Novel Corona Virus Pneumonia which put forward new standard for diagnosis and treatment. The present article is to provide a review of the characteristics of the COVID-19 including the epidemiology, clinical features, pathological changes, diagnosis, treatment, and the experience of prevention and control measures for this disease.
1. Epidemiology ofCOVID-19
Since December 2019, the first 27 case of unidentified pneumonia with a history of exposure in the Huanan Seafood Market were reported by the Wuhan Municipal Health Commission(Ashour, Elkhatib, Rahman, & Elshabrawy, 2020). On Jan 11, 2020, the pathogen of the pneumonia was initially confirmed as a new coronavirus. On January 20, the human-to-human transmission and nosocomial infection were official confirmed firstly(J. F.-W. Chan, Yuan, Kok, To, Chu, Yang, Xing, Liu, Yip, Poon, et al., 2020). And the same day, 4 confirmed cases of 2019-nCoV have been reported from three countries outside of China including Thailand (2 cases), Japan (1 case) and the Republic of Korea (1 case). With the epidemic further expanding, daily confirmed case increased to 3892, and then fluctuated to 2022 on February 11. Because of the improvement of diagnosis standard for confirmed cases, there were 14109 new clinically diagnosed cases were reported on February 12, 2020. Since then, the number of daily emerging cases gradually declined. However, the daily emerged cases were increased rapidly in abroad. Data released on February 25, 2020 showed foreign countries had overtaken China in confirmed cases per day for the first time (Figure.1) . Up to April 13, 2020, there were 83696 confirmed cases in China, and 1837079 cases were totally confirmed in 209 countries outside of China (Figure. 2, 3 ). From November 2019 to April 13, 2020, the number of cumulative deaths caused by the COVID-19 was 119138, and the overall case-fatality rate (CFR) was 6.20 % which was lower than that of the SARS (9.60%) and MERS (34.4%) (Figure.4, Table.1).
The transmission of infectious diseases must rely on three requirement conditions: sources of infection, routes of transmission, and susceptible hosts(Evans, 2013; Keeling & Rohani, 2011). A growing body of scientific evidence suggests that COVID-19 is a zoonotic disease as with SARS and MERS, and originated from wild bat(Evans, 2013). And pangolins and snakes were likely to be intermediate hosts of SARS-CoV-2. According to the Diagnosis and Treatment Scheme for Novel Corona Virus Pneumonia (Trial) 7th Edition, close contact with symptomatic cases and asymptomatic cases with silent infection are the main transmission routes of 2019-nCoV infection. It suggested that SARS-CoV-2 can be transmitted through respiratory aspirates, droplets, contacts, and digestive tract transmission remained to be confirmed(Peng et al., 2020). Vertical transmission was sporadically reported in some media but not yet proved(Aldohyan et al., 2019). Reports showed that the basic reproductive values (R0) of COVID-19 were calculated between 2 and 3.5, which means that one patient could transmit the disease to two to three other people. Therefore, SARS-CoV-2 appears to be more infectious than SARS-CoV or MERS-CoV based on R0 values at the early stage of this outbreak(Ying, A, Annelies, & Joacim, 2020)(Table.1 ). Similar to SARS and MERS, nosocomial transmission was a severe problem or even worse. COVID-19 had posed a difficult challenge to healthcare facilities from both the impact of healthcare-associated transmission and the resource burden of controlling and preventing further spread. It has been reported that a total of 3019 health workers were infected, accounting for 4.17% of total cases. Unfortunately, 14.8% of confirmed cases were classified as severe or critical and 5 deaths were observed. In terms of susceptible populations, people are generally susceptible to COVID-19 regardless of age or gender(Surveillances, 2020). 86.6 % of all patients were aged from 30 to79, and the median age of the patients was 47 years. The elderly and those with underlying chronic diseases are more likely to become severe cases(Shen et al., 2020).
2.The etiology of COVID-19
Coronavirus is comprised of single-stranded positive RNA virus that belongs to an order Nidovirales, family Coronaviridae, and subfamily Orthocoronavirinae (Jie, Fang, & Zheng-Li, 2019). Coronavirus can be divided to four genera: α-, β-, γ-, δ-coronavirus according to the characteristics of serotype and genome(P, Xin, P, & Y, 2019). Genome sequences analysis showed that the coronavirus is a new type of coronavirus (SARS-CoV-2) and belongs to beta-CoV strain(Li-Li et al., 2020; Ren et al., 2020). A recent study demonstrated that SARS-CoV-2 can survive in human respiratory epithelial cells for 96 hours in vitro(Huang et al., 2020). Current studies have revealed that SARS-CoV-2 shared the same receptors with SARS-CoV and MERS-CoV for invading the host cells(Huang et al., 2020). And the spike (S) protein serves as the main antigenic proteins for binding to angiotensin-converting enzyme 2 (ACE2) receptor and mediates subsequent fusion between the envelope and host cell membranes to aid viral entry into the host cell(X. Xu et al., 2020). In fact, SARS-CoV-2 also shared the same physical and chemical characteristics with SARS-CoV and MERS-CoV. Coronaviruses are sensitive to ultraviolet ray and heat. And it can be killed easily by exposed to 56 ℃ for 30 mins, 75% ethanol, chlorine disinfectant, peracetic acid and chloroform(Duan et al., 2003).
3. Clinical characteristics ofCOVID-19