Propagation sources of SARS-CoV-2 compared to SARS-CoV
SARS-CoV-2 and SARS-CoV belong to the virus group β-coronavirus. SARS-CoV-2 has 78% nucleotide homology with human SARS virus, about 50% homology with MERS virus [12, 13]and more than 85% homology with bat-SL-CoVZC45 based on the phylogenetic tree of coronavirus . Studies have shown that bats are natural hosts of more than 100 kinds highly pathogenic viruses, among which SARS-CoV is hosted in Chinese chrysanthemum-headed bat . Maybe, it spreads from bat to humans due to evolutionary mutation  by intermediate hosts like pangolin . The evidence about the transmission source came from original cluster confirmed cases with contact history to Seafood Market of Wuhan South China [18-21]. While, transmission pathway includes initial zoonotic (animal-to-human) transmission [22, 23] to secondary human-to-human transmission supported by increasing exportation cases of Wuhan across China fueled by human migration [24, 25]. Moreover, patients with asymptomatic infection and incubation infection maybe were potential important risk factors resulting in rapid spread . Other study found that convalescent patients maybe also were contagious detected with SARS-CoV-2 . We need further evidence on infectivity of latent and convalescent patients. In contrast, SARS might be an animal-borne infectious disease due to contact history with wild animal for the first case of SARS  and bats as a food ingredient for business transaction in Guangdong [26, 27]. Additionally, SARS-CoV emerged with high human-to-human transmission capability, characterized by family and medical staff infections . Moreover, evidence about being capability of multiple and continuous human-to-human transmission of SARS-CoV came from one original patient resulting in at least four generations of cases. 
Certainly, both exist initial zoonotic transmission to secondary human-to-human transmission. There was evidence of SARS that a few cases highly had the ability of over-transmission with high contagiousness called "first" cases. However, there is lack of evidence about "first" case in outbreak of COVID-19, which suggests that 2019-nCoV is too insidious and fierce to difficultly track the chain of transmission. Additionally, there are two risk factors in COVID-19 for asymptomatic infection and incubation infection accelerating its rapid transmission (Table 1). Thus, the fight of prevention and control for COVID-19 should be tougher than SARS due to unclear propagation chain.
Transmission route of COVID-19 compared to SARS
Transmission routes of COVID-19 predominantly have respiratory droplets, close contact and even air transmission by aerosols . Recently, SARS-CoV-2 was detected in the feces of confirmed cases in Wuhan, Shenzhen and the United States, indicating possibility of fecal-oral transmission [30, 31] due to virus replication and existence in digestive tract. Moreover, mother-to-child vertical transmission was supported by reports showed that cases presented positive nucleic acid in pharyngeal swabs 30 hours after birth . At present, no evidence of virus mutation has been found . Amounts of clinical isolates unrelated to time and geography are needed to assess the extent of viral mutations and whether these mutations indicate adaptability to human hosts . People are generally susceptible to COVID-19 especially those who are low immune function, and children are no exception. In contrast, the route transmission of SARS includes respiratory transmission by close droplets and aerosols, and contact transmission through contaminated hands or even objects [35, 36]. SARS leads to outbreaks in hospitals and families due to close contact in a confined space [35, 37]. While, nosocomial infection is prominent. Additionally, possibility of digestive tract transmission was found by indirect evidence that coronavirus RNA could be detected in the feces of patients with SARS . Different nationalities are generally susceptible. In a word, transmission routes of both include droplets, close contact and even air transmission by aerosols, but mother-to-child vertical transmission was merely supported in COVID-19. Thus, COVID-19 almost covers all kinds of transmission routes, which has higher probability of propagation than SARS (Table 1).
Dynamics of COVID-19 compared to SARS
SARS-CoV-2 were highly contagious than SARS-CoV supported by estimated data of basic reproduction number (R0) ranging from 2 to 6.5[39-42]. Julien Riou et al found that R0 was around 2.2 (1.4-3.8) indicating the potential for sustained human-to-human transmission . Additionally, there were growing evidence of human-to-human transmission to support higher contagiousness than SARS-CoV and MERS-CoV [20, 44-47]. Generally, incubation period of COVID-19 is 1 -14 days . At the early epidemic stage, mean incubation period was 5.2 days . Other study showed that median incubation period was 4.75 days based on 4 021 confirmed patients survey . Contrastingly, there is limited literature to estimate dynamics of SARS, but one study showed that R0 was estimated from 2 to 4. Although these evidences are based on different sample sizes, COVID-19 obviously has higher probability of propagation than SARS (Table 1).
Incidence and fatality rate of COVID-19 compared to SARS
Based on available data, fatality rate of COVID-19 in China was lower than SARS about 9.6% (774/8 098) . Incidence line of COVID-19 was dynamic by time in the research with 72 314 cases. Initially, new cases remained low and sporadic until 1st January; there was an abrupt jump followed by an exponential growth until 23 January, and reached first incidence peak during 24th and 28th January, and then declined slowly . During this period, an abnormal high value of one-day onset occurred on 1st February. Since 3rd February, the number of new cases has dropped for 15 consecutive days in China except Hubei Province . Until 12nd February, 15 152 new confirmed cases were reported nationwide (Figure 2). Among them, 97.9% came from Hubei province . For this incidence peak, the reason maybe was improvement of diagnosis standards, because Pneumonia Diagnosis and Treatment Program of Novel Coronavirus Infection (trial Fifth Edition) was issued adding clinical diagnosis to the case diagnosis classification in Hubei Province. Therefore, the number of clinical diagnosed cases was contributed into confirmed cases . Crude fatality rate is the highest of 14.8% at over 80 years old group with higher fatality rate of males than females (2.8 vs. 1.7%), followed by middle-aged group. Majority of them died due to complications. The infection risk possibly increased associated with the elder with underlying diseases such as asthma, diabetes and heart disease. Among children, the age range were from 30 hours to 17 years old . Medical staff and family members of patients were at high risk of infection due to close contact [56, 57]. Supporting Data showed that the proportion of medical staff was 2.09% among 1 099 COVID-19 cases from 552 hospitals in 31 provinces .
In contrast, more than 95% cases among 8 273 were Chinese with fatality rate of nearly 10% among 775 deaths that clustered in those over 40 years old or with underlying diseases . SARS stroked in winter and spring, and morbidity peak occurred in early February in Guangzhou and late April in other cities . Based on the official issued data , new cases reached at the peak on 29th April 2003, then consecutively decreased till to zero new case on 1st June 2003 (Figure 2). In summary, incidence of COVID-19 is significantly higher than that of SARS, and while fatality rate across age groups of COVID-19 is lower that SARS based on cut-off study date. However, at over 80 years old group the fatality rate is the highest of 14.8%. Thus, COVID-19 is characterized by higher incidence and moderate fatality rate than SARS.
Gender and age distribution of COVID-19 compared to SARS
Cases of COVID-19 mainly were males in the middle and elder ages. Among the first 425 COVID-19 patients, 56% were male with median age of 59 years old . Another study on 2nd January showed that 73% (30/41) cases confirmed by laboratory also were men with median age of 49 (IQR: 41.0-58.0) years old . Yang Yang et al study across China on 26th January  suggested that people at all ages were generally susceptible, and those who were aged 30 to 65 years old accounted for 71.45% and children (<10 years) for 0.35%. While, incidence of male was higher than female (0.31 vs. 0.27 per 100 000, P<0.001) among 4 021 confirmed patients . A study of 72 314 cases  further confirmed that population is generally susceptible. 86.6% and 31.2%were those aged 30-79 and over 60 years old, separately. The ratio of diagnosed male to female was 1.06.
In contrast, medical workers and retirees are highly at risk. While, proportion of medical staff reached 24.5% without gender difference . Hospitals and families became the predominantly areas . SARS occurred in any age groups focused on young and middle-aged group accounting for 80%. However, elderly death cases accounted for 40% at the early stage of SARS, and fatality rate of the elder was significantly higher than other groups . Severities of SARS were proportional to the amount and time of detoxification . Those who had close contacts with patients were at the higher risk. Therefore，the attacked population of COVID-19 focused on males in the middle and elder ages with those who had underlying diseases; while, SARS mainly attacked young and middle-aged group without significant sex difference. Both entirely put retirees and healthcare providers at the highest risk (Table 1).
Region distribution of COVID-19 compared to SARS
A total of 74 185 confirmed cases and 2 004 deaths of COVID-19 have been reported with a fatality rate of about 2.70%  in China on 18th February 2020. Majority of them were imported cases from Wuhan, and others were related to Wuhan . Wuhan is one of the major transportation hubs in China with a population of more than 11 million. The spread of 2019-nCoV was accelerated due to population movement during the Spring Festival transportation. Promptly, COVID-19 disseminated across China. There was a positive correlation between numbers of confirmed cases and population mobility in Wuhan . On January 15, 2020, CCDC confirmed the first known exported case of 2019-nCoV in the US state of Washington, who had recently returned from Wuhan City . While, COVID-19 rapidly spread over the world. A total of 804 confirmed cases and 3 deaths have been reported in 25 countries except China with a case fatality rate of about 0.37% . Contrastingly, SARS firstly was reported in 2002 in Shenzhen Guangdong Province. It last for almost eight months, and spread from Guangdong into 29 countries and regions around the world . Summarily, Wuhan is the starting place of CoVID-19, and for SARS is Shenzhen. Both of two cities had history of contact with wild animals (Table 1).
Clinical characteristics of COVID-19 compared to SARS
Common clinical symptoms of COVID-19 were fever, fatigue and dry cough, and majority of severe cases developed into dyspnea or hypoxemia after one week and further rapid progression of acute respiratory distress syndrome, septic shock, and refractory metabolic . Generally, symptoms of children are relatively mild. One study showed that severe and mild pneumonia accounted for 25.5% and 69.9%, respectively. However, others suggested that majority of cases were mild to moderate reaching 80.9% . There are partially different symptoms and chest imaging between severe/critical and mild patients. Moderate and low fever or even no obvious fever occurred among severe/critical patients; low fever and mild fatigue without pneumonia among mild cases. In chest imaging, there were multiple small plaques and interstitial changes in the extra pulmonary zone at the early stage, and then multiple ground glass and infiltration shadow in both lungs for mild cases; In severe cases, lung consolidation is possible but pleural effusion were rare. Contrastingly, SARS outbreak characterized by atypical pneumonia with symptoms of fever, cough, headache, muscle pain, and respiratory infection . Therefore, fever mainly is the starting symptom of both, and while severe and mild pneumonia is the predominant symptom with changes in lung imaging. However, symptoms of SARS was obviously severe than COVID-19 resulting in difficult diagnosis of COVID-19 (Table 1).
Diagnosis criteria of COVID-19 compared to SARS
Based on Diagnosis and treatment protocol of novel coronavirus pneumonia (Trial Version 6) , diagnosis criteria included suspected case and confirmed case. Criteria of suspected cases are those who have 1) COVID-19 epidemiological history, clinical manifestations with moderate or low fever and respiratory symptoms; 2) pulmonary imaging changes with small plaques shadow and interstitial changes or ground glass density shadows and infiltration shadow; 3) and blood routine changes with the total number of leukocytes in the early stage normal or decreased and lymphoid cells decreased. Meanwhile, diagnosis of confirmed case included that 1) detection of 2019-nCoV RNA was positive by photoluminescence RT-PCR technique of breath traces; or 2) blood samples and the gene sequence of detected virus in repertory tract or blood sample was highly homologous with the known gene of 2019-nCoV based on the diagnosis of suspected case. Totally, there are six versions of diagnosis and treatment protocol of NCP issued by National Health Commission of the People’s Republic of China (NHC, PRC) as of 19th February 2020. Among them, diagnosis criteria of the 5th edition was different between Hubei province and other provinces in China, which added clinical diagnosis criteria. Fluorescence PCR technology has become the preferred screening method; however, the sensitivity of the existing novel coronavirus fluorescence PCR detection kit was low, and the detection rate was only 30-40% .
In contrast, based on Diagnosis and treatment protocol of Severe Acute Respiratory Syndrome(SARS) (2004 Edition), clinical diagnosis of SARS included those who 1) had SARS epidemiological features, corresponding clinical manifestations with fever exceeding 38℃; 2) respiratory symptoms and pulmonary X-ray changes with ground glass density shadows and lung consolidation shadows; and 3) ruled out the diagnosis of other diseases. Laboratory diagnosis included that the secretion was positive for SARS-CoV RNA detection, or the serum (or plasma) SARS-CoV specific antigen N protein was positive, or the serum SARS-CoV antibody was from negative to positive, or the antibody titer was increased more than four times.
Compared to the two diagnosis criteria, both of clinical diagnosis were supported by epidemiological history, clinical manifestations, and pulmonary imaging changes. While, the technique of laboratory diagnosis is significantly developed and advanced in COVID-19. However, early diagnosis rate of COVID-19 keeps low especially among asymptomatic infection patients and suspected cases during incubation period. it is more important to develop rapid and accurate differential diagnosis technique.
Comparison of guidelines between COVID-19 and SARS
Table 2 showed that there are 7 guidelines issued at the global level by WHO from 17th to 31st January [67-73], and while 23 guidelines at the Chinese national level from 20th January to 22nd February [74-95], and 13 ones at the local level from 26th January to 20th February during the period of prevention and control of COVID-19[96-108]. The first document named 2019 temporary guidance document for laboratory testing of suspected cases of new coronavirus infection focused on testing of NCP among suspected cases on 17th January . After three days, another document from WHO paid attention to home care and contact management for 2019-nCoV . At the national level, Chinese government issued two documents in the laws on 20th January, and they focused on health quarantine and management of infectious diseases . After that, guidelines issued involved in prevention and control [74,83,89,94], diagnosis and treatment [75,76,81,84,90,93], laboratory biosafety , public transport , and psychological assistance hotline aspects . Moreover, documents focused on the elderly key population and public areas [78,79,80,86]. In the local level, documents focused on technical guidance [97,98,104,108], measures [96,105,106], command and notice about prevention and control of COVID-19[99,100,101,102,103,107].
Comparison of policies between COVID-19 and SARS
Figure 2 and Table 3 showed the similarities and differences among strategies and policies between COVID-19 and SARS. Similarities between them reflected in 12 aspects, which involved in outbreak notification [109,110], laws and regulations [95,111], Ministry of Education Notice, leader adjustments, outbreak report, disposal of patients` remains , notice about medical staff, public health emergency, and laboratory management [91,112,113]. The first case of COVID-19 found on 16th November was reported on 31st December . While, the first SARS case was reported after four months . In the outbreak of SARS, technical guideline was issued, and the whole preventive and control plan was implemented which acted by multi-sectoral cooperation. Moreover, both were included in the People's Republic of China on the Prevention and Control of Infectious Diseases. Outbreaks are reported one day at a time, but it started after near five months from the first case on 20th April 2003 for SARS and already carried out on 21st January 2020 after two months. Meanwhile, all country entered in the state of public health emergency on 9th May 2003 for SARS and on 20th January 2020 for COVID-19, separately. Furthermore, multi-sectors of cooperation mainly involved in CCDC, Health commission, Ministry of Transport, Ministry of Education, and even People's Liberation Army of China. Faced with outbreaks, our whole country was united fighting COVID-19.