Clinical Basis of COVID-19
COVID-19 is an acute infectious disease caused by the novel coronavirus. It is mainly transmitted by respiratory droplets and can also be transmitted by contact. The clinical symptoms are mainly fever, dry cough, fatigue, and gradual dyspnea. Severe cases may develop into acute respiratory distress syndrome. The World Health Organization (WHO) has officially named the disease COVID-19. This disease is a new infectious and highly contagious disease.
Etiology
2019-nCoV is an RNA virus that is widely found in humans and animals. It belongs to the coronavirus family of the nest virus order and belongs to the p genus of coronavirus. The coronavirus genus has an envelope. The particles are round, oval, or pleomorphic, with a diameter of 60 nm-140 nm. Its genetic characteristics are obviously different from those of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV). Current research has shown that it is most similar to the bat SARS-like coronavirus (bat-SL-CoVZC45) of the Chinese chrysanthemum bat, with nucleotide homology reaching more than 85%. It shares approximately 78% and 50% homology to SARS virus and MERS virus, respectively, which once brought grave disasters to China.
How does the 2019-nCoV work? The spike protein (S protein) on the surface of the virus enters the host cell by interacting with specific receptors on the cell surface. Then it enters the cell through membrane fusion and releases its genome into the cytoplasm. The virus mainly binds to angiotensin-converting enzyme 2 (ACE2) via the S protein on its surface. During fusion, the S protein undergoes structural rearrangement to fuse the viral membrane with the host cell membrane, thereby infecting human respiratory epithelial cells. It has a higher affinity than SARS-CoV and, therefore, is more infectious.
When isolated and cultured in vitro, 2019-nCoV can be found in human respiratory epithelial cells in about 96 hours, while it takes about 6 days to isolate and culture in Vero E6 and Huh-7 cell lines.
Hie physicochemical properties of COVID-19 are mainly understood from studies on SARS-CoV and MERS-CoV. The virus is sensitive to ultraviolet rays and heat and can be effectively inactivated by 56°C for 30 minutes, ethyl ether, 75% ethanol, chlorine-containing disinfectant, peracetic acid, chloroform, and other lipid solvents; however, chlorhexidine cannot kill the virus effectively.
Epidemiology
Source of Infection
It is currently believed that the source of infection is mainly COVID-19 patients, with an incubation period of 1-14 days, mostly 3-7 days. There are very few cases with an incubation period of more than 14 days, but the longest can even reach 24 days. Infected yet asymptomatic patients and patients who do not show obvious clinical symptoms due to weak immune system stress response or their own physical characteristics do carry the virus and can infect others. Because there are no clinical symptoms, asymptomatic infected persons are not easily detected, and even the patients themselves are not aware of the infection, which is difficult to control and get them isolated in time, thus it could easily cause large-scale transmission.
Route of Transmission
Transmission through respiratory droplets and close contact are the main route of transmission. There is a possibility of aerosol transmission in a relatively closed environment when exposed to high concentrations of aerosols for a long period of time. Because 2019-nCoV can be isolated in feces and urine, attention should be paid to the aerosol or contact transmission caused by feces and urine pollution to the environment. The possibilities of other routes of transmission require further research.
Respiratory Droplet Transmission
Respiratory droplet transmission is the main mode of the transmission of 2019-nCoV. The virus is spread through droplets produced when patients cough, sneeze, and talk, and those who are susceptible will be infected after inhalation.
Close Contact Transmission
2019-nCoV can also be transmitted through indirect contact with infected patients. Indirect contact transmission means that people come into contact with the droplets containing the virus through touching the surface of objects, and then touch their mouth, nose, eyes, and other mucous membranes, resulting in infection.
Fecal–Oral Transmission
Fecal-oral transmission occurs when bacteria or viruses found in the stool enter the human respiratory tract and digestive tract and thus infect people. Whether there is a fecal-oral transmission route for 2019-nCoV is to be determined. It is also believed that the virus in feces may be transmitted by aerosol formed by droplets containing the virus, which also requires further investigation.
Aerosol Transmission
Aerosol transmission refers to when the respiratory droplets lose water in the air, and the leftover proteins and pathogens form nuclei or dust that float far away in the form of aerosols, causing long-distance transmissions, and the range of transmission can vary from tens of meters to hundreds of meters.
Mother-to-Child Transmission
At present, a case has been reported in which the mother was a confirmed COVID-19 patient, and the throat swab for viral nucleic acid detection showed positive for the 30-hour-old infant, suggesting that 2019-nCoV may cause neonatal infection through mother-to-child transmission, and there is vertical mother-to-child transmission; however, preliminary evidence suggests that infection in the third trimester of pregnancy does not cause vertical transmission.
Other studies suggest that the urinary system might also be a potential route for COVID-19 infection. It has been proven through scientific experiments that the virus will not spread through skin penetration.
Susceptible Groups
As it is a new infectious disease, the mass population generally has no resistance. In terms of age, the ability to resist the virus is no different for people of all age groups, and everyone is susceptible under suitable conditions. The probability of infection increases among the elderly and people with underlying diseases. Children and pregnant and lying-in women are vulnerable to 2019-nCoV infection.
Mortality Rate
On the whole, COVID-19 spreads faster than SARS, with high risk and low mortality rate, but its mortality rate of severe patients is higher than that of SARS and MERS.
