SARS-CoV-2, the causative viral agent of the disease COVID-19, is a coronavirus which bears the transmembrane glycoprotein spikes (S protein) typical of viruses in its clade. These spikes are a prominent target of human immune responses and have been found to be highly immunogenic. The receptor-binding domain (RBD) of the S protein is particularly targeted by neutralising antibodies.
The spikes on SARS-CoV-2 allows the virus to enter host cells through the human receptor angiotensin converting enzyme 2 (ACE2), present in alveolar epithelial cells.
The time between initial viral exposure and symptom onset is known as the incubation period. For COVID-19, the average incubation period has been reported to be between five and six days. However, there is considerable variation in incubation time, with some studies suggesting symptoms can appear as soon as three days post-exposure or as late as thirteen days post-exposure.
Fever, fatigue, dry cough are the main symptoms. Some patients could present nasal congestion, runny nose, diarrhea and other symptoms. In severe cases, dyspnea occurred after one week which could lead to acute respiratory distress syndrome (ARDS), septic shock, refractory metabolic acidosis and coagulation dysfunction were rapidly advanced. Particularly, in the course of severe and critical cases, the fever can be moderate or low, and sometimes it is not even obvious. Some patients who showed only low fever, slight fatigue, and etc. without pneumonia manifestations, are able to recover in one week.