Scientists decode how COVID-19 begins to appear in humans

Researchers have found the likely order in which COVID-19 symptoms first appear -- fever, followed by cough, muscle pain, and then nausea, and/or vomiting and diarrhoea.

Knowing the order of the COVID-19 symptoms may help patients seek care promptly or decide sooner than later to self-isolate.

According to the study, published in the journal Frontiers in Public Health, recognising the order of symptoms also could help doctors plan how to treat patients, and perhaps intervene earlier in the disease.

"This order is especially important to know when we have overlapping cycles of illnesses like the flu that coincide with infections of COVID-19," said study author Peter Kuhn from the University of Southern California in the US.

"Given that there are now better approaches to treatments for COVID-19, identifying patients earlier could reduce hospitalisation time," said another study author Joseph Larsen.

Fever and cough are frequently associated with a variety of respiratory illnesses, including Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS).

But the timing and symptoms in the upper and lower gastrointestinal tract set COVID-19 apart.

"The upper GI tract (i.e., nausea/vomiting) seems to be affected before the lower GI tract (i.e., diarrhoea) in COVID-19, which is the opposite from MERS and SARS," the scientists wrote.

The authors predicted the order of symptoms this spring from the rates of symptom incidence of more than 55,000 confirmed coronavirus cases in China, all of which were collected by the World Health Organization.

They also studied a dataset of nearly 1,100 cases collected from December 11, 2019, through January 29, 2020, by the China Medical Treatment Expert Group via the National Health Commission of China.

To compare the order of COVID-19 symptoms to influenza, the researchers examined data from 2,470 cases in North America, Europe and the Southern Hemisphere, which were reported to health authorities from 1994 to 1998.

According to the researchers, the order of the symptoms matter.

"Knowing that each illness progresses differently means that doctors can identify sooner whether someone likely has COVID-19, or another illness, which can help them make better treatment decisions," Larsen noted.