(WGN Radio) – Researchers who conducted a study of more than 55,000 people say they’ve determined the order COVID-19 symptoms are most likely to show up. The University of Southern California study found fever was the most common first symptom of the coronavirus. A fever was likely to be followed by cough and muscle pain.

Then, those symptoms may be followed by nausea or vomiting. After that, they could be followed diarrhea.

That’s a difference from other respiratory illness, like SARS (severe acute respiratory syndrome) or MERS (Middle East respiratory syndrome), the USC researchers pointed out. “The upper GI tract (i.e., nausea/vomiting) seems to be affected before the lower GI tract (i.e., diarrhea) in COVID-19, which is the opposite from MERS and SARS.”

“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,” said study lead Joseph Larsen.

The study looked at the symptoms of more than 55,000 people in China with a confirmed COVID-19 case in February 2022. The data was collected by the World Health Organization.

The researchers compared the symptoms of those 55,000 people with COVID to symptoms from 2,470 influenza cases across several countries between 1994 and 1998.

“We want to look at what symptoms came when because as a physician it starts to give you what we call a differential diagnosis. It allows us to eliminate some thing and say if this illness happened to you, you wouldn’t have that first,” said Dr. Kevin Most, chief medical officer at Northwestern Medicine’s Central DuPage Hospital, in an interview with WGN Radio.

While the study’s findings are important to the scientific field, they are less useful to the general public and medical field, Dr. Most said. That’s because the first symptom for many viruses – including monkeypox and the flu is fever – so testing is still the best way to determine what a patient is sickened with.

Dr. Most also noted the order and prevalence of symptoms changes as the virus mutates. With the BA.4 and BA.5 subvariants of omicron – currently dominant in the U.S. – symptoms tend to stay in the upper respiratory system, causing for many people a sickness that feels like a bad cold.

You can listen to the full interview with Dr. Kevin Most above.