COVID-19 has always been tricky to identify because its symptoms tend to overlap with colds, flu, and even allergies. Even more troubling,' some people have no symptoms at all.

To better understand how COVID-19 appears and progresses, a team of researchers at the University of Southern California attempted to figure out the most common order in which symptoms appear.

Their study, recently published in Frontiers in Public Health (opens in new tab), analyzed the symptom incidence of COVID-19. They used data from more than 55,000 novel coronavirus cases in China collected by the World Health Organization (WHO) and a dataset of about 1,100 cases collected by the Chinese National Health Commission.

Based on their findings, the researchers determined that this was the most likely order in which someone would experience COVID-19 symptoms:

When they further expanded their analysis to include other symptoms, the order was still similar:

The researchers also compared how COVID-19 symptoms progressed in response to influenza and found that influenza was more likely to begin with a cough instead of a fever. They also found that flu patients were more likely to have body aches, headaches, and sore throats before they developed a fever.

And these results "support the idea" that people with fever should be screened for COVID-19 before entering a building. The authors write, "Moreover, our findings suggest that documenting the sequence of symptom onset in COVID-19 and other diseases is good clinical practice."

William Schaffner, M.D., professor of medicine and infectious disease specialist at Vanderbilt University School of Medicine (open in new tab), called the findings "very interesting" but stressed that they are "not universal. First of all, we know that a large number of people do not have fever."

Additionally, there is the problem of recall bias. Says Amesh A. Adarja, M.D., a senior fellow at the Johns Hopkins Center for Health Security (opens in new tab). 'You can't say that because you have symptoms in this order you are infected with COVID-19, or because you have symptoms in a different order you are infected with COVID-19.'

Instead, experts say it is important to pay attention to any unusual symptoms associated with COVID-19, regardless of when they appear. These include chills, shortness of breath, fatigue, loss of taste or smell, runny or stuffy nose, in addition to the list above.

"Don't assume it's not COVID if you first have a cough and then a fever," says Thomas Russo, M.D., professor and chief of infectious diseases at the University at Buffalo (opens in new tab). 'It's an interesting guideline, but symptoms alone should not dictate whether or not a patient should be tested.' He adds that physicians should "lower the threshold" for any respiratory symptoms, regardless of their order.

Nevertheless, this data will be helpful in further understanding COVID-19, as it is caused by a newly discovered coronavirus and still requires extensive research. Says Rajiv Fernando, M.D., an infectious disease specialist in Southampton, N.Y. (opens in new tab). Fernando, M.D., has noticed that his patients tend to follow a similar course to the current study, with fever and cough followed by additional symptoms such as loss of sense of smell and taste.

CONCLUSION: If you see symptoms of COVID-19, ask if you should be tested, but keeping track of when symptoms began to appear and how they progressed may be useful information for your doctor.