Is it Flu or Covid-19? Fever or Cough First Could Tell the Difference
A new study finds an easy way to differentiate between similar respiratory viral diseases.
The early symptoms of Covid-19 are fever, cough, sore throat, headache, shortness of breath, nausea, diarrhea, and loss of taste or smell. Among these, cough and fever are the most common. But cough and fever are also the two top early signs of influenza, SARS, and MERS.
So if one has a cough or fever, it could be either the flu, SARS, MERS, or Covid-19. The question now is: How can we tell these similar respiratory viral diseases apart based on early symptoms only?
A paper published last week in the Frontiers of Public Health, “Modeling the Onset of Symptoms of COVID-19,” sought to find out. In this study led by Peter Kuhn, a professor of biological sciences, medicine, biomedical engineering, aerospace, and mechanical engineering at the University of Southern California, researchers calculate what is the most likely sequence of events (clinical symptoms) that best define influenza, SARS, MERS, and Covid-19.
Typical disease course of Covid-19
First, the researchers determine the most and least likely symptomatic paths for how Covid-19 progresses. Datasets used were 55,924 confirmed cases of Covid-19, followed by validation with a smaller dataset of 1,099 cases. Results unveiled that:
- A person with Covid-19 is most likely to face symptoms in the order of fever → cough → nausea/vomiting → diarrhea (see red arrows).
- The least likely symptom trajectory of Covid-19 starts in the order of diarrhea and nausea/vomiting → cough → fever (see blue arrows).
- These two paths are the same for both severe and non-severe Covid-19.
“The likelihoods of transitioning to fever, [76.9%], and then to cough, [95.8%], are high, and these observations indicate that a large portion of infected symptomatic patients may follow this path,” the authors explained. “These results suggest that severity does not affect the order of discernible symptoms, and they are consistent with the hypothesis of fever as the first symptom of Covid-19.”
Typical disease course of Covid-19 vs. others
Next, the researchers calculate what the disease courses of other respiratory viral infections look like. They repeat what they had done with Covid-19 to the flu, SARS, and MERS.
Results showed that all coronaviruses often begin with a fever and then cough. In contrast, influenza starts with a cough, followed by a fever. “The most likely order of occurrence of symptoms in Covid-19 is fever, cough, nausea/vomiting, and diarrhea,” the authors wrote. “This path is identical to influenza except for the order of the initial two symptoms is switched.”
This further reinforces the point that fever is central to coronaviruses and cough to influenza.
Factoring in three other subjective symptoms — sore throat, headache, and myalgia (muscle ache and pain) —into the model did not alter the first two symptoms for SARS, MERS, and Covid-19. In other words, fever is still the first presenting symptom in the majority of coronavirus cases, followed by a cough. For influenza, however, the most likely disease course changed considerably with cough first and fever as the fourth symptom (see bulleted points below).
This further reinforces the point that fever is central to coronaviruses and cough to influenza. “To slow the spread of Covid-19, our results support the practice that fever should be tested before allowing entry to facilities and that those with fever should immediately seek medical attention for diagnosis and contact tracing,” the study authors concluded. “We are not proposing initial symptom as a diagnostic test, but instead as a possible sign to get tested.”
In sum, the most likely seven-symptom disease courses for each respiratory viral diseases are as follows:
- Covid-19: Fever → cough → sore throat, headache, and myalgia → nausea/vomiting → diarrhea.
- SARS: Fever → cough and myalgia → headache → diarrhea, sore throat, and nausea/vomiting.
- MERS: Fever → cough → myalgia → headache and diarrhea → sore throat and nausea/vomiting.
- Influenza: Cough and myalgia → headache → sore throat → fever → diarrhea and nausea/vomiting.
And upon removing the subjective symptoms of sore throat, myalgia, and headache, the more accurate four-symptom disease courses are (as depicted by red arrows in the figure below):
- Covid-19: Fever → cough → nausea/vomiting → diarrhea.
- SARS: Fever → cough → diarrhea → nausea/vomiting.
- MERS: Fever → cough → nausea/vomiting → diarrhea.
- Influenza: Cough → fever → nausea/vomiting → diarrhea.