There are many reasons why coronavirus is difficult to control, but the fact it mimics the symptoms of other common viruses, including the flu, presents a major hurdle.
But new evidence suggests there is a certain order in which COVID-19 symptoms appear and researchers hope their findings will enable earlier testing.
The paper, published in the Frontiers in Public Health journal, by researchers from the University of South California (USC), is based on the symptoms of more than 55,000 confirmed cases of COVID-19.
The researchers found the initial symptoms often appear in the following specific order:
1.Fever2.Cough3.Nausea and/or vomiting4.Diarrhoea
This order differs only slightly from other respiratory illnesses, but there’s a critical difference that might help with the detection of COVID-19.
The researchers hypothesise that if you have contracted influenza (the flu), the first symptom you will observe is a cough, not fever.
The ability to discern, early in the illness, between the flu and coronavirus cannot be overstated as COVID-19 is two to three times more contagious than influenza.
“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 researcher Peter Kuhn, professor of medicine and biomedical engineering at USC.
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There is also a subtle difference in the timing of COVID-19 symptoms when compared to Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS).
The upper gastrointestinal (GI) tract seems to be affected before the lower GI tract in COVID-19 cases which is the opposite to MERS and SARS.
This is why nausea/vomiting often presents before diarrhoea among some COVID-19 patients.
What if I experience other symptoms?
The researchers predicted the order by analysing the presentations of 55,924 confirmed cases of COVID-19 in China.
This data was collected between February 16 and 24 by the World Health Organization.
They also studied a further 1,099 COVID-19 patients from China who were ranked on the severity of their symptoms on admission.
The researchers concluded that the order of symptoms was no different when comparing mild and serious cases.
They focused their study on four objective symptoms that are easy for patients to confirm (and common in other respiratory diseases) but also went on to analyse more subjective symptoms.
This was the most likely order of onset when additional (more biased) symptoms were factored in:
1.Fever2.Coughing3.Equal likelihood of sore throat, headache or muscle aches4.Nausea/vomiting5.Diarrhoea
Whereas this was the likely order of symptoms for influenza:
1.Cough or muscle aches2.Headache3.Sore throat4.Fever5.Equal likelihood of vomiting/nausea and diarrhoeaHealth in your Instagram feed
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Sacha Stelzer-Braid, a senior post-doctoral scientist and virologist from the University of NSW, said the findings were consistent with previous research on symptom onset.
“The order of symptoms that the authors have reported makes sense in terms of the body’s immune response,” she said.
“Fever occurs first because that is your body’s immune response to the virus and the virus probably infects your nasal passages first and then moves to the lungs (hence the cough).
“The virus may then travel to other cells in the body that have the right receptor, including cells in the gastrointestinal system, hence the gastro symptoms including diarrhoea.”
However Dr Stelzer-Braid urged some caution in relying on this study as the data is based on hospitalised patients, meaning those who were sick enough to present.
There is an incredibly wide range of severity in COVID-19 infections and people should not assume they will feel all these symptoms, if any, she said.
The research also provides no insight into those people who never show any symptoms.
The proportion of true asymptomatic cases is still to be determined, but several studies have estimated about 40 per cent of people never develop any symptoms.
Nevertheless, in concluding that fever is often the first symptom, the research validates temperature screening in public places as a useful early checking method, Dr Stelzer-Braid said.
“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 researchers said.