If you lose your sense of smell, what are the chances you have COVID-19?
A new study released today as a preprint suggests that if you lose your sense of taste or smell, you have a 61.7% chance of having COVID-19.
Preprints are studies destined for peer-reviewed journals that have yet to be peer-reviewed. Because COVID-19 is such a rapidly evolving disease and peer-review takes so long, most of the information circulating about the disease comes from preprints.
The 61.7% figure is a lot higher than the 5.3% chance predicted by having flu-like symptoms, as I reported on April 1. It might be inflated, though, so let's consider how they arrived at it.
On March 24th, Zoe Global Limited and King's College London released the COVID RADAR Symptom Tracker app in the UK. Between March 24th and March 29th, 1,573,103 people signed up.
26% of people reported one or more COVID-19 symptom. From among them:
11% had fever
29% had a persistent cough
28.2% had shortness of breath
53.1% had fatigue
18.2% had a loss of the sense of smell or taste.
1,702 of the app users had gotten tested for COVID-19, had the results back, and had also reported whether they lost their sense of smell or taste. 59.4% of those who tested positive had lost their sense of smell or taste, while 19% of those who tested negative had the same symptom.
Overall, someone who had lost their sense of smell or taste had a 61.7% chance of testing positive.
When the researchers combined all of the symptoms into a prediction model, the loss of taste or smell was about twice as important as a fever, 3.5 times as important as a loss of appetite, five times as important as a cough or diarrhea, and more than six times as important as fatigue.
We should be cautious about putting too much emphasis on the 61.7% likelihood. The 1702 people who got tested were not randomly selected from the app participants. Instead, they got tested for reasons unrelated to the study: they had compelling symptoms, or they were in contact with someone infected, or they traveled to a high-risk area. These 1702 people probably had a significantly higher risk of being infected than the average person using the app. The app-users themselves were not a random sample of the population, and were biased towards younger individuals and females.
Nevertheless, there is no particular reason to think the study was biased towards associating the loss of the sense of smell or taste with COVID-19. They found that, among those who had other COVID-19-related symptoms, having this one symptom increased the odds of a positive test by 3-fold.
Still, no one has every single symptom, and just over 40% of those who tested positive in this study had not experienced any loss of their sense of taste or smell.
We won't know the true predictive value of these symptoms until they are applied to random samples of the population that have not yet been tested. However, to loosely put some numbers on it, I would think of it like this:
If adding this symptom to the more traditional ones triples the risk of a positive test, and if flu-like symptoms in Los Angeles back in mid-March when COVID-19 was far less common there indicated a 5.3% risk, then we might imagine that the lower bound of the predictive value of this symptom is 16%.
We can take the current study, likely biased towards reporting a higher risk, as representing the upper bound at 62%.
So, until harder numbers come in, the loss of sense or taste should be taken to indicate a risk of having COVID-19 somewhere between 16% and 62%.
Stay safe,
Chris
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Disclaimer
I am not a medical doctor and this is not medical advice. I have a PhD in Nutritional Sciences and my expertise is in conducting and interpreting research related to my field. Please consult your physician before doing anything for prevention or treatment of COVID-19, and please seek the help of a physician immediately if you believe you may have COVID-19.
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