The Science Behind Smell Testing And Why Experts Say It Is The MOST Effective COVID-19 Screening Factor​

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“Many times, loss of smell is a COVID-19 patient’s only symptom. Any symptom that can be tied directly to the disease becomes an important one to be aware of so that it can be used to guide testing and keep people from unknowingly spreading the disease.”

– Prof James Schwob, developmental, molecular, and chemical biology, Tufts University School of Medicine ᶦ

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It’s The Most Reliable Indicator Of COVID-19 Infection​

Unlike symptoms such as aching, coughing, and fatigue, a smell identification test is not self-reported data that the user can easily influence. According to research, often, the dysfunction is the only COVID-19 symptom that people register. A study published October 2020 by the NIH found that self-reported changes in smell were a better marker of the spread of infection than were other indicators tracked by governments.​ᶦᶦ

It’s The Most Common Indicator Of COVID-19 Infection​​

We have long known that people can lose their sense of smell after other viral infections, such as the flu, but the percentage of people who have had this problem with COVID-19 is quite remarkable. Institute for Research in Fundamental Sciences administered a smell-identification test to 100 people with COVID-19 in which the patients sniffed odors and identified them on a multiple-choice basis. 96% of the participants had some olfactory dysfunction, and 18% had total smell loss (otherwise known as anosmia). Smell loss is so common in people with the disease that some researchers have recommended its use as a diagnostic test because it may be a more reliable marker than fever or other symptoms.ᶦᵛ

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It’s The Earliest Indicator Of COVID-19 Infection ​

Early in the COVID-19 pandemic, it emerged that many people infected with the SARS-CoV-2 virus (COVID-19) were losing their sense of smell — even without displaying other symptoms.ᶦᶦᶦ

Screening Body Temp Or Sense Of Smell, Which Is Best?​

It’s not even close. Experts from the Mayo Clinic, Harvard Medical School, NIH, CDC have all stated that testing for sense of smell is far more effective at identifying COVID-19 infections than temperature checks. In a recent study, Andrew Badley, physician and COVID research specialist at the Mayo Clinic, found that Covid-19 patients were 27 times more likely than others to have lost their sense of smell. But they were only 2.6 times more likely to have fever or chills, suggesting that anosmia produces a more precise signal and may therefore be a better Covid-catching net than fever. Anosmia is quite specific to Covid-19. Fever, in contrast, has many possible causes. Temperature checks are consequently far less reliable and far less effective.ᵛ Studies show that only about half of people with COVID-19 report loss of smell when simply asked about their symptoms. But when given a range of scents chosen to catch loss of smell, that number rises to eight in ten, even among people with no other symptoms. That’s far more prevalent than fever, which impacts fewer than one in four people with the virus. Plus, loss of smell lasts longer, affecting patients for a week or more, while fever may only last a day or two. "Fever Checks Are A Flawed Way To Flag Covid-19 Cases. Experts Say Smell Tests Might Help" . Stat News. 2 July 2020

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Could Anything Else Cause Someone To Lose Their Sense Of Smell?​

Allergies, colds, flu, and other viruses rarely, if ever, cause people to lose their sense of smell. When they do, it is accompanied by a runny nose, sneezing, and nasal congestion. It is rare for someone with COVID-19 to have any of those symptoms, let alone two or more. In a recent study of 1,480 patients led by otolaryngologist Carol Yan of UC San Diego Health, someone who lost their sense of smell was “more than 10 times more likely to have Covid-19 than other causes of infection. Nasal inflammation from some 200 cold, flu, and other viruses can cause it, but those causes come along with runny nose, sneezing, and watery eyes, which are exceptionally rare with COVID-19.ᵛᶦ Loss of smell without a stuffy nose is highly specific to COVID-19, possibly because the virus tends to enter the body and replicate via ACE2 receptors, which are highly abundant in cells in nasal passages believed to influence sense of smell.ᵛᶦᶦ

Could anything else cause someone to lose their sense of smell?

Allergies, colds, flu, and other viruses rarely, if ever, cause people to lose their sense of smell. When they do, it is accompanied by a runny nose, sneezing, and nasal congestion. It is rare for someone with COVID-19 to have any of those symptoms, let alone two or more. In a recent study of 1,480 patients led by otolaryngologist Carol Yan of UC San Diego Health, someone who lost their sense of smell was “more than 10 times more likely to have Covid-19 than other causes of infection. Nasal inflammation from some 200 cold, flu, and other viruses can cause it, but those causes come along with runny nose, sneezing, and watery eyes, which are exceptionally rare with COVID-19.ᵛᶦ Loss of smell without a stuffy nose is highly specific to COVID-19, possibly because the virus tends to enter the body and replicate via ACE2 receptors, which are highly abundant in cells in nasal passages believed to influence sense of smell.ᵛᶦᶦ
SYMPTOMSALLERGIESCOLDFLUCOVID-19
LOSS OF SMELL Mild Rarely Rarely √
BODY ACHES Rarely √ √ √
CHILLS No No √ √
FEVER No No √ √
RUNNY NOSE √ √ Sometimes Rarely
SNEEZING √ √ Sometimes Rarely
NASAL CONGESTION √ √ Sometimes Rarely
SHORTNESS OF BREATH Sometimes Sometimes Sometimes √
HEADACHE Sometimes Sometimes √ Sometimes
WHEEZING Sometimes Sometimes Sometimes Sometimes
ITCHY/WATERY EYES √ No No No
DRY COUGH Sometimes √ √ √
SORE THROAT Sometimes √ √ Sometimes
NAUSEA, VOMITING, DIARRHEA No Sometimes Sometimes Sometimes

Other Considerations

What About The Rest?​

For others, the symptoms are more serious. Some people whose senses do not return right away improve slowly over a long period — and this can have consequences. Although the condition is not as well studied as the loss of other senses such as vision and hearing, researchers know that the consequences can be severe. One effect is that it leaves people vulnerable to dangers such as food poisoning and fire. A 2014 study found that people with anosmia were more than twice as likely to experience a hazardous event, such as eating spoiled food, as people without smell loss. We encourage all users who fail the sense of smell test and have a positive COVID-19 test to ask their doctor about possible treatments.ᵛᶦᶦᶦ

Should you test sense of taste?

Although scientists have some understanding of the mechanisms involved in smell, they have little idea about how the coronavirus affects taste and chemesthesis. “Nobody has a good handle on that yet that I know of,” says John Hayes, a food scientist at Pennsylvania State University in University Park, who is studying COVID-19’s effects on chemical senses. Taste and chemesthesis are distinct from smell, even though all three combine to tell humans what ‘flavor’ a food or beverage has. Taste relies mainly on taste receptors on the tongue, whereas chemesthesis relies on ion channels on sensory nerves, among other mechanisms — and their response to COVID-19 has not been studied much.ᶦˣ

How Quickly Does Sense Of Smell Return?​​

Most people recover within weeks. In a study published July 2020, 72% of people with COVID-19 who had olfactory dysfunction reported that they regained their sense of smell after a month.

Why do people with COVID-19 lose their sensitivity to smells?

Although the mechanisms are not fully understood, there is an emerging consensus that smell loss occurs when the coronavirus infects cells that support neurons in the nose. Odors are now warped: unpleasant scents have taken the place of typically delightful ones. A team led by Harvard Medical School has found that cells that support sensory neurons in the nose, known as sustentacular cells, are most likely what the virus is infecting.

References

  1. “Something Wrong With Your Sniffer? It Could Be The Coronavirus”. Tufts Now. 11 December 2020.
  2. “Smell And Taste Changes Are Early Indicators Of The Covid-19 Pandemic And Political Decision Effectiveness”. NIH. 14 October 2020.
  3. “Covid’s Toll On Smell And Taste: What Scientists Do And Don’t Know”. Nature News. 14 January 2021.
  4. “Mysteries Of Covid Smell Loss Finally Yield Some Answers”. Scientific American. 18 November 2020.
  5. “Fever Checks Are A Flawed Way To Flag Covid-19 Cases. Experts Say Smell Tests Might Help” . Stat News. 2 July 2020.
  6. “Fever Checks Are A Flawed Way To Flag Covid-19 Cases. Experts Say Smell Tests Might Help” . Stat News. 2 July 2020.
  7. “How A Simple Smell Test Could Curb Covid-19 And Help Reopen The Economy”. Cu Boulder Today, 9 December 2020.
  8. “How Covid-19 Causes Loss Of Smell”. Harvard Medical School. 24 July 2020.
  9. “Covid’s Toll On Smell And Taste: What Scientists Do And Don’t Know”. Nature News. 14 January 2021.