The loss of smell that some people with COVID-19 experience is much different from the loss of smell that sometimes accompanies a bad cold or flu, according to a small study published online Tuesday in the journal Rhinology.

When people have COVID-19, the loss of smell tends to be sudden and severe, the study found. And, unlike with the cold or flu, the symptom does not usually occur in tandem with a stuffy or runny nose. Most COVID-19 patients who lose their sense of smell continue to breathe freely through their nose.

Perhaps even more significantly, COVID-19 patients also experience a “true” loss of taste — specifically, an inability to identify bitter or sweet tastes.

These findings suggest “that there are altogether different things going on when it comes to smell and taste loss for [COVID-19] patients and people with a regular cold or flu,” says Dr. Carl Philpott, the study’s lead author and an academic surgeon at the University of East Anglia, in a released statement.

“This is very exciting because it means that smell and taste tests could be used to discriminate between COVID-19 patients and people with a regular cold or flu,” he adds. “Although such tests could not replace formal diagnostic tools such as throat swabs, they could provide an alternative when conventional tests are not available or when rapid screening is needed — particularly at the level of primary care, in emergency departments or at airports.”

Study details

The study appears to be the first to directly compare smell and taste symptoms in people with COVID-19 with those in people with upper respiratory illnesses. The inability to smell — amnosia — has become recognized as a common symptom of COVID-19 and may be experienced by as many as half of all patients who become infected with the virus.

Of course, loss of smell can also be a symptom of a bad cold or the seasonal flu. “We wanted to find out exactly what differentiates COVID-19 smell loss with the kind of smell loss you might have with a cold and blocked-up nose,” says Philpott.

For the study, Philpott and his colleagues used standardized tests to assess the smell and taste abilities of 10 COVID-19 patients (about two weeks after the onset of the infection). They then compared those results with similar assessments that had been made of 10 people with a bad cold and 10 healthy people (the “controls”).

“We found that smell loss was much more profound in the COVID-19 patients,” says Philpott. “They were less able to identify smells, and they were not able to identify bitter or sweet tastes. In fact it was this loss of true taste which seemed to be present in the COVID-19 patients compared to those with a cold.”

The patients were followed for an average of 18 days. All reported some improvement in their smell and taste by the end of that period, although only a third of the COVID-19 patients reported a complete recovery. Philpott and his colleagues write in their paper that they expect these symptoms are likely to persist in some COVID-19 patients long after they test negative for the virus. Other research has suggested that about one in 10 patients experience a long-term loss of smell or taste.

Attacks on the central nervous system

As the researchers point out, COVID-19 is believed to affect the brain and nervous system, based on some of the neurological symptoms that some patients develop.

The findings from the current smell-and-taste study seem to support that point. The olfactory (smell) receptor cells, which are located on tissue high in the nose, are connected directly to the brain.

“Our results reflect, at least to some extent, a specific involvement at the level of central nervous system in some COVID-19 patients,” says Philpott. “It is particularly interesting that COVID-19 seems to particularly affect sweet and bitter taste receptors, because they are known to play an important role in innate immunity.”

“More research is needed to see whether genetic variation in people’s bitter and sweet taste receptors might predispose them to COVID-19,” he adds, “or conversely, whether COVID-19 infection changes how these receptors function, either directly or through a cytokine storm — the over-reaction of the body’s immune system.”

A sudden loss of smell or taste is listed as a symptom of COVID-19 on the Centers for Disease Control and Prevention’s and the World Health Organization’s website. It can be one of the first symptoms of the infection. If you experience it, call your doctor and ask if you should get tested for COVID-19.

FMI: You can read the study in full on Rhinology’s website.