Loss of smell is a recognised symptom of COVID-19 and a new study has shown how this differs from the loss of smell you might experience with a bad cold or flu.

The study is the first to compare loss of smell and taste related to COVID with other causes of upper respiratory tract infections.

According to the findings, the main differences are that while people with COVID-19 may lose their sense of smell, they can still breathe freely and they do not tend to have a runny or blocked nose.

Also, people with COVID cannot detect bitter or sweet tastes.

The researchers said that these findings lend weight to the theory that the virus infects the brain and central nervous system.

"The loss of smell and taste is a prominent symptom of COVID-19, however it is also a common symptom of having a bad cold. 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," explained lead researcher, Prof Carl Philpott, of the University of East Anglia in the UK.

The research team, which was made up of a group of European smell disorder experts, carried out smell and taste tests on 10 COVID-19 patients, 10 people with bad colds and a control group of 10 healthy people.

"We wanted to see if their smell and taste test scores could help discriminate between COVID-19 patients and those with a heavy cold. We know that COVID-19 behaves differently to other respiratory viruses, for example by causing the body's immune system to overreact, known as a cytokine storm, and by affecting the nervous system. So we suspected that patterns of smell loss would differ between the two groups," Prof Philpott noted.

The researchers found that the loss of smell was much more profound in those with COVID-19.

"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.

"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," Prof Philpott said.

He pointed out that while such tests could not replace formal diagnostic tools, such as nose and 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".

"This research also shows that there are altogether different things going on when it comes to smell and taste loss for COVID-19 patients, compared to those with a bad cold.

"It has previously been suggested that the COVID-19 virus affects the central nervous system, based on the neurological signs developed by some patients. There are also similarities with SARS, which has also been reported to enter the brain, possibly via smell receptors in the nose," Prof Philpott said.

He noted that these results reflect, to some extent, a specific involvement at the level of central nervous system in some COVID patients.

"It is particularly interesting that COVID-19 seems to particularly affect sweet and bitter taste receptors, because these 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, or conversely, whether COVID-19 infection changes how these receptors function, either directly or through a cytokine storm - the overreaction of the body's immune system," Prof Philpott added.

Details of these findings are published in the journal, Rhinology.