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Scientists say they have discovered why coronavirus causes people to lose their sense of smell.

The symptom is one of the earliest and most commonly reported indicators that a person has been infected with COVID-19, but the reasons for it have been unclear - until now.

Experts from Harvard Medical School in the US have identified the cell types used for smelling which are most vulnerable to infection by SARS-CoV-2, the virus that causes COVID-19.

They found that sensory neurons that detect and transmit the sense of smell to the brain are not among the vulnerable cell types.

Instead, the team found that coronavirus attacks the cells that provide metabolic and structural support to the sensory neurons, as well as certain stem and blood vessel cells.

The findings suggest that in most cases coronavirus is unlikely to lead to persistent loss of smell - medically known as anosmia - a condition that can be associated with mental health issues including depression and anxiety.

Study author Sandeep Robert Datta, a professor of neurobiology at Harvard Medical School, said: "Our findings indicate that the novel coronavirus changes the sense of smell in patients not by directly infecting neurons but by affecting the function of supporting cells.

"I think it's good news, because once the infection clears, olfactory neurons don't appear to need to be replaced or rebuilt from scratch.

"But we need more data and a better understanding of the underlying mechanisms to confirm this conclusion."

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Most COVID-19 patients experience some level of anosmia, which is usually temporary, according to the researchers.

COVID-19 patients are 27 times more likely to have smell loss than those without the disease, but are only around 2.2 to 2.6 times more likely to have fever, cough or respiratory difficulty, the experts said.

Some studies have also suggested that anosmia in COVID-19 differs from when the condition is caused by other viral infections, including other coronaviruses.

For example, COVID-19 patients typically recover their sense of smell over weeks, rather than the months it can take to recover from anosmia caused by other viral infections.

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Also, many viruses cause temporary loss of smell by triggering upper respiratory issues such as a stuffy nose.

However, some COVID-19 patients experience anosmia without any nasal obstruction.

The team behind the study hope their findings help efforts to better understand loss of smell in people with COVID-19, which could lead to treatments for anosmia.

Prof Datta said: "Anosmia seems like a curious phenomenon, but it can be devastating for the small fraction of people in whom it's persistent.

"It can have serious psychological consequences and could be a major public health problem if we have a growing population with permanent loss of smell."

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The researchers say they also hope the findings will help studies into whether the nose acts as a "reservoir" for coronavirus.

Loss of taste and smell were added to the UK government's official list of symptoms for COVID-19 in May.

The advice says people should isolate if they have a new continuous cough, or fever, or anosmia.

The Harvard study is published in the peer-reviewed journal Science Advances.