One of COVID-19's original symptoms, loss of smell, could be making a comeback as some patients are reporting it again

• Health professionals in the US have said more patients with COVID-19 are reporting a loss of smell.
• Loss of smell was a hallmark feature of the original virus, but it's less common with more recent variants.

Loss of smell could once again become a hallmark feature of COVID-19, as health professionals say increasing numbers of people infected with the virus are reporting the symptom again.

Back in 2020, if people lost their sense of smell, it was highly likely that they'd caught the original virus. But more recent variants, including other Omicron subvariants, haven't caused the symptom as often.

However, loss of smell may be making a comeback, as the BA.5 subvariant, which can easily evade the body's immune response, has become the most common cause of new COVID-19 cases in the US.

Valentina Parma, a psychologist at the Monell Chemical Senses Center in Philadelphia who works with COVID patients, told NBC News she was seeing more people with loss of smell, compared with the beginning of the year.

"What I am seeing in my corner of the world is a spike," she said.

Parma said that those who reported a loss of smell were the ones that couldn't smell at all. "But when you administer a smell test, we're seeing about 25% of people have an impaired sense of smell, which is not a small number," she said.

Parma added that the number of people reporting a loss of smell was "significantly less" than with the Delta variant, which was dominant in the US in 2021.

It's not clear why certain variants cause loss of smell more than others. However, research suggests the loss of smell is caused when the virus infects nerve cells in the nose.

Early data from French health authorities published on July 15 found people infected with BA.4 or BA.5 were more likely to present with loss of smell or taste than those infected with BA.1, one of the earliest Omicron subvariants. BA.5 is not currently the most common cause of new COVID-19 cases there.

However, there isn't any data to confirm loss of smell is a feature of a BA.5 infection.

Dr. Lora Bankova, an allergist and immunologist at Brigham and Women's Hospital in Boston, told NBC News: "I have talked to people overall about losing their taste and smell lately and it seems that there is an uptick, but the data isn't there yet."

The symptom can be treated with smell re-training and nasal steroids, if given immediately.

If people get a sudden loss of smell and don't have a positive COVID-19 test, they should talk to a doctor, because other illnesses can cause loss of smell, such as epilepsy and rhinitis.