A growing volume of medical experts around the world are suggesting a loss of the sense of smell may be an early indication of COVID-19. However, the preliminary, and still relatively anecdotal, observation is not officially recognized as a COVID-19 symptom by the World Health Organization at this stage.

The American Academy of Otolaryngology, the nation’s association of head and neck surgeons, recently released a statement calling for anosmia, hyposmia, and dysgeusia to be added to official list of symptoms for COVID-19, the coronavirus disease currently spreading the world. These three conditions refer to reductions, or complete losses, of the senses of smell and taste.

Anosmia, the loss of the sense of smell, in particular has been reported by a number of doctors around the world as a potential early symptom of COVID-19. Not many prior epidemiological studies into COVID-19 cases have collected data on anosmia, so at this stage it is unclear how common this symptom may be.

Just to give you guys an update, loss of smell and taste is definitely one of the symptoms, haven’t been able to smell anything for the last 4 days. Anyone experiencing the same thing? — Rudy Gobert (@rudygobert27) March 22, 2020

A small study surveying 100 patients in a German hospital found nearly two-thirds reported some kind of disruption to their senses of smell and taste during the course of the disease. Another small study from South Korea suggested around 30 percent of positive COVID-19 cases noted a loss of smell.

A joint statement from Claire Hopkins, president of the British Rhinological Society, and Nirmal Kumar, president of ENT UK, suggests this symptomatic observation is being seen in patients suffering from very mild, or even asymptomatic, COVID-19 cases.

“… there have been a rapidly growing number of reports of a significant increase in the number of patients presenting with anosmia in the absence of other symptoms,” the statement notes. “I have personally seen four patients this week, all under 40, and otherwise asymptomatic except for the recent onset of anosmia.”

[Read more: Coronavirus survives on surfaces for days, but what does that really mean?]

Flinders University ENT specialist Simon Carney hypothesizes anosmia may be an early sign of the disease and, if verified in future studies, could be a way to detect what he refers to as "silent carriers."

“It is these ‘silent carriers’ who may remain undetected by current screening procedures, which may explain why the disease has progressed so rapidly in so many countries around the world,” says Carney.

It is important to note that a reduction in one’s sense of smell is a relatively common symptom associated with a number of viral and bacterial infections, from the flu to sinusitis. This novel virus is also a member of the coronavirus family which, alongside the more deadly SARS and MERS viruses, includes four strains known to cause a mild form of the common cold. What this means is that it is entirely plausible losing one’s sense of smell is an early symptom of COVID-19, however, it also could very easily be a sign of a simple common cold infection.

Although COVID-19 is fundamentally a respiratory disease, the initial point of viral infection is often through the nose and throat. Prior studies have found that very early in an infection, the virus sheds from the nose and throat at incredibly high levels. This is why the virus is so contagious early on, and it becomes less infectious as it progresses deeper into the lungs. Again, this affirms the potential for anosmia to be an early symptom, appearing initially as the virus takes hold in a patient’s nose and throat.

Loss of smell is still only anecdotally verified as a COVID-19 symptom, and it is not known how common it is across all confirmed cases. The three primary symptoms verified by the World Health Organization are fever, dry cough and tiredness or fatigue. Secondary symptoms to watch out for are shortness of breath, body aches and a sore throat.

The WHO does note a small number of cases have reported symptoms such as diarrhea, nausea and a runny nose. These other symptoms are still only reported in a minority of cases and should not be considered as common as the primary symptoms.

Although ENT experts are calling for anosmia to be added as a symptomatic trigger for testing and self-isolation, it is too early to tell how common the symptom is, and whether it regularly presents in mild or even asymptomatic cases. Speaking to STATNews, rhinologist Eric Holbrook says this new observation should not be used as a single screening factor, but rather it can be incorporated into broader considerations when evaluating potentially infected patients.

“I wouldn’t use it alone [to screen patients], but I think if it’s added to some of the other symptoms, it adds to the possibility,” he says. “If someone says yes, it would definitely make me more suspicious that they need further work-up.”

Sources: ENTUK, Flinders University