Harvard researchers say they have discovered the reason why COVID-19 causes loss of smell.

BOSTON – Temporary loss of smell is one of the earliest and most commonly reported symptoms of COVID-19.

Although studies reported that it is a better indicator of the new coronavirus than other common symptoms, such as high fever and cough, the scientists were still unsure why the disease caused loss of smell.

Now, an international team of researchers at Harvard Medical School is beginning to understand why.

In a study published in the scientific journal Science Advances, researchers identified the types of olfactory cells in the upper nasal cavity that are most vulnerable to SARS-CoV-2, the virus that leads to COVID-19.

To their surprise, the researchers discovered that the sensory neurons that detect and transmit smell to the brain are not among the vulnerable cell types. Instead, it is the cells that provide metabolic and structural support to these sensory neurons that are most vulnerable to the ACE2 receptor protein, which SARS-CoV-2 uses to enter human cells.

“Our findings indicate that the new coronavirus changes the sense of smell in patients not by directly infecting neurons but by affecting the function of supporting cells,” said Sandeep Robert Datta, lead author and associate professor of neurobiology at Harvard.

The good news is that this means that, in most cases, SARS-CoV-2 infection is unlikely to lead to permanent damage or persistent anosmia, the medical term for loss of smell.

“I think that is good news, because once the infection clears up, olfactory neurons don’t seem to need to be replaced or rebuilt from scratch,” Datta said. “But we need more data and a better understanding of the underlying mechanisms to confirm this conclusion.”

The research data suggests that COVID-19-related anosmia may arise from a temporary loss of support cell function in the olfactory epithelium, which indirectly causes changes in sensory neurons for smell. Scientists don’t fully understand the changes, but Datta added that while support cells have been ignored in the past, it is clear that they play a critical role.

The study observations will help accelerate efforts to better understand loss of smell in COVID-19 patients, which in turn could lead to treatments for anosmia and better diagnoses for the virus.

“Anosmia seems like a curious phenomenon, but it can be devastating for the small fraction of people in whom it is persistent,” Datta said. “It can have serious psychological consequences and it could be a major public health problem if we have a growing population with permanent loss of smell.”