Harvard researchers say they have figured out the reason why COVID-19 causes loss of smell

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

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

Now, an international team of researchers at Harvard Medical School are beginning to understand the reason.

In a study published in the peer-reviewed journal Science Advances, the researchers identified the olfactory cell types 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 the sense of 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 the most vulnerable to the ACE2 receptor protein, which SARS-CoV-2 uses to enter human cells.

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

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

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

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

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

“Anosmia seems like a curious phenomenon, but it can be devastating for the small fraction of people in whom it’s persistent,” said Datta. “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.”