New research led by scientists at Harvard University Medical School explains one of the many mysteries of COVID-19: why people infected with the virus temporarily lose their sense of smell.

The loss of smell has been the most common neurological symptom of the virus. The study's lead author, Dr. Sandeep Robert Datta, said their findings about why that happens was a surprise and could lead towards eventual treatments for a range of neurological disorders caused by the virus.

Harvard neuroscientists took a close look at a specialized type of sensory neurons in the nose that detect and transmit smells to the brain.

"Our intuition, and I think the intuition of many other people, would be that the virus would attack these sensory neurons and damage or kill these neurons, and that's how we lose our sense of smell," said Datta, an associate professor of neurobiology in the Blavatnik Institute at Harvard Medical School. The study Datta led was published Friday in the journal Science Advances.

"But in looking at our data, we got a big surprise," he said. "Which is that it seems like the virus is not actually capable of attacking the neurons that live in your nose."

Instead, the scientists discovered that two other kinds of cells that support those neurons are being attacked. Those cells can regenerate more quickly.

"And so we think, on the whole, this is good news, and suggests that people who lose their sense of smell, for the most part, are going to go on to get their sense of smell back," Datta said.

That's what doctors have seen as the epidemic has progressed — most patients regain their sense of smell in several weeks.

"We finally have clues that lead us to understand how it is the virus might attack your sense of smell," Datta said, "which leads us to theories about how it might attack your neurological systems more generally."

In addition to a loss of smell, the virus has caused a number of other neurological symptoms, including altered consciousness, difficulty concentrating, sensory motor deficits, and strokes. Datta said he's hopeful that this new understanding of what cells the virus attacks in the nose might prove useful in understanding those other symptoms.

"I think that points the way to important strategies for treatments, both for people who've lost their sense of smell and for people who are suffering from various neurological consequences of COVID, as well," he said.

The study also looked at a part of the brain called the olfactory bulb that's responsible for getting information from the nose, and found the neurons there are also not infected by the coronavirus.

"It's possible, even given our research, that the virus will infect some types of neurons," Datta acknowledged. "At least for now, our best guess is that it's mostly attacking vascular cells that help to feed the brain. I think that changes our strategies for how we might manage the virus and manage neurological symptoms that are associated with COVID going down the road."

COVID-19 patients have also lost their sense of taste, and Datta said that's more than just a side effect of losing the ability to smell.

"It's clear from the data that are available now that the coronavirus independently attacks your sense of taste as well," he said, adding that the same sorts of supporting cells may be attacked for taste, too.

"In preliminary analysis that has been done by colleagues of mine, it appears that the virus does not attack neurons that transmit information about taste," he said.

More research needs to be done to verify the paper's conclusion, Datta said, and to understand how the virus attacks the brain. He said a lack of access to autopsy studies has hampered that kind of study.

And while most patients who survive COVID-19 regain a sense of smell, Datta said some do not.

"And although I think to most people, that doesn't seem like very much of a big deal, we know from other people who have lost our sense of smell that it's a huge risk factor for depression and other kinds of psychological changes," he said. "Because we, as humans, even though we don't think about our sense of smell very much, are deeply emotionally dependent on our sense of smell for our day-to-day well-being."