what scientists do and don’t know

At the onset of the COVID-19 pandemic, it appeared that many people infected with the SARS-CoV-2 virus were losing their sense of smell – even without showing other symptoms. Researchers also found that infected people could lose their sense of taste and their ability to sense chemically triggered sensations such as prickling, called chemesthesia.

Almost a year later, some still have not regained these senses, and for some of the people who do, the smells are now distorted: the unpleasant smells have replaced the normally delicious smells. Nature studies the science behind this potentially long-lasting and debilitating phenomenon.

How many people with COVID-19 lose their sense of smell?

The exact percentage varies from study to study, but most suggest loss of odor is a common symptom.

A review published last June¹ compiled data from 8,438 people with COVID-19 and found that 41% reported experiencing loss of odor. In another study, published in August², a team led by researcher Shima T. Moein of the Basic Sciences Research Institute in Tehran, Iran, administered an odor identification test to 100 people with COVID-19 in which subjects sniffed the smells and identified them on a multiple choice basis. Ninety-six percent of the participants had olfactory dysfunction and 18% had complete loss of odor (otherwise known as anosmia).

“Usually these patients say they lost their smell suddenly,” a clue that the symptom is linked to COVID-19, Moein says. And often the dysfunction is the only symptom of COVID-19 that people experience, suggesting that the phenomenon is distinct from the nasal congestion induced by the virus.

Some researchers say the loss of odor should be used as a diagnostic test for COVID-19. A study published last October³ found that self-reported changes in smell or taste were a better marker of the spread of infection than other indicators tracked by governments, such as hospital and emergency department arrivals.

Why do people with COVID-19 lose their sensitivity to smells?

While the mechanisms are not fully understood, there is an emerging consensus that loss of odor occurs when the coronavirus infects cells that support neurons in the nose.

When researchers first identified odor loss as a symptom of COVID-19, they feared that the virus would infect odor-sensitive neurons in the nose that send signals to the olfactory bulb in the brain – and that the virus can therefore access the brain. . However, post mortem studies⁴ of people who have had COVID-19 have shown that the virus rarely reaches the brain.

A team led by Sandeep Robert Datta, a neurobiologist at Harvard Medical School in Boston, Massachusetts, found instead⁵ that the cells that support sensory neurons in the nose – called sustentacular cells – are likely what the virus infects.

Datta and his colleagues focused on sustentacular cells because SARS-CoV-2 attacks by targeting a receptor called ACE2 on the surface of cells, and sustentacular cells have many such receptors. Olfactory sensory neurons do not. This suggests that the coronavirus infects supporting cells, leaving neurons vulnerable and starved of nutrients.

But COVID-19 can induce odor loss in other ways. For example, a research team in Italy showed⁶ this loss of smell and taste occurs at the same time as an increase in the blood levels of an inflammation signaling molecule called interleukin-6. And a post-mortem study published last December showed clear signs of inflammation, such as leaky blood vessels, in the scent bulbs of people who had had COVID-19^seven.

Although scientists have some understanding of the mechanisms involved in smell, they have little idea of ​​how the coronavirus affects taste and chemotherapy. “No one has a good idea of ​​this yet to my knowledge,” says John Hayes, professor of food science at Pennsylvania State University at University Park, who studies the effects of COVID-19 on chemical senses. Taste and chemesthesia are senses that are distinct from smell, even though all three combine to tell humans the “flavor” of a food or drink. Taste relies primarily on taste receptors on the tongue, while chemesthesia relies on ion channels on sensory nerves, among other mechanisms – and their response to COVID-19 has not been studied much.

How quickly do the altered senses return?

For most people, the smell, taste and chemesthesia recover within a few weeks. In a study published last July⁸72% of people with COVID-19 who had olfactory dysfunction reported recovering their sense of smell after a month, as did 84% of people with taste dysfunction. Claire Hopkins, ear, nose and throat consultant at Guy’s and St Thomas’ Hospital in London, UK, and colleagues have similarly observed⁹ a rapid return of the senses: they followed 202 patients for a month and found that 49% reported complete recovery during this period, and 41% reported improvement.

But, for others, the symptoms are more serious. Some people whose senses don’t return right away slowly improve over a long period of time – and that can have consequences, says Hopkins. As a person regains their sense of smell, the smells are often considered unpleasant and different from how they remembered them, a phenomenon called parosmia. “Everything smells rancid” to these people, says Hopkins, and the effect can last for months. This could be because olfactory sensory neurons are rewiring themselves as they recover, she says.

Other patients remain totally anosmic for months, and it is not known why. Hopkins suggests that, in these cases, the coronavirus infection could have killed the olfactory sensory neurons.

How does the permanent loss of chemical senses affect a person?

While the condition is not as well studied as the loss of other senses such as vision and hearing, researchers know the consequences can be serious.

One of the effects is that it makes people vulnerable to dangers such as food poisoning and fires. For example, people with anosmia are less able to detect spoiled food and smoke. A 2014 study found that people with anosmia were more than twice as likely to experience a dangerous event, such as eating spoiled food, than people without loss of smell.^ten.

Other effects are more difficult to measure. “Most people don’t recognize the importance of smell in their life – until they lose it,” Moein says. Being unable to appreciate the flavor of food is obviously a major loss, but other sensations are also important. Hayes points out, for example, the loss a parent would feel if they couldn’t connect to their child through “the scent of the newborn.” And Moein says the dysfunction of smell has been linked to depression, although the biological mechanism involved is unclear.

Are there treatments to restore these senses?

A lack of research means that there are few established treatments. But one option is scent training, in which people regularly sniff out prescribed scents to relearn them. Hopkins is working with a charity called AbScent in Andover, UK to publicize this training. There is evidence¹¹ before the pandemic, it may improve olfactory function in some people with such impairments, but it doesn’t seem to work for everyone.

The drugs available are even more limited, says Hopkins. But for people in the early stages of COVID-19 infection, when the loss of smell could be largely due to inflammation of cells in the nose, steroids could be helpful, according to a preliminary test by the Hopkins team¹².

As part of longer-term research, Richard Costanzo and Daniel Coelho at Virginia Commonwealth University in Richmond are developing an olfactory implant – a device built into the nose that detects smelling chemicals and sends electrical signals to the brain. However, the device is still “many years away” to be offered in clinics, Coelho says. In particular, researchers need to determine which areas of the brain the implant should stimulate, he adds, so “there is still scientific data to be worked out.”