Still struggling with your sense of smell after a bout with Covid-19? You're far from alone. 

About 5% of patients with confirmed cases of Covid-19 — some 27 million people worldwide — are estimated to have suffered a long-lasting loss of smell or taste, a new analysis suggests.   

In the analysis published Wednesday in The BMJ (the peer-reviewed medical journal of the British Medical Association), researchers evaluated  18 previous studies of smell and taste loss across several continents and in varying demographic groups. About three quarters of those affected by loss of taste or smell regained those senses within 30 days.

Rates of recovery improved over time, but about 5% of people reported "persistent dysfunction" six months after their infection with Covid-19. 

The analysis suggests loss of smell and taste could be a prolonged concern that requires more research and health resources for patients struggling with long-term symptoms. 

Losing smell has been linked to higher death rates in older adults and has been shown to have major impacts on people's emotional and psychological well-being, said Dr. Zara Patel, a rhinologist at Stanford University who was not involved in The BMJ research. 

"Having these now millions more people worldwide with decreased ability to smell — that may simply be a new public health crisis," Patel said. 

Loss of smell was one of the most distinct markers of Covid-19 in the pandemic's beginning days. 

"You could track the pandemic across the globe" by analyzing Google searches about smell loss, Patel said.

The BMJ analysis gives a broad review of smell studies across the world and over time. Data from nearly 3,700 patients was included in the analysis. 

Studies from North America, Europe and Asia were all included in the analysis, which noted that women were less likely to regain their senses of smell and taste than men. Patients with greater nasal congestion were less likely to recover, also. 

The analysis showed steady increases in the proportion of patients who recovered their sense of smell over time. After 30 days, about 74% of patients had recovered it; after 90 days that number was up to 90%. After six months, about 96% of patients said they were able to smell again. 

Scientists are beginning to grasp how Covid-19 affects the smelling function. 

The coronavirus often causes swelling in the olfactory cleft, that is, the passages in the upper part of the nasal cavity where humans perceive the sense of smell and process flavor beyond basic tastes like sour or bitter.   

Researchers think the virus does not initially infect the olfactory neurons but instead latches on to support cells, which help the neurons provide a signaling pathway.

Patients who suffered smell loss after Covid-19 make up a unique subset, said Dr. Aria Jafari, a rhinologist at the UW Medicine Sinus Center in Seattle, who was not involved in the new analysis. "They tend to get better and kind of quickly, which makes sense based on the cells that are affected." 

Jafari said about half of his patients deprived from the sense of smell likely had Covid-19 at some point. Many experienced dramatic impacts on their well-being because of the loss. 

"They tend to be distraught about the loss of sense of smell. It's such an important part of our every day and what makes us human," Jafari said, adding that he's treated a professional chef, a chocolatier and others whose livelihoods depend on their ability to determine smell and flavor. "The most common thing I hear is that it leads to social isolation and feeling disconnected from the world and society as they know it. And that can be really bothersome."

Jafari said many patients also describe a transition period "that can be distressing" as their sense returns in which they smell things that aren't present — like burning rubber or smoke — or experience abnormally foul smells. 

People who are unable to smell or sense flavors can have higher rates of psychiatric illness, depression and anxiety, Jafari said. In an extreme case, Jafari said he treated a patient who became malnourished after losing the senses of smell and taste. 

Smell underlies the way we interact with each other and make our way in the world, dictating "your first impressions of other people, the people we choose for sexual encounters or for lifelong partners," Patel said. Cues from scent could subconsciously influence people's attraction to others based on their underlying genetics, studies suggest.  

The analysis relies on studies using data that was self-reported by patients. Patel said that could underestimate the true toll of smell dysfunction and skew some of the research findings because people are sometimes unable to perceive how much sensitivity they have lost. 

The study's authors agreed. 

"Many previous studies have shown that objective smell testing can identify far more people with smell loss than if we asked them to self-report," wrote Professor Song Tar Toh, an author of the study and head of the the department of Otolaryngology-Head & Neck Surgery at Singapore General Hospital, in an email. "The true number of people affected is likely to be far higher than our estimate."  

Patel suspects the true rate of smell dysfunction among those who have experienced Covid-19 could be above 20%. It could be that women are not more likely to struggle with recovery, but are more perceptive of a prolonged deficit in their ability to smell. 

"Women overall, have, on average, a more acute sense of smell than men," Patel said. "We know people with a more acute sense of smell and taste are much more likely to recognize when they have a loss and are more likely to seek care for a loss."

Jafari said The BMJ analysis generally tracks with his clinical experience and his observations of patients' recoveries. 

"It's nice to collate data from all around the world to better understand what's going on and take some variability out of these analyses specific to a patient population or institution," Jafari said. "It increases the level of evidence, overall, to support what we, as sinus surgeons, see in our offices." 

Initial versions of the omicron variant seemed to affect the sense of smell less than previous waves of Covid-19, Patel said. 

But the latest subvariant, BA.5, could be reversing that trend. 

"We don't have enough data yet to know for sure," Patel said. "I'm now, in my clinic, starting to see an uptick again." 

Treatments are available for people who have lost their sense of taste and smell due to Covid-19. 

Structured olfactory training — in which patients twice a day sniff essential oils like lemon, clove, eucalyptus and rose to stimulate different types of neurons — can reteach the brain to recognize different scents. Doctors will often prescribe a steroid rinse for the sinuses to decrease inflammation and aid training. 

Some emerging evidence suggests supplements of Omega 3 fatty acids could be helpful for patients with smell dysfunction.

Patel and others are exploring other treatments, including nasal injections of platelet-rich plasma and electrical stimulation. 

Patel said she hopes research funding and public interest in smell and taste dysfunction continue to grow so researchers can dive deeper and unlock new treatments. 

Before the pandemic, "it was the orphan, Cinderella sense," Patel said. "It's only after so many millions of people have been affected or had loved ones affected that people are coming to understand the huge impacts smell and taste have on your quality of life." 

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