Nearly half of those who became ill with Covid in the first wave of infections may have long-term and even permanent changes to their sense of smell, according to preliminary research from Sweden.
A sudden loss of smell, or an impaired or distorted perception of odours, emerged as an unusual symptom of Covid early on in the pandemic. While many people swiftly recovered, others found that their sense of smell never quite returned to normal.
To find out how common the impairments might be, scientists at the Karolinska Institute in Stockholm ran comprehensive tests on 100 individuals who caught Covid in the first wave of infections that swept through Sweden in spring 2020.
Their early findings show that 18 months after recovering from Covid, very few people – only 4% – had lost their sense of smell entirely, but a third had a reduced ability to detect odours, and nearly half complained of parosmia, where the sense of smell is distorted. Most of those with a reduced sense of smell were unaware of it before they joined the study.
The scientists then ran the same tests on a control group of people who tested negative for Covid antibodies, indicating that they had managed to avoid the virus. About a fifth were found to have similar deficiencies in their sense of smell, implying that smell disorders were common in the general population before Covid struck.
Writing in a preprint that has not yet been peer reviewed, the scientists conclude that 65% of those who recovered from Covid displayed either a loss of smell, a reduced sense of smell, or distortions to the sense 18 months after the infection, compared with 20% of those who had not caught the virus. “Given the amount of time since [the] initial insult to the olfactory system, it is likely that these olfactory problems are permanent,” they wrote.
The volunteers recruited for the study were healthcare workers who were regularly tested for Covid from the start of the epidemic in Sweden. Because the study focused on people who caught Covid in the first wave, none of the volunteers had been vaccinated at the time. For the same reason, their infections were caused by older versions of the virus, not the Omicron variant that is now spreading rapidly around the world.
Analysis by the UK Health Security Agency suggests that a loss of smell or taste is less than half as common with Omicron than the Delta variant, but Dr Johan Lundström, who led the research at the Karolinska Institute, said there was no reliable data demonstrating that Omicron was less dangerous to the olfactory system.
A minor loss of smell, or noticing that certain odours smell weird, may not be life changing for many, but Lundström said a severe loss of smell could lead to depression and to people changing their diets, often for the worse, causing them to put on weight.
“When you cannot smell, all you can sense is the five basic taste qualities, tactile sensations and spices,” he said. “Unconsciously, people start to add more sugar and fat, or have an increased urge for fried food for the texture, all to get some enjoyment out of eating.”
Lundström said the greatest surprise of the study was that nearly half of people who had recovered from Covid reported a distorted sense of smell so long after the infection. “Many of these individuals can get help by doing olfactory training,” he added. “They might not regain 100% of past performance, but most of them will, with training, get back to a point where their reduced sense of smell will not affect their lives.”