Scientists have said the reason some people lose their sense of smell after contracting COVID could be down to genetics.
Loss of smell was added to the UK government’s official list of symptoms for COVID-19 in May 2020.
Some people who lost their sense of smell after testing positive for coronavirus have said it still hadn’t returned six months after becoming infected.
The precise cause of this sensory loss related to coronavirus is not known, but scientists believe it is caused by damage to infected cells in part of the nose called the olfactory epithelium – which is found in the nasal cavity.
These cells protect olfactory neurons, which help humans smell.
A genetic locus, meaning the specific location of a gene on a chromosome, near two olfactory genes is associated with COVID-induced loss of smell and taste, according to a study published in the Nature Genetics journal.
This genetic risk factor increases the likelihood a person infected with COVID will experience a loss of smell or taste by 11%, the study says.
Researchers at the genomics and biotechnology company 23andMe conducted the study as part of a larger COVID project, with 69,841 participants living in either the US or the UK.
The study doesn’t reveal how many were from each country.
Of those who took part, 68% reported a loss of smell or taste as a symptom.
After comparing the genetic differences between those who lost their sense of smell and those who reported that they did not suffer this symptom, the study team found a region of the genome associated with this split that’s situated near two genes, UGT2A1 and UGT2A2.
Both of these genes are expressed within tissue inside the nose involved in smell and play a role in metabolising odorants.
“It was this really beautiful example of science where, starting with a large body of activated research participants who have done this 23andMe test, we were able to very quickly gain some biological insights into this disease that would otherwise be very, very difficult to do,” said Adam Auton, vice president of human genetics at 23andMe and the lead author of the study.
How UGT2A1 and UGT2A2 are involved in this process is unclear, though Mr Auton and his colleagues hypothesise the genes “may play a role in the physiology of infected cells” and the resulting impairment that leads to smell loss.
Trends also emerged among participants who reported the loss of smell and taste.
Women, for example, were 11% more likely than men to experience this. Meanwhile, adults between the ages of 26 and 35 made up 73% of this group.
The study team also found people of “East Asian or African American ancestry were significantly less likely to report loss of smell or taste”.
Loss of smell and taste have been recognised as symptoms of coronavirus throughout the global pandemic.
Early research suggests loss of smell and taste is rarer with the Omicron variant, but not entirely unlikely.
In a study of 81 Omicron cases in Norway, 12 percent reported reduced smell and 23 reported reduced taste.