There’s more to the nose than meets the eye. Swedish psychology professor Jonas Olofsson smells it like it is in a new book. You can read Part I here, or jump straight into the edited extract from the book.
There were early reports that the Sars-CoV-2 coronavirus affected the sense of smell. However, this was not something that was initially given much attention. The loss of smell was initially thought to be similar to that which occurred in colds, where nasal congestion and mucus secretions help to disrupt the sense of smell. But eventually it was realised that the new virus was different. It was more aggressive and seemed to lead to an immediate, complete elimination of the sense of smell.
Olfactory scientist Danielle Reed estimated in February 2022 that the Covid-19 pandemic might have left 20 million people with a permanently impaired sense of smell. We smell and taste researchers quickly came together to meet this new challenge. In March 2024, I participated in a videoconference on the new reports, along with hundreds of smell and taste researchers. My colleagues from Europe and the US created an effective organisation and, just one week after the meeting, the number of participants had grown from 100 to nearly 500.
The team created a survey that asked participants if they had experienced symptoms of the disease in the nose and throat, and if their sensory abilities had changed as a result of the disease. Thanks to the commitment of the various participants around the world, the survey was soon translated into 35 languages and published online. I did the Swedish translation together with my colleagues, and we could soon see how more and more Swedish participants reported a loss of sense of smell.
The survey showed that the sense of smell in patients with confirmed Covid-19 deteriorated by an average of 80%. More than half of the people with confirmed Covid-19 lost their sense of smell completely, making loss of smell a particularly strong symptom of Covid-19. Other symptoms, such as headache or fatigue, were also present in many patients, but as these symptoms were also present in those who did not have Covid-19, but only a common cold, they were not such good markers for the coronavirus.
The sense of taste and the trigeminal sense, which reacts to chili peppers and other hot or pungent substances, were also severely impaired. The loss of taste was particularly difficult to assess because people often report “losing taste” when in fact it is the sense of smell that has been lost. Common viruses can cause loss of smell, but rarely loss of taste. Here, our survey had an advantage because we asked about individual tastes. A full 45% of Covid patients reported a loss of salty taste, and this taste is rarely confused with smell, indicating that there was a real loss of taste. What distinguished the loss of smell in Covid-19 from that in common colds was that it was immediate, usually total and absent of other nasal symptoms, such as swelling and runny nose. Newspapers, television and radio reported on the new findings. But the authorities were slower to act. The Public Health Agency of Sweden described loss of smell as an “unusual” symptom – even though it was the most common. The World Health Organisation (WHO) did the same, and many countries’ public health authorities downplayed the role of the sense of smell. Perhaps it was thought that people could not really judge whether they themselves had lost their sense of smell. After all, it had long been described as a subjective and unreliable sense.
Perhaps because of the prejudice against the human sense of smell, many countries’ public health authorities ignored the reports, focusing instead on fever, cough and headache. But the sudden loss of smell was obvious to those affected. And I like to think I helped protect the public when I repeated to journalists in an almost parrot-like manner that readers and listeners who suddenly lose their sense of smell should isolate themselves immediately. It was long a mystery why the Covid-19 infection hit the sense of smell so hard. But finally, researchers discovered how the virus finds its way into the body, via so-called support cells found around the olfactory receptor cells in the olfactory mucosa of the nasal cavity.
The support cells keep the mucosa in order, helping to maintain the chemical balance, but they also clean the mucosa by taking care of substances that could otherwise cause damage. The support cells contain two genes that influence the shape of the outer membrane of the cells. The coronavirus has found a way to attach itself to the membrane of these cells, and from there it enters the cells and breaks them down. When the coronavirus breaks down the support cells, the consequence is that all other cells are thrown into chaos.
In this eroded environment, which can now be compared to the ruins of an ancient city, new receptor cells will not always be able to find their way back to the brain. Communication is interrupted. The total loss of smell lasted for different lengths of time for those affected. On average, there was a 50% recovery in just over a month, according to our study. But this figure hides a wide range. For some, it went away in a few days. For others, the sense of smell has not returned at all.
The long-term consequences of Covid-19 are still not fully understood. It has proved difficult to make predictions about this new and elusive disease. However, research is ongoing about the long-term effects of the disease on the sense of smell.
Staff at Danderyd Hospital in Stockholm were recruited for a large study to map the impact of Covid-19 on them. It has become one of the best Swedish data sources from the pandemic, thanks to the loyalty of the staff who, despite great hardship, contributed their time to help science. A clear lesson learned is that it is mainly those who had severe symptoms at the time of infection who have long-term problems. The patients who had severe Covid-19 had significantly more long-term symptoms after eight months, and loss of smell, anosmia, was the most common.
Anosmia was present in 25% of those with severe Covid-19 but only in 10% of those with mild Covid-19. A full year into the study, 6% of Covid-19 patients had not yet recovered their sense of smell.
The Forgotten Sense: The New Science of Smell, by Jonas Olofsson (Harper Collins, $38), is out now.
