COVID-19 and the sense of smell

Professor S. Craig Roberts, Professor of Social Psychology, Faculty of Natural Sciences, University of Stirling

A loss of sense of smell and taste is a particular symptom of COVID-19. Professor Craig Roberts explores the issue further in a COVID-19 bite-sized lecture.

Watch the lecture online or read the transcript below.

Hello and thank you for your interest in my short talk about COVID-19 and the sense of smell. I’m Craig Roberts and I’ve been working on smell for much of my career and at Stirling for the last 10 years.

You’ve probably been hearing much more about smell in the past months than ever before, because as you’ll know, one of the three core symptoms of COVID-19 is a sudden loss or change in one sense of smell or taste. At least that’s been the case since the 18th of May and probably should have been much earlier, as I’ll discuss. It’s also been in the news a lot. In late March. NBA star Rudy Gobert was tweeting about his sudden loss of smell while a son of Gary Lineker, a footballer, drank neat vinegar to demonstrate his alter sense of taste – not recommended, by the way. More recently, Prince Charles commented that his sense of smell hasn’t returned after COVID infection, something we’ll also come on to at the end.

So in this talk on COVID and smell, I’ll start by briefly introducing the senses of smell and taste, describe how the evidence-based relating to COVID developed earlier this year, and why it came to be recognized as a key symptom. Then I’ll end with some practical ideas if you’re interested in testing and tracking your sense of smell, or you want to know about how you can improve it, especially after COVID infection.

The importance of smell

We use a sense of smell for three main functions. Smell first helps us to learn about, prefer and choose different kinds of food. Secondly, it acts as a detector of hazards in the environment. For example, we use it to tell if milk or other food has spoiled or to detect gas leaks or burning toast. And finally, we use smell for social communication, whether that’s playing a role in who we prefer as an intimate partner or in, for example, facilitating breastfeeding of a new born infant. Although it’s often been said that humans have a rather poor sense of smell, especially compared with dogs and other animals, we’re realizing that we are much better at smell tasks in each of these three areas than we used to think. For example, recent studies have shown that people can track a chocolate trail through grass in a similar way that a dog follows the trail of a pheasant, or that we can smell sick people by detecting subtle changes in someone’s armpit odour when they receive the vaccine shot that activates their immune system. Or finally, that new born babies learn and discriminate their mother’s odour from that of other breastfeeding women. All of these tasks require that we can discriminate between many thousands of very complex mixtures of detectable chemical compounds.

In contrast, the sense of taste is really quite simple. We have just five or possibly six types of taste receptors. Sweet, sour, salty, bitter and savoury. Taste helps us assess food value. If something is sweet, it probably has lots of energy. If it’s better, it might be poisonous. And if you’re wondering how such a simple system allows you to distinguish between so many different kinds of food, say, between chicken and beef to smell because chewing food releases odour compounds which travel through the back of the mouth to the smell receptors at the very top of the nose. This is called retro nasal olfaction and this combination of the taste and smell senses can lead to some confusion. If you feel you can’t taste anything, it might in fact mean that you’ve lost your sense of smell.

There are many different terms for sensory loss in smell and taste, but these are the four ones. Anosmia is the total loss of smell and hyposmia means a partial loss or reduced sensitivity. The equivalent terms for taste loss are Ageusia and Hypogeusia. In normal times, full or partial smell loss is really quite common, with many estimates placing around 15 to 20 percent of the population to have some degree of hyposmia and perhaps surprisingly, a high prevalence of full of anosmia to total small loss than for loss of either sight or hearing.


So after this brief introduction, what is the evidence for the effects of COVID on smell and taste? We should start by looking at the usual known causes for anosmia. This pie chart shows the various causes from a German sample from fifteen or so years ago. As you can see at the top, congenital anosmia that’s in anosmia at birth accounts for only 1 percent of all announcement’s, meaning that most anosmia is what we call acquired during the course of our lives. And the majority of this arises from nasal inflammation and respiratory illness. So, anosima as a result of post-fire infection accounts for about eleven percent of cases, and the irresponsible viral agents include rhinoviruses and influenza, but also coronaviruses such as the common cold. So, we shouldn’t be surprised that COVID due to another form of coronavirus should be associated with smell loss.

The evidence for this specific link between smell and COVID starts with this report from Wuhan, China, the very first epicentre of the disease on the 25th of February, it was reported that about 10 percent of a sample of hospitalized patients suffering from one of the chemical sensory losses  – taste or smell. This was followed by a report from Iran, another early epicentre, which suggested that about two thirds of participants reported some olfactory dysfunction. Of these, loss of smell was usually very sudden and presumably otherwise unexplained. From there, a raft of studies reported much the same thing, including this one from 12 hospitals across four European countries at the heart of the pandemic now reporting a very high proportion, 86 percent or 88 percent for  smell or taste in mild to moderate patients. Importantly, this loss was the first symptom in about twelve percent of cases. We’ll come back to this point.

I want to mention in particular that this study by Valentine Parma and colleagues, not only because it reports interesting results, which I’ll mention in a minute, but because it shows how the global pandemic has brought about a lot of highly collaborative research. So, in the wake of the first reports of small loss in COVID, a global consortium of researchers interested in the chemical senses was hatched and then grew. Currently, it stands at over 500 scientists from 56 countries who are working together on the issue. And through me, the University of Stirling is also involved in this endeavour. In the consortium’s first effort, we launched a survey which currently has more than 40,000 responses in one of 32 valuable survey languages.

In its first paper, we report that relative to pre-COIVID levels, participants report about an 80 percent loss of smell, a 70 percent loss of taste function and also significant chemesthesis loss. This relates to a third category of sensory experience relating to pain receptors in the mouth, for example, detecting burning from chilli or cooling from mint toothpaste or tingling from carbonated drinks. In addition, we asked about and found that such losses are not due at all to nasal obstruction or blockage, which is often seen in other post-viral infections, including influenza, so that odour molecules are prevented physically from reaching odour receptors at the top of the nose in flu and other such infections, but not in COVID. This suggests a different mechanism for sensory loss and we think it happens through direct interference with the support cells around the receptors in the nose and the mouth. It also seems in some cases to be invading the nerves leading to olfactory bulb, which may lead to much longer lasting and anosmia in some cases. So, COVID it is different from other post-fire olfactory loss.


So with growing evidence that COVID effects sensory loss, why are these symptoms so important for our response to COVID infection in the population? The short answer is that smell or taste loss is an unusually specific symptom of this particular disease and one that occurs both early and often. It might be the only symptom.

This is most clearly illustrated by results from a mobile app based study run by researchers at King’s College, London. Clearly, it’s a very large study with over two and a half million participants. They reported incidents of a very wide range of possible symptoms. In the paper, eventually published in the journal Nature Medicine, the study showed in both samples from the UK and the US that smell and taste loss was about three times more common in people who tested positive for COVID compared to those who were tested but had a negative result. In contrast, the other two core symptoms (persistent cough and fever) were only a little more likely to occur in people testing positive and negative.

The paper also compared the odds ratio for different possible symptoms. In this case, the odds ratio is a statistical measure of the likelihood of having or not having the symptom when testing positive for COVID relative to the same likelihood when testing negative. In short, the further to the right, the more precise the association is between the symptom and the disease. This graph shows how smell is such a very specific symptom, much more so than fever and persistent cough.

So, smell and taste loss may be the single most discriminating symptom and as we saw earlier, it can kick in very suddenly and possibly before other symptoms. Further data from the app study reports that in 15.9 nine percent of people with COVID positive tests, it’s the only symptom. This Venn diagram shows the degree of overlap or non-overlap between the three main symptoms and smell and taste loss is present in total in over 64 percent of the sample. But, as the researchers argue in this paper in The Lancet, almost 16 percent of the population would have been missed by ignoring this symptom: smell and taste loss, and these people should clearly be instructed to self isolate and be entered into testing and tracking procedures. It’s very important that these symptoms are recognized. Unfortunately, it took UK at least much longer than other countries to accept that sudden loss of smell and taste was a useful discriminating symptom. The calendar here shows some key dates for official recognition of these symptoms, beginning with the chief of the French health survey saying in a press conference on the 20th of March that people with sensory law should self isolate and contact their doctor. Four weeks later, as more reports came in, the World Health Organization officially listed it as a symptom, closely followed by the US. Other countries in the EU did the same thing during April.

If we then considered the UK, we can go back to March for official warnings to the government, notably an open letter to Public Health England jointly written by the leaders of two organisations representing Ear Nose and Throat surgeons in the UK and sent on the 23rd of March. The issue was then discussed by SAGE on the 25th of March, the government’s scientific body. But, despite this and the growing evidence, it wasn’t until the 18th of March, four and a half weeks after the World Health Organization, that the UK finally added it to the symptoms list. Anyway, here we are.

Tracking and improving your own sense of smell

I’ll end the lecture by touching on two implications that you may be interested in applying to your own experience. The first is that you might want to track your own sense of smell as a potential marker for COVID infection. You could do this at a very simple level by simply paying more attention to your smell and taste: finding something at home as a distinct aroma or taste and simply checking on it each day. If you’d like to do this more systematically and at the same time contribute to our collaborative research effort, you could head to the GCCR website and participate in one or both of our currently running studies. So, on arriving at the website, you will see this invitation to participate by checking one of the five situations they return you to one or both of our studies: the survey, which would normally do just once or tracking self-check, which you can do as many times as you like. We would suggest once a week to do the self-check. It’s very easy to do because it uses things that you should have at home. For the smell test, it would suggest that you find one item from each of these four category lists. It should be an item that you regularly have in or around.

So you might, for example, pick lemon from the first list, perhaps lemon juice that you always have in the fridge and then say cloves or oregano from your spice rack, a bottle of laundry detergent from the third list for example, or finally, some peanut butter or some coffee from the other items. It will ask you questions about how you experience these smells as you check in each time and there’s a similar procedure for taste and taste items.

The second thing to do is to try to improve your sense of smell by training. This might be particularly useful if you’re recovering from COVID, although the same applies also to any hopefully temporary loss of smell. While the evidence today suggests that most people suffering from COVID will regain their sense of smell within one or two weeks, post infection in about one in 10 people is taking quite a bit longer. Training might be useful, articularly in that latter group. Whatever your position, training most commonly works by taking for essential oils and sniffing at least twice a day for several weeks. Controlled experiments indicate that this can lead to a very much speedier recovery than not doing training at all.

Thank you for listening to my lecture. If you’d like to know more about what I’ve been talking about, these sites might be useful. The top two Abscent and Fifth Sense for his sense of sites packed full of information, advice and resources for anosmics. I included GCCR site again for taking part in studies. And finally, if you find it interesting that we use smell to communicate, and want to find more about the social function and using smell, you’ll find a link on my web page for rural society meeting held last year where there are recorded talks from some of the world’s leading experts at freely available, as well as links to an attached special journal issue which has free access to 17 papers published on this topic in April 2020. And once again, thanks very much for listening.

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