The first results from our COVID-19 symptom tracking app show that losing your sense of smell or taste is a stronger predictor of coronavirus infection than fever. 

According to the NHS, the most common symptoms of coronavirus infection are fever (high temperature) and a new continuous cough. 

However, many people are also reporting symptoms such as aches and pains, chills, tiredness, headaches and diarrhoea. But there have also been plenty of anecdotal reports of patients suddenly losing their sense of smell or taste, particularly in the early stages of infection. 

Is loss of smell or taste a symptom of COVID-19?

To find out more about the early warning signs of COVID-19, our researchers at King’s College London and ZOE have been analysing data from by nearly 2 million 400,000 ‘stay-at-home scientists’ who have been submitting daily health updates through the COVID-19 symptom tracking app. 

We found that of 400,000 people reporting one or more symptoms between 24-29th March::

• 53% had fatigue or tiredness
• 29% had a persistent cough
• 28% suffered from shortness of breath
• 18% had lost their sense of smell (anosmia) or taste
• 10.5% were suffering from fever

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Of these 400,000 people, 1,702 had been given a test for COVID-19. 579 had tested positive, while 1,123 were negative. 

Doing a quick bit of maths tells us that of the people who were definitely infected by coronavirus (i.e. tested positive), nearly six in ten (59%) reported losing their sense of smell or taste compared with only around two in ten (18%) who tested negative.

This map shows the hotspots in the UK where app users are reporting loss of smell or taste:

Our team then crunched all the symptom data together to build a computer programme (model) combining data on loss of smell and taste, fever, persistent cough, fatigue, diarrhoea, abdominal pain and loss of appetite to predict whether someone is likely to be suffering from COVID-19 or not, even if they haven’t been tested for the virus.

When we used the model to look across the 400,000 people who had reported symptoms but not yet had a COVID-19 test, we found that more than one in ten (13%) are likely to be infected by the virus based on their combination of symptoms. 

That adds up to an extra 50,000 individuals who are likely to have as yet unconfirmed COVID-19 infections.

Why does COVID-19 cause loss of smell and taste?

Our senses of smell and taste are closely intertwined. Most of what you taste when you eat and drink actually comes from its smell, rather than its effects on the tastebuds on your tongue.It’s not entirely clear why coronavirus infection leads to a loss of smell and taste. 

The key probably lies in the olfactory epithelium: a layer of neurons (nerves) and other supporting cells at the back of the nose that detect smelly chemicals in the air and relay this information into the brain. 

One idea is that the virus causes inflammation and swelling in the back of the nose, interfering with normal smelly sensations. Scientists at Harvard have also just published a pre-print (non-peer-reviewed results) suggesting that the coronavirus responsible for COVID-19, known as SARS-CoV-2, may infect the supporting cells in the olfactory epithelium that surround the neurons responsible for detecting scents.

What should I do if I lose my sense of smell or taste?

Loss of smell or taste is one of many symptoms of COVID-19, and people respond in different ways to infection (something we’re working hard to understand from the data we’re collecting). 

It’s important to remember that the common cold and other respiratory infections can also make you lose your sense of smell. Conversely, you may be infected with coronavirus without suffering any loss of smell or taste at all. And while loss of smell does seem to be a genuine early-warning sign of COVID-19, fever and coughing are still the most important symptoms to look out for. 

Our lead researcher Professor Tim Spector says, "When combined with other symptoms, people with loss of smell and taste appear to be three times more likely to have contracted COVID-19 according to our data, and should therefore self-isolate for seven days to reduce the spread of the disease."

Given the circumstances, the best advice for anyone noticing loss of smell or taste is to treat yourself as being infectious. That means following the Government guidelines and starting self-isolation to protect your family, the NHS and the wider community by stopping the spread.