• How do COVID-19 symptoms compare across countries and platforms? 
• My sense of smell has changed - should I get a COVID test?
• I feel ill but my sense of smell is normal - does that mean I don’t have COVID-19?
• How important is community science for public health?

Early in the pandemic during 2020, we used symptom reports and testing data from millions of ZOE COVID Study app contributors to confirm that symptoms like loss of smell (anosmia), fever, and cough could predict whether someone was likely to have COVID-19, even without a test.

Here’s why it’s still worth watching out for any changes in smell or taste, especially if you don’t have access to testing.

How do COVID-19 symptoms compare across countries and platforms? 

Our research colleagues at King’s College London - together with scientists in Israel and the US - have been looking back over our data from the first wave of the pandemic and comparing it with information from other community science platforms in the US and Israel to find out whether the same symptoms and predictions hold up in different populations.

The team combined ZOE COVID Study data from the UK and US with data from The University of Maryland Facebook COVID-19 Symptom Survey and the Israel Corona Study. Although each of the platforms is slightly different, they all ask users to self-report symptoms and test results. 

The researchers looked at data from more than 10 million respondents from April 1st 2020 to July 31st 2020, analysing half a million COVID tests and millions of health reports.

The analysis showed that loss of smell (anosmia) or loss of taste (ageusia) was consistently the strongest predictor of a COVID-19 infection across all platforms, regions, and populations. In fact, someone with either of those symptoms was 17 times more likely to test positive for COVID than someone without. 

What’s more, loss of smell or taste remained the best predictor of a positive test result regardless of a person’s age, sex, or illness severity. However, the team’s analysis also showed that having a high temperature (fever) and lung symptoms like shortness of breath or coughing were also strongly associated with having a positive COVID test.

The analysis confirms our findings early in the pandemic that loss of smell is a strong predictor of a COVID infection. Thanks to these findings, anosmia was added to the list of official symptoms to get an NHS PCR test, alongside cough and fever.

The results of the study are  in a paper published in the Lancet Digital Health. 

My sense of smell has changed - should I get a COVID test?

We know that the symptoms of COVID-19 are changing as the pandemic unfolds and new variants emerge. Back in Spring 2020, the key symptoms were cough, fever and loss of smell. 

Today we know that there are more than 20 symptoms of COVID, and for many people it feels like nothing more than a mild cold. This is especially true if you’ve been vaccinated, which can change the symptoms you experience.

The top five most common symptoms in double-vaccinated people are:

1. Headache
2. Runny nose
3. Sneezing
4. Sore throat
5. Loss of smell
   

Although loss of smell is fifth on the list, the other four often have other causes, such as hayfever or a regular cold. But loss of smell is much more likely to be due to COVID-19 than another cause, especially when rates are still so high.

Taken together, all the evidence points to one conclusion: you should self-isolate and get a COVID test if you notice any changes to your sense of smell or taste, even if you feel well otherwise. ‍

It’s particularly important to stay home and get tested if you have any other symptoms, such as tiredness, headache, fever or a new, persistent cough, especially if you’ve been ‘pinged’ for having been in contact with someone who has tested positive.

It can be difficult to notice changes to your sense of smell as you go about your daily activities, but you can easily check for anosmia at home using things with distinctive smells like coffee, garlic, coconut or orange. 

I feel ill but my sense of smell is normal - does that mean I don’t have COVID-19?

Although this analysis shows that anosmia is the best predictor of having COVID-19, you can still be infected even if your sense of smell is perfectly normal. ‍

If you feel under the weather, stay home and get a test, even if your sense of smell seems normal and you’ve been vaccinated.

How important is community science for public health?

This analysis demonstrates the power of community science apps like the ZOE COVID Study for providing reliable, near real time insights into public health on a mass scale.

Importantly, our research shows that loss of smell is a strong predictor of COVID-19 infection, even in the absence of a test. Although that information may be less useful here in the UK where we have good availability of free testing through the ZOE COVID Study app and the NHS, it could be life-saving in countries with high rates of COVID-19 but little access to tests.

The ZOE COVID Study is the biggest study of its kind globally and has made huge contributions to the COVID response in the UK and abroad. But we couldn’t have done any of this without you -  our amazing contributors. 

Throughout the pandemic, your data has provided valuable insights about what’s going on with COVID-19 on the ground and helped to change the course of the pandemic.

We’re not yet at the end of the pandemic. If you haven’t already, download the ZOE COVID Study app and join a million other users logging daily health reports and playing their role in the fight against COVID-19.

Stay safe and keep logging.