Taking a deep look at data on thousands of twins has shown how some Covid-19 symptoms might be more influenced by our genetics than others.

Researchers at King’s College London analyzed data on 2,633 identical and fraternal twins who have been using their Covid-19 Symptom Tracker app, which also includes the data of 2.7 million other users. The study, which has not yet been peer-reviewed, can be found on the preprint server medRxiv.

Their preliminary findings suggest that genetic factors could be responsible for about 50 percent of the differences between people’s symptoms of Covid-19, the list of which seems to be ever-growing. The development of some key Covid-19 symptoms, such as fever, fatigue, shortness of breath, diarrhea, and the loss of taste and smell, appear to be strongly influenced by genes. On the other hand, environmental factors appear to explain the development of symptoms such as a hoarse voice, cough, chest pain, and abdominal pain. This potentially explains why the virus appears to impact some people severely while others experience relatively mild or no symptoms.

The Covid-19 Symptom Tracker app asks people in the UK on a daily basis about the presence or absence of common symptoms. You can check out the latest figures from the app on its interactive map. The +2,600 twins using the app were recruited from TwinsUK, one of the most detailed research projects on twins in the world. Together, this information was used to see whether typical symptoms of a likely COVID-19 infection were more or less common in identical twins (who share 100 percent of their genes) compared with non-identical twins (who share 50 percent of their genes, just like regular full siblings).

“The idea was to basically look at the similarities in symptoms or non-symptoms between the identical twins, who share 100 percent of their genes, and the non-identical twins, who only share half of their genes,” Professor Tim Spector, a genetic epidemiologist at King’s College London, told The Guardian. “This disease is very weird, the way it has a very different presentation in the population in different people – what we are showing is that [it] isn’t random. It is not mainly due to where you live or who you have seen; a lot of it is something innate about you."

There are some drawbacks to the research, namely because all the results are based on self-reporting, which means a fair amount of subjectivity might sneak into the results. Nevertheless, the unique project offers a rare opportunity for scientists to study large amounts of data on Covid-19 from people who have not sought out medical attention.

“Our twins are fantastically committed, enthusiastic health research participants who have already been studied in unprecedented detail, putting us in a unique position to provide vital answers to support the global fight against Covid-19,” Professor Spector said in a statement.

“The more of the public that also use the app, the better the real-time data we will have to combat the outbreak.”