The story of Britain’s most successful Covid-19 app – yes, there is such a thing – started with twins.

Tim Spector, a professor of genetic epidemiology at King’s College London, has been probing and observing identical twins for a long time – and in Just 1992 he launched the registry TwinsUK to aid research on the subject. His interest was investigating how people with identical genes could grow up to develop radically different traits and illnesses. Take nutrition: identical twins might eat the same food, but react very differently – maybe one will develop an allergy while the other won’t; or their blood sugar level would draw dissimilar curves, with different repercussions on each sibling’s metabolism.

After decades spent researching twins at King’s College, in 2018 Spector teamed up with technologists Jonathan Wolf and George Hadjigeorgiou to launch a startup called ZOE. In partnership with research institutions and universities in various countries, the company started carrying out its own studies about twins and nutrition. Its first study, gauged how over 1,000 people (mostly twins) responded to various kinds of food. A similar study, with a larger sample, is currently underway.

ZOE's business plan was to combine the data gathered through these studies with machine learning, and eventually develop a consumer product able to predict every person’s individual response to what they eat. According to a company spokesperson, the product would be “like [genetic testing service] 23andme, but for eating.”

Then the coronavirus outbreak started. As the virus ripped through the globe, on March 20 Spector resolved to turn to TwinsUK’s volunteers to track whether they were showing any coronavirus symptoms, and how those symptoms progressed over time. He roped in ZOE, too – which was tasked with creating an app through which twins could report their symptoms. The initial plan was to have all volunteers tested for Covid-19, but the spokesperson says that it has since proved “impossible” to get hold of the testing kits.

While designing the app, the ZOE team thought that its utility could go well beyond keeping tabs on twins. “We realised that this was something that we could make even more useful overall – by getting everybody in the public to participate,” ZOE president George Hadjigeorgiou says. “[This can] help understand how the virus is progressing [in the UK] and also to help understand whether a particular area is at a high risk.”

Hadjigeorgiou says the app was developed and inaugurated with a sense of extreme urgency. “We just put together a ‘SWAT team’ and we worked through the weekend to get it live.” The result? The Covid Symptom Tracker. The app went live on Tuesday, both for androids and ioses phoness. By Wednesday, Britons had downloaded it over a million times.

The app is extremely simple. Users are asked to create an account, and provide information, including age, sex at birth, height, weight, postcode of residence, pre-existing health conditions, and habits such as smoking. Then, they’re asked daily to report any symptom that could be associated with Covid-19. The list currently includes fever, persistent cough, fatigue, shortness of breath, loss of smell or taste, hoarseness, chest pain, abdominal pain, diarrhoea, drowsiness and confusion, and lack of appetite. Hadjigeorgiou says that as more symptoms get flagged by the international research community as potentially relevant to Covid-19 detection, they will be added to the app.

The data gathered this way will be shared with researchers at King’s College and at the Guys and St Thomas' Hospitals, to get a better sense of how the pandemic is spreading through the UK, and which symptoms should be regarded as telltale signs of a novel coronavirus infection as opposed to the common cold or the seasonal flu. Hadjigeorgiou says that the company has also been trying to get in touch with the UK government and the NHS in order to share their data with them, although whether that would result in a partnership was unclear as of Wednesday. The company spokesperson says that ZOE is talking with NHSX, the NHS’s digital unit.

According to Hadjigeorgiou, information about how symptoms are being reported in various parts of the UK might be even made public. “What we will try to do [Wednesday night] or [Thursday] at the latest is publish the first set of data that we collected,” he says. “That will be geographical data but whether we'll be able to publish down to the postcode level – maybe not yet. But the intent is to share all this information in a way that's useful for people.”

Read more: Don't believe the headlines saying half the UK has had coronavirus

Yet, a banner published on the app’s website this morning reads: “We are not sharing any analysis or data from the app except with researchers at King's College London and the NHS. As soon as they have analysis they are happy with, they will share it.” It is unclear whether that will affect the timetable of the publication of the locations data.

The company is planning to launch the app in the US in the near future. Hadjigeorgiou says that partner institutions there might include high-profile universities and hospitals with which ZOE – which opened a Boston office last year – had already been working for its nutrition studies.

The app privacy policy mentions that anonymised data might be shared with US-based organisations including Harvard University, Stanford University, Massachusetts General Hospital, Tufts University, and Berkeley University. The spokesperson also says that the company has been in touch with American hospitals. One paragraph in the app’s privacy policy underlines that the data will be handled in accordance with GDPR, but that they might not enjoy similar levels of protection when transferred to countries like the US.

Beyond that, Hadjigeorgiou cannot bring himself to make prediction about Covid Symptom Tracker’s future. “I don't think anybody will know the answer of how the world will look like in two weeks,” he says. “It would be ideal if this app can be part of the NHS’s efforts because that's the only way we can really deliver something that will have everything it needs to be integrated with what public health systems are doing.”

As of Thursday morning, the app ranked first on Google Play Store's list of most downloaded health apps in the UK. That is despite the fact that, currently, searching for “Covid” on Google Play returns no result apart from an app run by the World Health Organisation.

Gian Volpicelli is WIRED's politics editor. He tweets from @Gmvolpi

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This article was originally published by WIRED UK