A coronavirus patient's phone may be swarming with the infection.
After more than a year of restrictions, social distancing is set to end in England on 19 July, freeing people up to attend nightclubs, festivals and concerts.
With no vaccine 100% effective, and new coronavirus variants emerging, screening for the infection may be required to prevent venues from becoming "superspreading" events.
Scientists from University College London have now found swabbing smartphone screens for the virus detected the infection up to 100% of the time.
This may offer an alternative to "physically unpleasant" swabs, which have to be inserted up the nostril or rubbed against the tonsils, putting some people off the test.
"Like many, I was very worried about the economic and social impact the pandemic would leave behind, particularly in lower-income countries," said lead author Dr Rodrigo Young.
"We knew the only effective way to stop the spreading is to regularly test as many people as possible, but this was not happening because it's too expensive and uncomfortable.
"We immediately knew this was something special, as PoST [phone screen testing] is a method that would not only make COVID-19 [the disease caused by the coronavirus] mass testing much easier but could also be used to contain outbreaks of new viruses, to avoid future pandemics".
Dr Young founded the Chilean based start-up Diagnosis Biotech, which has been used to screen people in offices and schools in the South American country.
The company is developing a machine that could sample phones for the coronavirus, sending the results via text to minimise contact.
Putting a prototype to the test, the UCL scientists swabbed the screen of 540 people's phones.
Fifty-one of the participants later tested positive via a standard polymerase chain reaction (PCR) test, considered the gold-standard method of diagnosis.
Held close to the mouth, phone screens may become contaminated with coronavirus-laden droplets or "secretions".
The participants' swabs were then also assessed via a PCR test.
Between 81% and 100% of the coronavirus patients with a "high viral load" – a lot of the virus in their body – tested positive via PoST, "suggesting this method is effective in identifying COVID-19 contagious individuals".
Of all the positive samples, just over three quarters (76%) "corresponded to individuals with no specific COVID-19 symptoms", suggesting screen swabs may also have potential among asymptomatic coronavirus carriers.
The phone swabs detected when a participant did not have the coronavirus 98% of the time, the results show.
They also picked up on the Alpha, Beta and Gamma coronavirus variants, which emerged in Kent, South Africa and Brazil, respectively.
Cleaning or sanitising the phone before the sampling was not found to affect the results.
"Overall, we report PoST is a new non-invasive, cost-effective and easy to implement smartphone-based smart alternative for [coronavirus] testing", the scientists wrote in the journal eLife.
The test "could help contain COVID-19 outbreaks and identification of variants of concern in the years to come".
PoST could replace PCR swabs, which "require specialised staff" and cost up to $25 (£17.99) in Chile, compared to as little as $1 (71p) for the smartphone test.
Nevertheless, PCR swabs – including those from phones – have to be processed in a laboratory, which usually takes several hours.
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