Thousands of people have been assessed in the community with likely covid-19. To avoid overwhelming hospitals and to prevent infection of staff and other patients, the majority are sent home with little structured follow-up other than advice to get in contact with healthcare providers if their symptoms worsen. However, some patients are at risk of rapid deterioration, with hospital admission sometimes too late for effective treatment. This has led to calls for more active home monitoring. 
Proactive telemonitoring is widely used in long-term conditions where patients self-monitor and electronically report their symptoms and physiological readings once or twice daily to their clinician. These systems have the potential to identify early deterioration of covid-19 patients, thereby improving safety while still minimising admission.
How telemonitoring should be delivered during covid-19 will vary according to local care pathways and infrastructure, but all implementations have to address common challenges including usability, choice of data to collect, data transmission, and evaluation and optimisation.
Systems should be selected that are as simple as possible for clinicians to deploy and explain to patients, for patients (or carers) to interact with and for clinicians to access the data sent back by patients. Unnecessary features or complexity will reduce widespread adoption. 
Early data from the pandemic suggest that symptoms such as increasing breathlessness and high fever are predictive of serious illness as are physiological measures such as pulse rate and oxygen saturation (SpO2), which are easily measured. Respiratory rate, a strong predictor of outcome, is more challenging to measure remotely, but recently pulse oximeters, which estimate respiratory rate from pulse-wave variability, and apps which use mobile phone cameras to visualise thoracic movements are becoming available. [3-7]
Ideally, data transmission should work across different types of phones and computers and, though challenging, link to health service systems using open standards to provide immediate robust data. Telemonitoring systems that require personal smartphones or tablet PCs and wi-fi access could exclude older and poorer people who are more vulnerable.  Simpler SMS-based systems are available now, but other systems (usually smartphone or tablet-based) may, in principle, provide better integration with NHS electronic health records.  A telemonitoring system is being piloted in clinical care in Scotland by the Scottish Technology Enabled Care Programme. Patients with covid-19 considered at risk of deterioration are given a pulse-oximeter and thermometer. They are sent a series of texts each day asking them to text back information on their temperature and any symptoms of breathlessness daily and pulse-oximetry readings twice daily. They are automatically advised to contact services urgently if their symptoms or physiological signs suggest deterioration.
Covid-19 requires a rapid implementation at scale of a new kind of telemonitoring. Any implementation therefore needs to be within an evaluative framework which examines impact on clinician workload, usability, patient acceptability and equity of access. Time-stamped telemonitoring data, linked to outcomes, can facilitate development of prediction tools and optimise the intervention to determine the utility and safety of different measures. For example, it is uncertain what level of SpO2 and respiratory symptoms should trigger action in covid-19. Emerging reports suggest that some people experience little breathlessness despite a very low SpO2 while others with significant disease maintain SpO2 through increased respiratory effort but then suffer rapid decompensation. [1,10] Asking patients to walk briskly for a minute may demonstrate desaturation in borderline patients or its absence reassure, but this needs to be established.
Routine telemonitoring of those at greater risk of severe disease offers the potential to detect deterioration early, and to provide regular support and advice without compromising quarantine and exposing healthcare workers to unnecessary risk.
Brian McKinstry, Emeritus Professor of Primary Care eHealth, Usher Institute, University of Edinburgh
Lionel Tarassenko, Professor of Electrical Engineering, The institute of Biomedical Engineering, Department of Engineering Science, University of Oxford
Chris Paton, Head of Global Informatics Group, Nuffield Department of Medicine, University of Oxford
Bruce Guthrie, Professor of General Practice, Usher Institute, University of Edinburgh
Competing interest statement: All authors have completed the Unified Competing Interest form (available on request from the corresponding author) and declare: BM is in receipt of consultancies from the Scottish Government to provide clinical leadership in scaling up telemonitoring of high blood pressure and from Pharmatics a tech company developing AI based telemonitoring apps. He is also in receipt of grants related to telemonitoring from the British Heart Foundation, Chief Scientist Office of Scotland and the Stroke Association. LT is the R&D Director for Sensyne Health and holds share options in the company. He is also a non-executive (Founder) Director of Oxehealth and holds shares in the company. His University Department receives funding from Sensyne Health for unrelated research. CP has had travel, accommodation and speaker at the PedMedDev and SMIT 2019 conferences in Germany, and ICLR 2020 in Ethiopia and he is the director of New Media Medicine Ltd., a digital health consultancy company.
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