Introduction
COVID-19 is a new infection in humans. The symptom profile, disease progression and complication rates are still relatively unknown.1 From the available evidence, four broad categories of illness have been postulated. ‘Mild COVID-19’ makes up over 80% of cases and is typically a self-limiting infection similar to the common cold, resolving without intervention. ‘moderate COVID-19’ typically has features of viral pneumonia in the absence of hypoxia, progressing to ‘severe COVID-19’ typically when patients require oxygen therapy. ‘Critical COVID-19’, where ventilatory support is typically required, occurs in less than 5% of cases.2 The rate of disease progression is not fixed: early intervention and various management strategies can reduce the rate of progression to critical illness and death.2–6
While the infection fatality rate is yet to be determined, COVID-19 is associated with a substantive mortality. Over a period of 5 months, COVID-19 has led to more than 300 000 deaths, with more than half these deaths occurring within the last month.7
The risk of mortality is affected by a number of risk factors. Coexisting health problems such as diabetes, heart disease and cancer have been implicated as conferring a higher risk of mortality in COVID-19.8 Age appears to be the most striking and consistent risk factor for COVID-19 related mortality.9 Based on current data, the mortality rate in patients under 50 years of age is thought to be less than 1.1%, rising to around 14% in those over 80 years of age.10
Variation in mortality also seems to exist between countries.11 Initially, this variation was thought to be predominantly related to the method of recording deaths and the total number of tests conducted (ie, the detection of milder cases).12 As the pandemic spreads across the globe, it is becoming increasingly clear that how a country responds to the pandemic impacts the number of deaths their locality will experience.6 11
The national response to the COVID-19 pandemic has many important tenets. On the public health side, infection control initiatives attempt, in part, to mitigate the surge of infections that can accompany new pathogens where there is little circulating immunity. This reduces mortality by preventing the healthcare services from being overwhelmed, thus permitting improved access to medical management for those who need it.6 The clinical response to COVID-19 also centres on access to treatment. To successfully reduce the mortality rate, those patients who are developing more severe disease must be identified.3
Identifying those patients with COVID-19 that require treatment is challenging. First, COVID-19 has a broad range of presentations that can mimic common conditions that rarely require clinical assessment (eg, the common cold).1 Second, there are no clinical signs or symptoms that reliably predict who will progress to severe disease.3 As such, the clinical community is left with a large number of potential cases without any clear symptom indicators for: (1) who has the disease and (2) who is developing more severe disease. The problem is compounded further as more serious, life-threatening conditions (eg, bacterial pneumonia and sepsis) can mimic any stage of COVID-19 disease.13 14
National ‘Symptom Checkers’ have been implemented in many countries in the hope of reducing this burden faced by healthcare services. Symptom checkers are self-assessment tools. The individual—typically online or via computer application—enters their symptoms into a predetermined platform and from there a predetermined algorithm produces an outcome (usually advice). This is a form of self-led triage. It is hoped that such self-directed assessments will enable the identification of potential cases15 and will correctly triage those individuals who would benefit from clinical assessment and/or management into further care.16 For such a hope to be realised, symptom checkers must be able to determine mild conditions from severe conditions.
While self-triage has been used for some years in non-emergency conditions to varying degrees of success,17 self-triage has never before been used in a pandemic setting and as yet the efficacy and safety has not be formally studied. Caution must be exercised as, to date, studies examining symptom checkers have had mixed and disappointing results in general—demonstrating poor diagnostic performance (34%–58%) and questionable triage performance (55%–80%).18 The stakes are high, in that a failure to triage serious medical conditions (such as severe COVID-19, bacterial pneumonia or sepsis) in for further assessment will inevitably lead to delayed treatment and higher mortality.19–22
Here, we test the performance of four nationwide symptom checkers from four nations to ascertain how safe and efficient each symptom checker is in differentiating mild from severe COVID-19 cases, and how well they detect time-sensitive COVID-19 mimickers such as bacterial pneumonia and sepsis.