Improving State-Level Influenza Surveillance by Incorporating Real-Time Smartphone-Connected Thermometer Readings Across Different Geographic Domains

Background. Timely estimates of influenza activity are important for clinical and public health practice. However, traditional surveillance sources may be associated with reporting delays. Smartphone-connected thermometers can capture real-time illness symptoms, and these geolocated readings may help improve state-level forecast accuracy. Methods. Temperature recordings were collected from smart thermometers and an associated mobile phone application. Using temperature recordings, we developed forecasting models of real-time, state-reported influenza-like illness (ILI) 2 weeks before the availability of published reports. We compared time-series models that incorporated thermometer readings at various levels of spatial aggregation and evaluated out-of-sample model performance in an adaptive manner comparing each model to baseline models without thermometer information. Results. More than 12 million temperature readings were recorded from over 500 000 devices from August 30, 2015 to April 15, 2018. Readings were voluntarily reported from anonymous device users, with potentially multiple users for a single device. We developed forecasting models of real-time outpatient ILI for 46 states with sufficient state-reported ILI data. Forecast accuracy improved considerably when information from thermometers was incorporated. On average, thermometer readings reduced the squared error of state-level forecasting by 43% during influenza season and more than 50% in many states. In general, best-performing models tended to result from incorporating thermometer information at multiple levels of spatial aggregation. Conclusions. Local forecasts of current influenza activity, measured by outpatient ILI, can be improved by incorporating realtime information from mobile devices. Information aggregated across neighboring states, regions, and the nation can lead to more reliable forecasts, benefiting local surveillance efforts.