## 3 Future Perspectives of Wearable Devices during Pandemics  
Wearable technologies for monitoring patients during the COVID-19 pandemic include the following: (1) respiratory rate, respiratory airflow sensing thermistor, humidity/carbon dioxide sensor, chest wall movement detection by strain/triboelectric/accelerometer, and derivation from cardiac signals using electrocardiography and photoplethysmography; (2) heart/lung sound, piezoelectric acoustic sensing; (3) oxygen saturation, optical sensing; (4) heart rate, electrical/capacitive sensing; (5) blood pressure, multimodal sensing; (6) body temperature, thermal sensing; and (7) cough, mechanical or piezoelectric sensing [5]. Wearable devices have been put to practical use for the management of patients with COVID-19; however, most of them are still in the process of development in terms of their validity and reliability in clinical practice. Real-time cardiovascular disease monitoring requires a highly powerful wireless communication technology for processing the large amount of data; however, currently, wearable medical devices cannot fully satisfy the real-time data requirements [33]. Furthermore, the diagnosis and instructions provided by doctors are manual processes; therefore, it is challenging to develop an efficient monitoring platform that can automatically detect an abnormality and provide prompt and accurate diagnosis based on the data from wearable devices. Arterial intelligence will play a crucial role in monitoring patients with chronic diseases as well as COVID-19 [34]. Wearable activity trackers may not be very useful in the management of patients with COVID-19; however, previous studies suggest that they make a significant contribution to lowering the risk of severe illness from COVID-19 through self-management of physical activity. In this review, wearable activity trackers were used only to measure physical activity during self-isolation, social distancing, or lockdown. In contrast, [PERSON] et al. [35] developed a smartphone application that collects wearable activity tracker data and assessed whether self-reported symptoms and sensor data can differentiate patients with COVID-19 from other symptomatic subjects. The area under the curve for differentiating patients with COVID-19 and symptomatic subjects who were negative for COVID-19 was 0.80 (interquartile range, 0.73-0.86) when using the application, which suggests that the combination of wearable device data and clinical symptoms improves the diagnostic accuracy of COVID-19. This study is highly suggestive of a promising future of wearable devices. It is essential for clinicians to have a perspective on how to utilize wearable device data in the management of patients during the pandemic. This review has shown that the pandemic and/or non-pharmaceutical public health measures significantly reduce people's daily physical activity, which should be harmful for metabolic health. Indeed, the pandemic and/or non-pharmaceutical public health measures adversely impact patients with diabetes by physical inactivity, unhealthy diet, and limited access to healthcare [36]. In addition, obesity, of which physical inactivity is one of the major causes, increases the risk of other metabolic diseases such as type 2 diabetes and nonalcoholic fatty liver disease. A comparative risk assessment analysis in the US reported that 30% of COVID-19 hospitalizations were attributable to obesity and 21% were attributable to diabetes [37]. Thus, to increase or at least maintain daily physical activity during the pandemic benefits not only individual health but also public health by reducing the burden on healthcare systems. The problem is when and how to intervene in such sedentary individuals. The studies included in this review showed that the daily step count was decreased by approximately 1000-4000 steps per day during the pandemic. Considering that the risks of all-cause mortality (6-36%) and cardiovascular disease (5-21%) are reduced by 1000 daily step counts [38], early intervention is preferred.