Now-casting the COVID-19 epidemic: The use case of Japan, March 2020

Stephan Gloeckner; Gerard Krause; Michael Hoehle

Reporting delay of cases is a common obstacle for real-time risk assessment and management of epidemics. In continuously updated epidemic curves, case numbers for the most recent epidemic week or day usually appear to be lower than the previous, suggesting a decline of the epidemic. In reality however, the epidemic curve may still be on the rise, because reporting delay prevents the most recent cases to already appear in the case count. This phenomenon often introduces misperceptions in the general public and in media coverage as can again be seen in the current COVID-19 epidemic. 1 Reporting delay can be decomposed into several parts: (a) the time passing from symptom onset until the patient seeks medical service, (b) the time passing between this service and initiation of a laboratory test, (c) the time between this test initiation and completion of its result, (d) the time between test completion and its registration at the local health authority level and (e) the time passing from transmitting the case report from that local level to the (usually national) health authority level that will generate and publish the epidemic curve. 2 Technology and rapid procedures can minimize the delay occurring from the first contact with the health care system and appearance of a case in the daily case count at national level, but they can hardly reduce the delay between appearance of first symptoms and contact to the health care service. 2 The latter is subject to a variety of factors such as severity of the disease, public awareness for it, individual benefit to seek health services, accessibility to those services, some of which may also be subject to change during an epidemic. For all those reasons, reporting delay is not only specific to a country and its surveillance and health care system, but to the disease in questions and the phase of the epidemic.