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

BackgroundReporting delays in disease surveillance impair the ability to assess the current dynamic of an epidemic. 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, the epidemic curve may still be on the rise, because reporting delay prevents the most recent cases to appear in the case count. In context of the COVID-19 epidemic and for countries planning large international gatherings, such as the Summer Olympic Games in Japan 2020, the ability to assess the actual stage of an epidemic is of outmost importance. MethodsWe applied now-casting onto COVID-19 data provided by the nCoV-2019 Data Working Group to evaluate the true count of cases, by taking into account reporting delays occurring between date of symptom onset and date of confirmation. FindingsWe calculated a decrease of reporting delay, from a median delay of ten days in calendar week four 2020 to six days in calendar week eight, resulting in an overall mean of 4.3 days. The confidence intervals of the now-casting indicated an increase of cases in the last reporting days, while case country in that same time period suggested a decline. InterpretationAs a specific use case this tool may be of particular value for the challenging risk assessment and risk communication in the context of the Summer Olympic Games in Japan 2020 and similar situations elsewhere.