Usefulness of a Pluralistic Approach in Sentinel Surveillance: Seasonal Influenza Activity Based on Case Counts per Sentinel Site in the National Epidemiological Surveillance of Infectious Diseases Program and Test Counts, Case Counts, and Test Positivity from the National Hospital Organization.

In Japan, based on the National Epidemiological Surveillance of Infectious Diseases (NESID) Program, influenza cases from approximately 5,000 sentinel sites are monitored weekly as part of influenza surveillance (as the number of influenza cases per sentinel site). One limitation is that the number of influenza tests conducted is not reported. However, the National Hospital Organization (NHO), with approximately 140 hospitals, routinely publishes three indicators: the number of influenza tests, the number of influenza-positive cases, and test positivity. We used the NESID and NHO data from April 2011 to June 2022 to assess the usefulness of multiple indicators for monitoring influenza activity. The temporal trends of the NHO and NESID indicator were similar, and the NHO indicator levels correlated well with those of the NESID indicators. However, influenza positivity in the NHO data showed an earlier rise and peak time than that in the NESID indicator. Importantly, during the non-epidemic summer periods and the coronavirus disease 2019 pandemic, a sizable number of influenza tests were still performed at NHO hospitals, with results showing considerably low case counts and test positivity. These data show that a relatively small number of sentinel sites is sufficient to monitor influenza activity nationally and that utilizing multiple indicators can increase our confidence in situational awareness and data interpretations.