Study design, population and setting
This study was a retrospective, observational study conducted over a study period of six years from January 2012 to December 2017. Osaka prefecture is the largest urban area in western Japan, with an area of 1905.14 km2 and 2.3 million elderly people over 65 years old among the population of 8.8 million people . We included the cases of calls for the telephone triage service and the cases for which the triage nurse performed telephone triage using software in this study. This study was approved by the ethics committee of Osaka University Graduate School of Medicine (approval number: 16070). The requirement for informed consent was waved because the telephone triage data were anonymized. This article was written based on the STROBE statement to assess the reporting of cohort and cross-sectional studies .
Outpatient surveillance of influenza-like illness in Japan
The surveillance program for infectious diseases in Japan, which was begun in 1981 and forms the basis for influenza surveillance of outpatients [12,13], was revised and updated to its current format following the revision of the Infectious Disease Control Law in 2014 [12-15]. The system requires mandatory reporting of nationally notifiable diseases and sentinel surveillance systems for various types of infectious diseases .
Influenza is included in the sentinel surveillance system. The numbers of influenza patients from 5000 medical institutions across Japan are reported weekly to local health centers. Sentinel sites were chosen on the basis of their geographic distribution, whether a clinic or hospital, and population densities. These sentinel sites report influenza-like illness according to the following criteria: (1) sudden onset of illness, (2) fever >38℃, (3) symptoms of upper respiratory inflammation, and (4) systemic symptoms such as general fatigue. A case meets the reporting criteria if the patient meets all four of the above symptoms or has at least one of the four symptoms along with a positive rapid diagnostic test.16 Information on the age group and sex of the patients is reported every week by the sentinel sites and is then transferred from the local health centers to each prefectural government’s Department of Public Health, which aggregates it into a prefectural report. The report is then received by the National Institute of Infectious Diseases in Tokyo, which is affiliated to the Ministry of Health, Labour and Welfare. Within Osaka, 300 medical institutions report influenza patients to 10 local health centers .
Telephone triage service in Osaka, Japan
As with the telephone triage service in Tokyo, that in Osaka prefecture is also a public service  and can be freely used by anyone. A triage nurse at the service uses software with a protocol for telephone triage in Japan and determines the urgency of the client. There are 97 different protocols of telephone triage for chief complaints in Japan, and the urgency of the client is determined by selecting the signs and symptoms related to each chief complaint. As with telephone triage service in the departments of veteran affairs in the United States , Canada and United Kingdom [20-22], the telephone service in Osaka provides the client necessary services such as ambulance dispatch and guidance of medical institutions based on the result of the urgency . The software records the sex and age group of the client, the time when the telephone triage was started and ended, the chief complaint and selected signs and symptoms, the urgency of the client and whether an ambulance was dispatched.
The main endpoint was the weekly number of influenza patients in Osaka. These data were acquired from data on the website of the Osaka Institute of Public Health .
Using a linear regression model, we calculated the R2 of the regression model to assess the relationship between the number of telephone triages and the number of influenza patients in Osaka. The covariates in the linear regression model were the weekly number of telephone triages for fever and the week number. We defined the week number as a binary variable, with the week including January 1st as “week number = 1”. Next, according to the season (January-March, April-June, July-September, October-December), we calculated the Spearman’s correlation coefficient, R2, between the predicted weekly number of influenza patients from the linear regression model and the weekly number of influenza patients for each season. Statistical significance was defined as P<0.05, and statistical analysis was performed by SPSS version 23.0J (IBM Crop., Armonk, NY).