2019 Volume 68 Issue 4 Pages 352-357
Objectives: It has been indicated that there is a correlation between the epidemic of seasonal influenza and absolute humidity. Studies on influenza epidemic in Japan mainly use weekly reports such as the Infectious Diseases Weekly Report from the National Institute of Infectious Diseases. Also, many of such studies are focused on large areas and are conducted at a prefectural level. This study analyzed trends in shorter-term influenza epidemic in small areas at a city level, based on existing knowledge. Particularly, the study compared the number of days of class closure and absolute humidity on the two days preceding the announcement of closure with the aim of examining the correlation between changes in absolute humidity and the number of days of subsequent class closure during the influenza epidemic season.
Methods: Data on the dates of class closure and the number of days of class closure were aggregated on a daily basis and weather data from thermo-hygrometers set at 26 sites in the city were collected. The relationship between influenza epidemic and absolute humidity were compared with reference to these data.
Results: An examination of the relationship between the number of days of class closure and absolute humidity on the preceding day of the announcement of the closure during the influenza epidemic season in the 2010, 2011 and 2014 academic years revealed a tendency for class closures to be associated with low absolute humidity. Large-scale class closures occurred when absolute humidity was 1.0-4.0g/kg (DA). Furthermore, an examination of the relationship between absolute humidity and the number of days of class closure with a threshold value of 4.2g/kg (DA) revealed significant correlations between the number of days of class closure and absolute humidity on both of the two days preceding the announcement of closure for the 2010 academic year. However, no significant correlations were observed for either of the two days preceding an announcement of closure for the 2011 and 2014 academic years.
Conclusion: Under the conditions investigated in this study, there was a spread between academic years in terms of correlations between absolute humidity prior to class closure and the number of days of class closure. Thus, the study suggested that other types of weather information and the impact of infection should be studied in future studies. Many preceding studies on influenza epidemics have drawn conclusions based on results from research in large areas, and this study indicated that such conclusions may apply to small areas as well.
