Abstract
We studied the frequency of occurrence of Norovirus food poisoning over 11 years (1998—2008) according to the year, month, week, day, and day of the week of occurrence using the data involving the date of occurrence, number of occurrences, and the number of patients, which were recorded in the 3rd part of Records of food poisoning cases in Japan.
The annual frequency of occurrence was higher than the mean from 2003 to 2008 in which the frequency in 2006 was abnormally high and exceeded the upper limit.
The occurrence of food poisoning was more in the winter season, and the monthly frequency of occurrence was as follows in a descending order: December, January, February, March, and November. Although no significant difference was observed in the frequency of occurrence between December and January, there was a significant difference when comparing these two months with other months. The frequency of occurrence was therefore the highest in December and January.
Regarding the weekly frequency of occurrence, the number of occurrences was higher than the mean between weeks 47 and 14, and the number of patients was greatest between weeks 47 and 13. The frequency of occurrence was significantly higher in week 51 and significantly lower during the New Year holidays between weeks 52 and 1.
Regarding the daily frequency of occurrence, the frequency of occurrence was greater than the mean between November 17 and March 31, and the number of patients increased between November 15 and April 2. The frequency of occurrence was especially high during week 51 (mid-December) and December 9, 10, 16, and 23. The frequency of occurrence was lower than the mean during the New Year holidays, including December 29—30 and January 2 and 3.
The frequency of occurrence according to the day of the week was as follows in the descending order: Saturday, Sunday, Friday, Monday, Tuesday, Thursday, and Wednesday. Although no significant difference was observed in the frequency of occurrence between Saturday and Sunday, there was a significant difference when comparing these two days with other days of the week. Therefore, the frequency of occurrence was highest on Saturday and Sunday.
