ましん(麻疹)流行の季節性

The seasonal variation in attack rates by measles in Japanese children has been examined by the method of harmonic analysis. The monthly attack rates per 100,000 children during 20years from September 1948 to August 1968 showed a characteristic seasonal pattern when the rates were transformed to logarithms. It was described effectively by a two-term Fourier curve equation, Y = 1.6549-0.5561u_1-0.3488v_1 -0.1443u_2-0.0180v_2.The average maximal incidence of measles occurred on May 25.47 ±4.66 and its minimum on October 8.19±4.49. By solving the two-term Fourier equation, the predicted log incidence was Y = 2.20268 for the average maximum and Y = 1.07795 for the average minimum. From their antilogarithms, the maximum number of cases per month averaged 159.47 per 100,000, and the minimum number 11.97 per 100,000, a 13.33 fold increase through the year. The rates during 4 years from September 1968 to August 1972 was described by a three-term Fourier equation, Y = 1.3488-0.1969u_1-0.1477v_1-0.1260u_2-0.0384v_2-0.0223u_3+0.0158v_3.The average peak of the measles epidemic delayed about 18 days, but the minimum date was almost similar to that of the former 20 years. The maxilmum number of cases per month averaged 42.693 per 100,000, and the minimum number 11.930 per 100,000, a 3.58 fold increase through the year. The clear decrease in attack rates and the change in the seasonal incidence pattern were recognized, compared with those of the former 20 years. The utility of preventive vaccination for measles in recent years is seemed to be significant for lowering the incidence, though the elevation of economical and social standards of people's living may be playing important roles in this matter. Similarly the seasonal distribution of monthly totals of measles cases in logarithms for 10 years from September 1958 to August 1968 in Shimane prefecture was able to describe by a two-term Fourier curve and that for 7 years from September 1968 to August 1975 was described by a three-term Fourier curve.