Epidemiology Epidemic Features Prior to the use of vaccine After the use of vaccine Morbidity Major epidemics: 500-1000 9.5in1990 (1/100000) Mild epidemics: 400 Peak season In winter and spring In any time of a year Epidemic cycle Major epidemics appear by Epidemic cycle 2-4 yr intervals disappeared, Sporadic cases increased Age 6m-5y(98%10y) Below 6m and above 5y Typical cases Atypical cases Clinical severe manifestations mild manifestations manifestations more complications less complications high mortality rarely death DIVISION OF INFECTIOUS DISEASE AND GASTROENTEROLOGY7 DIVISION OF INFECTIOUS DISEASE AND GASTROENTEROLOGY Prior to the use of vaccine After the use of vaccine Morbidity (1/100000) Major epidemics: 500-1000 Mild epidemics: 400 9.5 in 1990 Peak season In winter and spring In any time of a year Epidemic cycle Major epidemics appear by 2-4 yr intervals Epidemic cycle disappeared, Sporadic cases increased Age 6m-5y (98%<10y) Below 6m and above 5y Clinical manifestations Typical cases severe manifestations more complications high mortality Atypical cases mild manifestations less complications rarely death Epidemiology Epidemic Features