Incidence of influenza: Under the National Epidemiological Surveillance of Infectious Diseases, influenza cases are weekly reported from influenza sentinels, comprising approximately 5,000 clinics (3,000 pediatricians and 2,000 general physicians), diagnosing clinically or by rapid antigen detection with a kit. In 2001/02 season, after attaining 1.0 case per sentinel in the second week of 2002, cases rapidly increased. The number of patients peaked during the 6th to 8th weeks, and then decreased to 1.0 case per sentinel in the 15th week (Fig. 1). The incidence by prefecture is shown in Fig. 2. The epidemic season came early in Kyushu district and later in Tohoku district and ended in each district after 10-14 weeks. As described later, the epidemic due to type B virus was delayed. Reports of more than 1.0 case per sentinel carried on until the 22nd week in Tohoku and a part of Kyushu districts and until the 30th week in Okinawa district. Cases by age group are shown in Table 1. The 3-5 year group formed a peak and cases of 0-9 years accounted for 60%.
During December 1, 2001 to April 9, 2002, influenza cases were reported daily on the Internet with cooperation of about 400 clinics of influenza sentinels. Reports increased suddenly from January 15, and those of which increased realization of epidemics in a rapid matter than of the National Epidemiological Surveillance of Infectious Diseases.
By the new Alarming/Warning System for Influenza Epidemics started in 2000/01 season, warning was given to a health center in the 52nd week of 2001 and to another health center in the first week of 2002, to eight health centers in the 2nd week, and in the 3rd week of 2002, to other 45 health centers, and alarming was given to six health centers. Later on, warning and alarming have increased in the whole country.
According to school outbreak reports of influenza-like illness accompanying temporary closing of class work, cases reported during 2001/02 season totaled at 345,000, being 2.8 times of those in 2000/01 season.
Isolation of influenza viruses: During 2001/02 season, isolation of 3,253 type AH1, 3,095 type AH3, and 1,820 type B influenza viruses were reported from prefectural and municipal public health institutes (PHIs) (as of November 26, 2002) (Table 2). Type A viruses were seldom tested for N subtype; nevertheless two strains isolated from outbreaks occurring in Yokohama in February were confirmed to be A/H1N2 type (see IASR, Vol. 23, No.8).
Type AH3 was isolated for the first time in the 42nd week and type AH1 in the 44th week of 2001. Both types increased rapidly from the first week of 2002 and peaked in the 6th and 5-6th weeks, respectively. Type B virus isolated late in the 50th week for the first time, started to increase from the 3rd week of 2002, isolation continued until the 28th week with a peak in the 11th week (Fig. 1). Although all types were isolated in most prefectures, reports of isolation of type B virus lasted even after the 20th week*. Influenza virus types AH1 and AH3 were isolated mainly from the younger generation and the reports of virus isolation in 2001/02 season outnumbered those of the preceding season in all age groups. Type B virus was isolated from children aged older than 7 years, showing a slight increase of the age of the patients in 2000/01 season. It was seldom isolated from those aged more than 20 years (Fig. 3).
Antigenic characteristics of 2001/02 isolates: Ninety-six percent of type AH1 viruses isolated during 2001/02 season were similar to A/New Caledonia/20/99 (the vaccine strain for 2001/02 season); 97% of type AH3 viruses were similar to A/Panama/2007/99 (the vaccine strain for 2001/02 season). Type B virus was mainly of Yamagata lineage after 1989/90 season, nevertheless viruses of Victoria lineage became the mainstream, and most type B viruses isolated during 2001/02 season were of Victoria lineage. Therefore, the type B vaccine strain was changed from B/Johannesburg/5/99 (Yamagata lineage) to B/Shandong/7/97 (Victoria lineage) (see IASR, Vol. 23, Nos. 10 & 11).
The excess mortality due to influenza epidemics: There was no excess mortality from the total deaths in Japan in 2001/02 season as was in 2000/01 season. From 2001/02 season, the weekly reporting system for influenza-related deaths is being operated on the Intranet by the Ministry of Health, Labour and Welfare (MHLW) and data of deaths due to pneumonia and influenza are reported on a weekly basis from 13 major cities. The estimated excess mortality based on this system is in accordance with the retrospective nationwide results.
The influenza vaccine coverage rate among elderly people: By the amendment of the Preventive Vaccination Law in 2001, the aged (principally those older than 65 years) can receive influenza vaccination at partial public expense (see IASR, Vol. 22, No. 12). The vaccine coverage rate among the aged in 2001/02 season in compliance with the Preventive Vaccination Law was 27% (see p. 309 of this issue).
Acute encephalopathy among influenza patients: A study group of MHLW (headed by Professor T. Morishima, Nagoya University) conducted a nationwide survey and detected 217 cases in 1999, 109 in 2000, and 63 in 2001. In 2002, 118 cases reported by each prefecture as usual and directly reported 109 cases by the hospitals with pediatric wards accounted for 227 cases. The 2002 investigation has found a case-fatality rate of 15% and sequelae in 21%, showing a slight improvement (see p. 310 of this issue).
Antibody prevalence prior to 2002/03 season: During July - September 2002, a seroepidemiological survey was conducted under the National Epidemiological Surveillance of Vaccine-Preventable Diseases by using four kinds of antigens including three 2002/03 season vaccine strains (see IASR, Vol. 23, No. 10). Rapid reports from 18 prefectures show that the influenza hemagglutination-inhibition (HI) antibody prevalence rates (HI titer of 40 or higher) to A/New Caledonia/20/99 (H1N1) accounted for 40-50% among healthy individuals of 5-19 years of age, about 20% among 0-4 years as well as 20s, and about 10% among over 30 years. To A/Panama/2007/99 (H3N2), the rates were slightly lower than 70% among 5-9 years, 55-65% among teens, 25% among 0-4 years, about 20% among 20s-40s, about 10% among 50s, and 31% among 60 years and over. To B/Shandong/7/97 (the 2002/03 season vaccine strain of Victoria lineage), the rates were slightly lower than 20% among 20s and lower than 10% among all other age groups. To B/Shenzhen/407/2001 (a Yamagata lineage strain genetically different from the strains prevailing in 2001/02 season), the rates were slightly lower than 20% among teens and lower than 10% among all other age groups (see p. 309 of this issue). A low type B antibody prevalence is seen.
Virus isolation in 2002/03 season: Type B virus was isolated from an outbreak at an elementary school in Saitama on November 11, 2002 (see p. 315 of this issue), from a sporadic case in Shizuoka on November 28, AH3 type from a sporadic case in Osaka on November 12 (see p. 314 of this issue), in Toyama on November 14, in Hamamatsu on November 18, and from an outbreak in Ishikawa on November 26 (as of December 3, 2002)*.
*Refer to http://idsc.nih.go.jp/iasr/index.html