Incidence of clinically diagnosed influenza patients: The number of case reports in 2000/01 season exceeded an indicator of the influenza epidemic, one case per sentinel, in the fifth week of 2001, 5-6 weeks later than usual, and resulted in a small peak in the 11th week (Fig. 1). Cases then decreased gradually down to less than one case per sentinel in the 18th week. The incidence by prefecture (Fig. 2) shows that epidemics started earlier in Kochi and Wakayama Prefectures, attaining a peak in the 8th week (31.0 and 16.2 cases per sentinel, respectively), while a peak was attained in the 11th week in Tohoku and many other districts. As for the ages of cases (Table 1); those aged 4 years were predominant in the age group under 20 years, and in the age group over 20 years, aged 30s were predominant, following 1999/2000 season (see IASR, Vol. 21, No. 12).
Isolation of influenza viruses: Fig. 3 shows weekly reports of influenza virus isolation/detection at prefectural and municipal public health institutes (PHIs) nationwide. In 2000/01 season, epidemics of the three subtypes occurred at nearly the same time, attaining the peak in the 9th week of 2001 for type A (H3N2), in the 10th week for type A (H1N1) and in the 10-11th week for type B. The number of reports of type B was larger than that of type A (H1N1); since the start of the Infectious Agents Surveillance in 1982, many reports of isolation of both types could be seen for the first time. It was a mixed epidemic; of type B virus occurring after 1998/99 season, of type A (H1N1) virus occurring successively from 1999/2000 season, and of type A (H3N2) virus occurring successively for four consecutive seasons (Table 2 and see IASR Vol. 21, No. 12). Type B, type A (H1N1), and type A (H3N2) viruses were isolated in 45, 44 and 41 of the prefectures, respectively. In Kochi Prefecture, in the 8th week when the incidence was at peak (Fig. 2), type B virus was exclusively isolated. The ages of cases from which influenza virus was isolated (Fig. 4) peaked at 5-7 years for type B, and 4-5 years for type A (H1N1) and further from adults including the aged. On the other hand, type A (H3N2) virus was isolated mainly from younger generations, with a peak at one year of age.
Antigenic characteristics of 2000/01 isolates: Type B virus is grouped largely into the B/Yamagata and the B/Victoria lineage. The isolates in 2000/01 season belong mostly to the B/Yamagata lineage and isolates antigenically closely related to B/Sichuan/379/99 and B/Johannesburg/5/99 (the vaccine strain for 2001/02 season) were dominant. Isolates similar to B/Yamanashi/166/98 (the vaccine strain for 2000/01 season) accounted for approximately 13%. A few viruses of the B/Victoria lineage were also isolated. Eighty percent of type A (H1N1) isolates were similar to A/New Caledonia/20/99 (the vaccine strain for 2000/01 and 2001/02 seasons) and a few hemagglutination-antigenic variants were also isolated. Ninety-two percent of A (H3N2) isolates were similar to A/Panama/2007/99 (the vaccine strain for 2000/01 and 2001/02 seasons) (see IASR, Vol. 22, No. 10).
Antibody prevalence: In the autumn of 2001 prior to 2001/02 season, a seroepidemiological survey was conducted under the National Epidemiological Surveillance of Vaccine-Preventable Diseases with four influenza virus antigens including the three vaccine strains for 2001/02 season. The influenza hemagglutination-inhibition (HI) antibody prevalence rates (HI titer of 40 or higher) of healthy individuals were extremely low to A/New Caledonia/20/99 (H1N1) according to the results from 14 prefectures (Fig. 5), being 33-48% of those aged 5-19 years, 19% of those aged 0-4 years, and lower than 10% of all age groups over 20 years. To A/Panama/2007/99(H3N2), the rate was high, being 55-82% of those aged 5-19 years, but low in other age groups, being 17-27%. To B/Johannesburg/5/99, the rate was 39-59% of the age group of 5-19 years, but lower than 26% in any of the other age groups, being particularly low in the age groups of 0-4 years and over 40 years. The antibody prevalence rate to B/Akita/27/2001 included in the B/Victoria lineage was very low in all age groups (see p. 311 of this issue).
The excess mortality: An excess mortality is seen in a season of relatively large influenza epidemics and its increase is an issue in an aging society like Japan (see IASR, Vol. 21, No. 12). In 2000/01 season, since the epidemics were small in scale, no statistically significant excess mortality was seen.
Encephalopathy: It has recently been notified that many cases of acute encephalopathy occur among infants and children in an influenza-epidemic season. A study group of the Ministry of Health, Labour and Welfare (MHLW) (headed by T. Morishima) conducted a nationwide surveillance. The study group accepted 217 of the 238 cases reported from January 1 to March 31, 1999, 109 of the 142 cases during the same period of 2000, and 55 of the 61 cases during the same period of 2001 as encephalopathy. The mechanism of the encephalopathy occurrence is not fully understood yet; however, it has been suggested that the use of some antipyretics (diclofenac sodium and mefenamic acid) be possibly related to worsening encephalopathy. On May 30, 2001, the MHLW issued a notice on "antipyretics to be administered to influenza patients" (see p. 312 of this issue).
In 2000/01 season, there were 32 reports of isolation/detection of influenza virus from acute encephalopathy patients at PHIs (half of those in 1999/2000 season). Isolation was from the pharynx in 26 cases (in addition, two cases were detected by PCR only) and from the cerebrospinal fluid in one case (in addition, two cases were detected by PCR only). Type B virus was isolated from 17 cases, A(H3N2) from eight cases, and A(H1N1) from seven cases.
Amendment of the Preventive Vaccination Law: On November 7, 2001, the Preventive Vaccination Law was partially amended, and influenza was placed under the Category II* Infectious Diseases (*to put a stress upon immunization of each person to prevent from the disease or its worsening rather than on herd immunity). If (1) one aged over 65 years, or (2) a patient of chronic or severe heart/lung/kidney failure or acquired immunodeficiency syndrome (AIDS) aged between 60 and 65 years desires influenza vaccination, he/she will be regarded as an official recipient of routine immunization and cost will be covered partially by public expenditure. The schedule of injection, target of immunization, charge for each vaccinee will be decided locally by each government (city, ward, town or village). If any health damage results from regular vaccination, official compensation will be made (see p. 313 of this issue). On November 12, 2001, the MHLW issued a notice "A comprehensive strategy for influenza in this winter". Guidelines for influenza control and prevention will also be amended.
Virus isolation in 2001/02 season: As of December 6, 2001, a strain of type B virus was isolated on September 26 in Nagoya City. A strain of type A (H3N2) virus was isolated on October 5 in Sendai City (see IASR, Vol. 22, No. 11) and four strains during October 19-23 in Okinawa Prefecture.
The up-to-date information on influenza is available on the Infectious Disease Surveillance Center homepage (http://idsc.nih.go.jp/index.html).