The Topic of This Month Vol.20 No.12(No.238)

Influenza, 1998/99 Season, Japan

The following is a summary of the epidemiological feature of influenza in 1998/99 season, based mainly on the data obtained by the National Epidemiological Surveillance of Infectious Diseases (NESID).

Incidence of clinically diagnosed influenza patients: The weekly incidence of influenza-like illness (ILI) during three seasons starting from 1996 is summarized in Fig. 1-a. The 1998/99 season is characterized by the start of epidemics of ILI in December and sudden increase in number of patients in the beginning of 1999, forming a peak in the 3rd-4th weeks of 1999 followed by a sudden decrease. During the 6th-9th weeks, influenza patients remained temporarily on the same level, and then from the 10th week toward the 15th week, they decreased at a slow pace.

The age distribution of the patients reported as ILI during the first quarter of 1999 (the 1st to the 13th weeks) was compared with that during the same period of 1998 (Fig. 2). In the group of 1-9 years of age, the number of patients was the same at any age, but the reports of 0 year patients were no more than 1/3-1/4 of those of these age groups. A similar tendency was seen in 1998, but increased reports of elders aged over 15 years especially those aged over 30 years were conspicuous. (Since most sentinel clinics for NESID were those of pediatrics until March 1999, patients at ages over 15 years must have been more than reported.)

Isolation of influenza viruses: Fig. 1-b shows weekly reports of influenza viruses isolated at prefectural and municipal public health institutes (PHIs) in the last three seasons. As seen in the figure, type A (H3N2) prevailed consecutively for the last three seasons. In the first half of 1998/99 season, type A (H3N2) prevailed, but after the 6th week, type B gradually replaced type A (H3N2). Influenza virus isolates totaled at 8,526 in 1998/99 season, including 4,690 type A (H3N2), 3,826 type B, and 10 type A (H1N1).

Patients of known ages from which influenza virus was isolated are summarized in Fig. 3. In 1997/98 season, type A (H3N2) was isolated in a bimodal fashion with peaks at ages of one and 9 years. In 1998/99 season, the frequency of virus isolation from patients of all ages but those of 9 years of age was higher than that in the preceding season. Type A (H3N2) was isolated much more frequently from those of 0-4 years, especially from those of one year of age in 1998/99 season than in the preceding season. Isolation from those aged 5-9 years and 10-14 years was, however, less frequent in 1998/99 season than in the preceding season. On the other hand, type B was isolated most frequently from patients aged 7 years, and more frequently than type A (H3N2) from those aged over 5 years to 10-14 years.

The most isolates of type A (H3N2) during 1998/99 season were similar to the A/Sydney/5/97-like strain included in the vaccine for 1998/99, and a few A/Fukushima/99/98-like strains, with a hemagglutination-inhibition (HI) antigenicity deviated from the homotiter to the vaccine strain by 23 or more. Type B isolates, provoking small-scale epidemics in the latter half of the season, were those belonging to the B/Yamagata/16/88 antigenic/genetic group and some coexisting strains belonging to B/Victoria/2/87 antigenic/genetic group (see p. 291 of this issue).

Antibody prevalence: The influenza HI antibody prevalence (HI titer >_1:10) among serum samples taken from healthy individuals in the autumn of 1998 (before 1998/99 season) examined by the National Epidemiological Surveillance of Vaccine-Preventable Diseases is shown in Fig. 4 (see IASR, Vol. 19, No. 12, 1998). The antibody prevalence rate against A/Sydney /5/97(H3N2) was high in the age groups of 5-9 and 10-14 years, while low in those of ages of around these years. The ages of patients from which type A (H3N2) was isolated in 1998/99 season, when type A/Sydney/5/97-like strains were prevalent, formed a peak at one year of age. Such cases were fewer in the age groups of 5 years to school children (Fig. 3), perhaps reflecting the antibody prevalence. With respect to type B, the virus was isolated mainly from the age group of school children, reflecting a low antibody prevalence against B/Beijing/184/93 in children under 14 years of age. The flash report of antibody prevalence before the 1999/2000 season can be found on p. 294 of this issue and in the Infectious Disease Surveillance Center (IDSC) homepage (

The excess mortality: The excess mortality of patients of influenza and/or pneumonia during 1998/99 season was 1.8 times higher than that in the preceding season. More than 90% of the death cases of influenza and/or pneumonia were the aged older than 65. Excess deaths of the aged are tending to increase due to the recent increase in the population of the aged (see p. 293 of this issue).

Encephalitis/encephalopathy: Such severe complications of influenza, involving the central nervous system, as encephalitis/encephalopathy are known to occur among children. Severe cases of these complications were increasing in 1997/98 season (see IASR, Vol. 19, No. 12, 1998). Fig. 5b shows the reports of encephalomyelitis patients (including acute encephalitis, encephalopathy, Reye syndrome, and myelitis) reported by about 500 sentinel hospitals in the whole country during the past 12 seasons. Patients of encephalomyelitis appeared to increase suddenly in 1998/99 season parallel to those of ILI (Fig. 5a), as was the case in the preceding season. Reye syndrome is encephalopathy well known to be complicated with influenza. Since the withdrawal of salicylic acid drugs as antipyretics for children in 1980s, patients of Reye syndrome have rapidly decreased in Japan as in European and North American countries. Cases reported as Reye syndrome are shown in thick lines in Fig. 5b. Since 1987, reports of Reye syndrome cases during December-March in every winter show only one digit number except 10 in 1987/88 season. The recent increase in encephalitis/encephalopathy cases may have excluded those of Reye syndrome.

Acute encephalitis/encephalopathy patients from which influenza virus was isolated or viral genome was detected by RT-PCR in the pharynx or cerebrospinal fluid at PHIs during 1998/99 season counted at 91, including 74 patients of type A (H3N2) and 17 of type B. The patients were the largest in number ever reported, outnumbering 75 reported in 1997/98 season. The largest number of patients was seen among children aged 1-3 years, as was the case in 1997/98 season. At the time that the reports were made, seven and two fatal cases of type A (H3N2) and type B, respectively, occurred.

The Ministry of Health and Welfare notified local health departments of "the overall control plan for influenza in forthcoming winter" on November 17, 1999, requiring influenza prevention to be thoroughgoing (see p.296 of this issue).

The reports of virus isolation in the 1999/2000 season could be found in the following homepage of the IDSC (

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