The Topic of This Month Vol.24 No.11(No.285)

Influenza in 2002/03 season, Japan

(IASR 2003; 24 : 281-282)

Influenza in 2002/03 season in Japan was characterized by mixed epidemics with type AH3 as the predominant virus in the first half and type B in the latter half. There was no epidemic due to type AH1. It has been the largest epidemic since the enactment of the Infectious Diseases Control Law in April 1999. Of the influenza AH3 viruses, 58% had a 4-times or more reduced HI titer compared to the 2002/03 influenza vaccine strain. Influenza B viruses belonged to the Victoria lineage and were similar to the 2002/03 influenza vaccine strain.

In Hong Kong, a fatal case due to A(H5N1) virus infection was reported in February 2003 and another fatal case due to A(H7N7) virus infection in April 2003 in the Netherlands, and the more strict alert against pandemic of new subtypes of influenza A for human has been carried out.

Incidence of influenza: Under the National Epidemiological Surveillance of Infectious Diseases, clinically diagnosed influenza cases are weekly reported from influenza sentinels, comprising approximately 5,000 clinics (3,000 pediatricians and 2,000 general physicians). During the past 11 seasons, the height of the peak in 2002/03 season ranked fourth; cases per sentinel (during the whole season) counted at 259.5, the third largest number after 1994/95 (288.3) and 1992/93 seasons (276.1). The national level heightened rapidly to more than 1.0 per sentinel from the 50th week of 2002. It lowered after the peak in the 4th week of 2003, becoming to less than 1.0 in the 15th week (Fig. 1). Case reports increased early in Kyushu district and so did late in Tohoku-Hokkaido district (Fig. 2). In Aomori and Akita, reports of more than one per sentinel lasted until the 17th week and Tottori until the 18th week. By age groups (Table 1), cases of 0-9 years accounted for about 60% with a peak at 3-5 years.

The excess mortality due to influenza epidemics: During 2000/01 and 2001/02 seasons, the excess mortality due to influenza epidemics, indicated by total deaths was low in Japan, whereas in 2002/03 season, the estimated deaths counted at about 11,000 (see p. 288 of this issue).

Isolation of influenza viruses: During 2002/03 season, reports of isolation of influenza viruses from prefectural and municipal public health institutes (PHIs) in the whole country counted at 4,850 for type AH3, 2,451 for type B and only one for type AH1 (see IASR, Vol. 24, No. 5) (as of October 17, 2003: Table 2). Both type AH3 and type B viruses were first isolated in the 46th week of 2002. Type AH3 started to increase from early December 2002, attaining a peak in the 4th week of 2003 and type B started later in the middle of January, attaining a peak in the 6th week of 2003. Type B virus was continually isolated until May (Fig. 1). Both AH3 and B types were isolated in many prefectures of the whole country. Reports of isolation of type B virus continued until the 20th-23rd weeks in Okinawa, Fukushima and Hokkaido ( In July, not in the epidemic season, type AH3 virus was isolated from returnees from Taiwan and Canada, and those from China (Shanghai) and Australia in August (see IASR, Vol. 24, No. 10). The age distribution of cases yielding type AH3 virus showed a high frequency from the younger generation through the aged, giving a peak at one year of age. In 2002/03 season, distribution of cases yielding type B virus shifted to lower age groups than the preceding season; isolation from 5-9 years old accounted for 45% (Fig. 3). From the age group of 6-8 years, reports of isolation of type B virus outnumbered those of AH3 virus; in other age groups, those of AH3 virus outnumbered those of type B virus. In adult groups, the higher the age of cases, the higher the ratio of reports of AH3 virus.

Antigenic characteristics of 2002/03 isolates: Forty-two percent of AH3 viruses isolated during 2002/03 season were similar to A/Panama/2007/99 (the vaccine strain for 2002/03 season). Viruses undergoing antigenic variation, showing 4-times or more reduction in HI titer from A/Panama/2007/99, increased to 58%. The virus of Yamagata lineage was dominant among type B viruses after 1989/90 season, however in the latter half of 2001/02 season the virus of Victoria lineage, represented by B/Shandong/7/97 (the vaccine strain for 2002/03 season), became dominant; most of type B viruses isolated during 2002/03 season were similar to B/Shandong/7/97 (see p. 283 of this issue).

From the prevalence of influenza viruses in Japan and other countries and the requirements of the vaccine production, the same vaccine strains as those for 2002/03 season, A/New Caledonia/20/99 (H1N1), A/Panama/2007/99 (H3N2), and B/Shandong/7/97, have been chosen for 2003/04 season (see IASR, Vol. 24, No. 9).

The influenza vaccine coverage rate among elderly people: The vaccine coverage rates among elderly people (principally older than 65 years) complying with the amendment of the Preventive Vaccination Law in 2001 were 27% in 2001/02 season and 35% in 2002/03 season (the Blood and Blood Products Division, Pharmaceutical and Food Safety Bureau, the Ministry of Health, Labour and Welfare).

New subtypes of influenza A for human: In February 2003, avian influenza virus A(H5N1) was isolated from father and son returning from Fujian, China and developing pneumonia in Hong Kong (the father died) (see WER, Vol. 78, No. 10, 2003). Besides, outbreaks due to a highly pathogenic strain of avian influenza virus A(H7N7) were reported on various poultry farms in the Netherlands since late February 2003. A(H7N7) virus was detected from a veterinarian died of pneumonia in April. A(H7N7) virus infection was confirmed in 82 workers participating in elimination of chickens and person-to-person transmissions among family members were also confirmed (see Eurosurveillance Weekly, Vol. 7, No. 18, 2003).

Antibody prevalence prior to 2003/04 season: A seroepidemiological survey was conducted under the National Epidemiological Surveillance of Vaccine-Preventable Diseases (see p. 289 of this issue). Among healthy individuals, from which blood samples were withdrawn during July-September 2003, the prevalence rates of influenza hemagglutination-inhibition (HI) antibody (HI titer of 40 or higher) for A/New Caledonia/20/99(H1N1) were about 50% of 5-19 years old, about 20% of those of 0-4 years, 20s and older than 60 years, and 5-20% of 30-50 years. For A/Panama/2007/99(H3N2), the rates were 60-80% of 5-19 years, 20-40% of 0-4 years and 20-50 years, and about 45% of older than 60 years. For B/Shandong/7/97 (Victoria-lineage), the rates were about 20% of 20s-30s and older than 60 years, and lower than 10% of 0-19 years and 40s-50s. For B/Shanghai/44/2003 (Yamagata-lineage), the rates in 5-19 years were higher than those for B/Shandong/7/97, but lower in other age groups.

Virus isolation in 2003/04 season ( Type B virus was isolated in Okinawa on September 4, 2003, (see IASR, Vol. 24, No. 10), and in Aichi on October 25, each from an adult case. On 12-19 September 2003, AH3 virus was isolated from sporadic cases of four children in Nagasaki (see p. 294 of this issue) (as of November 7, 2003).

In forthcoming winter, watching and countermeasure for emergence of new subtypes of human influenza A and re-emergence of severe acute respiratory syndrome (SARS) are the most important subjects (see IASR, Vol. 24, No. 10 and p. 291 of this issue). In Japan, the Infectious Diseases Control Law was amended in November 5, 2003, and SARS has been designated to the category I notifiable infectious diseases and human infection of highly pathogenic avian influenza to the new category IV notifiable infectious diseases. For differentiation of influenza virus from many other etiological agents of respiratory diseases, such as not only SARS coronavirus but also human metapneumovirus (see IASR, Vol. 24, Nos. 3 & 7) and respiratory syncytial (RS) virus, further intensification is desired on the infectious agents surveillance based on the laboratory examinations and the information of laboratory findings.

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