The Topic of This Month Vol. 28, No. 11 (No. 333)
Incidence of influenza: Under the National Epidemiological Surveillance of Infectious Diseases, clinically diagnosed influenza cases have been reported weekly by approximately 5,000 influenza sentinel clinics all over the country (3,000 of pediatrics and 2,000 of internal medicine). Weekly cases per sentinel exceeded 1.0 on the nationwide level during week 3 of 2007, which further increased gradually until a peak (32.9) in week 11 of 2007. The number of cases decreased notably during weeks 13-14 and slowly after week 15 (Fig. 1). During the recent 10 seasons, epidemic started second latest, and the week showing the peak and that when cases per sentinel became less than 1.0 (week 21) was the latest (http://idsc.nih.go.jp/idwr/kanja/weeklygraph/01flu.html). Although the height of the peak was low, only the 7th high, the total number of cases per sentinel in the whole season was 225.8, being the medium-sized epidemic or the 5th largest epidemic during the recent 10 seasons.
Cases per sentinel increased early in Aichi and Miyazaki Prefectures, whereas epidemics continued until June in Kagoshima, Akita, Iwate, and Miyagi Prefectures (http://idsc.nih.go.jp/disease/influenza/inf-keiho/index.html). In Okinawa Prefecture, continuing from 2004/05 and 2005/06 seasons, epidemics occurred also during summer in 2006/07 season and the epidemic was still continuing, without cessation, even at the end of October (see p. 322 & 324 of this issue).
By the national reporting requested for all cases of "acute encephalitis" of category V infectious diseases, 42 cases of influenza encephalopathy were reported (51 cases each in 2004/05 and 2005/06 seasons).
Isolation of influenza viruses: During the 2006/07 season, prefectural and municipal public health institutes (PHIs) isolated 2,287 influenza AH3, 1,987 influenza B, and 576 influenza AH1 viruses (reports as of October 23, 2007; Table 1). These figures include cases, from which virus was isolated after overseas traveling (Table 2).
The report of the first isolation during 2006/07 season was on influenza B viruses in week 38 in Hiroshima Prefecture, in which small local epidemics were reported (IASR 27: 268-269, 2006). Influenza AH3 virus was first isolated in week 42 from a kindergartner in Saitama Prefecture (IASR 27: 337, 2006) and AH1 viruses in week 46 from four cases of a familial outbreak in Yamanashi Prefecture (IASR 27: 337-338, 2006). The first school outbreak occurred in week 45 with influenza B viruses at a primary school, accompanying temporary closing of class work in Shiga Prefecture (IASR 28: 12-13, 2007). Weekly isolation and isolation by prefecture are shown in Fig. 1 and in Fig. 2, respectively. Until the end of 2006, isolation of a few strains of all three types was reported every week; isolation of influenza AH3 virus gradually increased after entering 2007, later than usual, and after week 3, reports of influenza B virus increased, exceeding that of influenza AH3 virus after week 10. A small number of influenza AH1 viruses were isolated throughout the season. With a delay of the increase in number, the peak of influenza AH3 virus isolation was late than usual, being in week 9 at the same time as for both influenza B and AH1. After week 23 of 2007, chiefly AH1 virus was isolated and AH3 virus was also isolated in small number. Isolation of influenza B virus in week 24 was the last.
The age distribution of influenza virus-isolated cases shows that influenza AH3 viruses were isolated at lower frequencies than 2005/06 season for all age groups, the largest number of cases in one-year group and more cases in 30s than in 20s among those over 15 years. On the other hand, more influenza B viruses were isolated in all age groups than 2005/06 season, from most cases being younger than 19 years, mainly 7-13 years. Influenza AH1 viruses were isolated fewer in all age groups than 2005/06 season, mostly from those younger than 11 years (Fig. 3).
Antigenic characteristics of 2006/07 isolates and the vaccine strains for the 2007/08 season: Influenza AH1 viruses shifted to A/Solomon Islands/3/2006-like strain, an antigenic variant of A/New Caledonia/20/99-like strain (the vaccine strain for 2000/01-2006/07 season), which occupied the majority of the isolates in the latter half of the season. Of influenza AH3 viruses, antigenic variants of A/Hiroshima/52/2005-like strain (the vaccine strain for 2006/07) were isolated from the beginning of the season and occupied the most in the latter half of the season. Most influenza B virus strains belonged to B/Victoria lineage, continuing from 2005/06 season, being B/Malaysia/2506/2004-like strain (the vaccine strain for 2006/07 season) (see p. 313 of this issue).
For the 2007/08 season, the vaccine strain of influenza AH1 virus was replaced to A/Solomon Islands/3/2006, differing from the preceding season, and of AH3 and B viruses, A/Hiroshima/52/2005 and B/Malaysia/2506/2004 of Victoria lineage, the same as the preceding season, were selected respectively (see p. 320 of this issue).
Production of influenza vaccine and influenza vaccine coverage rate among the elderly: In the 2006/07 season, 25,180,000 vaccine doses were produced and 18,770,000 doses of them were used. For the 2007/08 season, the demand of vaccine was estimated to be approximately 19,400,000-20,800,000 doses and a maximum production of 23,500,000 doses is planned. The vaccine coverage rate among the elderly (primarily those ≥65 years of age), in compliance with the Preventive Vaccination Law, has failed to rise over approximately 50% during the recent four seasons including the 2006/07 season (http://www.mhlw.go.jp/shingi/2007/06/s0628-20.html).
Preparedness for the next influenza pandemic: Current phase of alert in the WHO global influenza preparedness plan is phase 3. In foreign countries, human infection with A/H5N1 virus has been occurring continuously, and 334 cases (205 deaths) were reported to WHO as of November 5, 2007 (http://www.who.int/csr/disease/avian_influenza/country/en/).
In Japan, the guidelines for the next influenza pandemic (phases 4-6) were prepared on March 26, 2007 (http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou04/index.html).
The national and prefectural governments each started to stockpile of influenza antiviral agents for 10,500,000 doses totaling 21,000,000 doses.
A total of 10,000,000 doses of A/H5N1 pre-pandemic vaccine were stockpiled in 2006, and additional 10,000,000 doses are planned to be stockpiled in 2007. Further enrichment of the facilities for laboratory testing at PHIs is necessary and urgent budget compilation at each municipality is desired.
Conclusion: Local influenza epidemics during summer season, which used to be considered non-epidemic periods in Japan, occurred for three consecutive seasons. Influenza virus isolation from cases developing influenza after overseas traveling has been reported all year round. To get isolates certainly, year-round influenza surveillance including that in summer season is becoming more important for acquiring new candidate of vaccine strains and for selection of vaccine strains based on the virus analysis of epidemic strains.
Preliminary reports for 2007/08 season (http://idsc.nih.go.jp/iasr/prompt/graph-ke.html): As of November 6, 2007, influenza AH3 virus was isolated in Aichi and AH1 viruses in Kanagawa, Okinawa, Chiba, Hyogo, Shiga, Osaka and Hokkaido Prefectures and Tokyo Metropolitan (see p. 324-325 of this issue).
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