Incidence of influenza: Under the National Epidemiological Surveillance of Infectious Diseases (NESID), 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 in week 47 of 2007, the earliest since 1987 when the influenza surveillance under NESID started, and increased until week 51. After decreased once during yearend holidays, it increased again and peaked in week 5 of 2008 (17.7), then decreased notably during weeks 6-7 and slowly after week 8 becoming less than 1.0 in week 14 (Fig. 1 and http://idsc.nih.go.jp/idwr/kanja/weeklygraph/01flu.html). During recent 10 seasons, the peak incidence and the total number of cases per sentinel during the whole season (142.0) were the second smallest following 2000/01 season.
The 2007/08 epidemic started early in Hokkaido and Aomori Prefectures and expanded to Kyushu district in the latter part of the season (https://nesid3g.mhlw.go.jp/Hasseidoko/Levelmap/flu/index.html). In Okinawa Prefecture, continuing from past three seasons, an epidemic occurred also during summer in 2007/08 season, though its size was small (see p. 309 of this issue).
By the national reporting requested for all cases of "acute encephalitis" of category V infectious diseases, 34 cases of influenza encephalopathy were reported (51 cases in 2005/06 season and 45 in 2006/07 season).
Isolation of influenza viruses: During 2007/08 season, prefectural and municipal public health institutes (PHIs) isolated 3,646 influenza AH1, 504 influenza AH3, and 306 influenza B viruses (reports as of October 16, 2008; Table 1). These figures include 22 cases, from which virus was isolated after overseas traveling (Table 2).
The report of the first isolation during 2007/08 season was on influenza AH3 viruses in week 37 in Aichi Prefecture, in which isolation from 2 cases in a small outbreak including 10 cases of 5 families was reported (IASR 28: 324, 2007). Influenza AH1 virus was first isolated in weeks 39-40 from cases having visited sentinel clinics in Okinawa Prefecture (IASR 28: 324, 2007) and the reports of influenza AH1 virus isolation increased in week 39 in Kanagawa Prefecture (IASR 28: 351-352, 2007) and in week 41 in Chiba Prefecture (IASR 28: 324-325, 2007) (Fig. 1). Influenza B virus was first isolated in week 46 in Kyoto Prefecture (a Yamagata-lineage strain) (IASR 29: 15-16, 2008) and in week 47 in Hiroshima Prefecture (a Victoria-lineage strain) (IASR 29: 16, 2008). Influenza AH1 virus was dominant until the peak in week 4 of 2008, thereafter it decreased and influenza B and AH3 viruses slightly increased in turn (Fig. 1). Isolation of influenza AH1 virus from domestic cases was the last in week 16 (afterward, only from a returning traveler each in week 29 from Malaysia and in week 32 from China). Isolation by prefecture is shown in Fig. 2. In Hokkaido and many other prefectures, influenza AH1 virus was isolated in the first half of the season and influenza AH3 virus in the second half, and influenza B virus in mid-season.
The age distribution of influenza virus-isolated cases (Fig. 3) shows that influenza AH1 viruses were isolated at higher frequencies than 2006/07 season for all age groups; children aged 4-10 years accounted for 58% with a peak at 7 years and more cases were seen in the 30s followed by the 20s and the 40s among adults. On the other hand, isolation of influenza AH3 viruses was largely decreased from 2006/07 season in all age groups; slightly more cases were seen among children aged 3-7 years and among adults in the 20s and the 30s. Influenza B viruses were isolated mainly from children aged 4-7 years and the frequency of isolation among the 30-60s, which was rare in 2006/07 season, slightly increased.
Antigenic characteristics of 2007/08 isolates and the vaccine strains for the 2008/09 season: Of influenza AH1 viruses, A/Solomon Islands/3/2006-like strains (the vaccine strain for 2007/08 season) accounted for 70%; however, A/Brisbane/59/2007-like strains markedly increased from March 2008. Only 5% of Influenza AH3 viruses were A/Hiroshima/52/2005-like strains (the vaccine strain for 2006/07-2007/08 seasons) and A/Brisbane/10/2007-like strains accounted for 80%. Influenza B virus strains belonging to the Victoria lineage, being prevalent in 2006/07 season, decreased and 77% of isolates were belonging to the Yamagata lineage. Yamagata-lineage strains resembled B/Florida/4/2006 (the vaccine strain for 2008 in the southern hemisphere) and Victoria-lineage strains resembled B/Malaysia/2506/2004 (the vaccine strain for 2006/07-2007/08 seasons) (see p. 299 of this issue).
In Norway and some other countries in Europe and southern hemisphere, oseltamivir-resistant strains of influenza AH1 viruses were isolated in high frequency, but the frequency was low (2.6%) in Japan (see p. 299 of this issue and IASR 29: 155-159, 2008).
For 2008/09 season, all the three vaccine strains were replaced as follows: A/Brisbane/59/2007 for AH1, A/Uruguay/716/2007 (A/Brisbane/10/2007-like strain) for AH3, and B/Florida/4/2006 belonging to the Yamagata lineage for B (see p. 307 of this issue).
Production of influenza vaccine and influenza vaccine coverage rate among the elderly: For 2007/08 season, 25,500,000 vaccine doses were produced and 22,570,000 doses were used. The demand for 2008/09 season has been estimated at approximately 21,450,000-24,000,000 doses and a maximum production of 26,400,000 doses is planned. The vaccine coverage rate among the elderly (primarily those aged 65 years or older), in compliance with the Preventive Vaccination Law, has risen to 55% for 2007/08 season from 50% for 2006/07 season (http://www.mhlw.go.jp/shingi/2008/06/s0618-9.html).
Avian influenza: In foreign countries, human infection with A/H5N1 virus has been occurring continuously, and 387 cases (245 deaths) have been reported since 2003 to WHO as of September 10, 2008 (http://www.who.int/csr/disease/avian_influenza/en/).
In Japan, several cases were suspected of avian influenza virus infection after returning from Southeast Asia, however all resulted in A/H5N1-negative in the tests conducted by PHIs and National Institute of Infectious Diseases (NIID). Isolation of orthoreovirus from such a case was reported (see p. 310 of this issue). Cooperation between PHIs and NIID on laboratory diagnosis became more important, and a training course of A/H5N1 detection for laboratory staffs of PHIs was held in NIID in August 2008.
Conclusion: Since 2004/05 season, local influenza epidemics during the summer have been remarkable. In Okinawa, only influenza AH1 viruses were isolated after week 24 of 2007 preceding the nationwide outbreak of influenza AH1 virus in 2007/08 season and thus the influenza virus strain isolated in summer often became the prevalent strain next season. In this summer, outbreaks have been reported one after another; at a facility for the elderly in Aomori in June (AH3) (IASR 29: 228-229, 2008), a university in Okayama in June (AH3) (IASR 29: 253-254, 2008), a primary school in Chiba in July (B) (IASR 29: 254-255, 2008), and a dormitory in Kanagawa in August (AH3) (see p. 312 of this issue). Influenza virus isolation from cases developing influenza after overseas traveling has been reported all year round. More and more it becomes important to get isolates by year-round influenza surveillance including in summer for acquiring new candidates of vaccine strains and for selection of vaccine strains based on the virus analysis of epidemic strains.
Preliminary reports for 2008/09 season (http://idsc.nih.go.jp/iasr/influ-e.html): In week 35 of 2008, just before the start of this season, influenza AH3 viruses were isolated in Aichi from two cases of a family returning from the Philippines, and also from five cases at a dormitory in Kanagawa during weeks 35-36, three cases at a junior high school in Tochigi in week 36, one case in Hyogo in week 37, four cases in Osaka during weeks 41-43, and one case in Hyogo in week 42. Influenza B viruses (Victoria lineage) were isolated from two cases in week 40 and six cases at a primary school in week 41 in Osaka and detected by PCR from two cases at a junior high school in week 38 in Tochigi and four cases at a junior high school in week 41 in Tokyo. In week 43, influenza AH1 viruses were isolated from three cases at a kindergarten in Yamaguchi (reports as of November 4, 2008).