Incidence of Influenza: Under the National Epidemiological Surveillance of Infectious Disease (NESID), the influenza sentinels (3,000 pediatric and 2,000 internal medicine clinics) weekly report the number of clinically diagnosed influenza cases. Weekly cases per sentinel on the nationwide level outnumbered 1.0, an indicator of the start of the influenza epidemic, in week 49 of 2008, peaked in week 4 of 2009 (37.5), subsided temporarily in weeks 6-8, peaked again in week 11 (16.5), and gradually decreased to less than 1.0 in week 22 (Fig. 1 and http://idsc.nih.go.jp/idwr/kanja/weeklygraph/01flu.html). As for pandemic (H1N1) 2009, the sentinel clinics started to report increasingly from week 28 when the local governments stopped "fever clinic" and cases/sentinel exceeded 1.0 again in week 33 (1.7). In the past 10 seasons, the peak incidence of this season was the third highest following the 2004/05 and 2002/03 seasons; the total number of cases per sentinel of this season (288.70) was the second highest following the 2004/05 season.
Hokkaido Prefecture experienced the first wave of influenza earlier than other prefectures (https://nesid3g.mhlw.go.jp/Hasseidoko/Levelmap/flu/index.html). Okinawa Prefecture, where the influenza epidemic persisted till summer in the past four seasons, was attacked by the pandemic (H1N1) 2009 towards the end of July before termination of seasonal influenza (IASR 30: 264-265, 2009).
The number of influenza encephalopathy which is categorized as "acute encephalitis" (category V infectious disease that requires report of all the cases) reported by week 18 was total 53 (due to seasonal influenza; 42 for type A, 7 for type B, and 4 for type unknown). Between week 27 and 35, there were 14 additional reports (10 for AH1pdm, 3 for type A and 1 for type B).
Isolation/detection of influenza viruses: During 2008/09 season, the number of viruses detected by isolation by prefectural and municipal public health institutes (PHIs) was 9,963 (as of November 4, 2009, Table 1). The number of viruses detected by PCR alone was 6,471, in much larger number than usual, owing to the fact that PHIs took part in PCR differential diagnosis of AH1pdm since May. Among total 16,434 influenza virus isolation/ detection, those from specimens collected at non-sentinel clinics (8,838) exceeded those from specimens collected at influenza sentinel clinics (7,596) (Table 2). From people who came back from abroad, not only AH1pdm (692) but also seasonal AH1 (39), AH3 (154), and type B (3) were isolated/detected (Table 1).
From the start of 2008/09 season, AH3, B of Victoria lineage and AH1, all associated with outbreaks, were isolated one after another (IASR 29: 340-341, 2008). Oseltamivir resistance with H275Y mutation was initially found among AH1 isolates from Sendai City and Shiga Prefecture in October and in November, respectively (IASR 30: 47-49, 2009). The oseltamivir-resistant AH1 then became the majority of all the seasonal influenza viruses (IASR 30: 49-53, 2009). In 2008/09 season, 99% of AH1 isolates from Japan turned out to be oseltamivir-resistant (see p. 287 of this issue and IASR 30: 101-106, 2009).
AH3 and AH1 peaked at week 4 (Fig. 1), and AH3 peaked again at week 21 (IASR 30: 182-184, 2009). Type B peaked at week 10, and persisted till week 28 (Fig. 1 and http://idsc.nih.go.jp/iasr/prompt/graph-pke.html).
Isolation/detection of AH1pdm (first isolated in week 19) from the influenza sentinel clinics started to increase from week 32 (Fig. 1).
As for the age distribution (Fig. 2), AH1 that was dominant both in the preceding and this season had two peaks, one at the age of 6 among children and the other in the age of 30fs among adults. AH3 increased in all ages from 2007/08 season. Influenza B was most frequently isolated from children with the peak at 8 years. AH1pdm was most frequently isolated from 15-19 year olds.
Antigenic characteristics of 2008/09 isolates: Among AH1 isolates, 94% obtained in the first half of the season and 85% obtained in the latter half of the season resembled A/Brisbane/59/2007 (the vaccine strain for 2008/09 season). As for AH3 isolates, 72% obtained in the first half of the season resembled A/Uruguay/716/2007 (the vaccine strain for 2008/09 season), but later than March, A/Perth/16/2009-like viruses with entirely different antigenicity occupied 75% of the isolates. For type B, Victoria-lineage viruses prevailed in 2008/09 season (75%). Victoria-lineage isolates were largely deviated from B/Malaysia/2506/2004 (the vaccine strain for 2006/07-2007/08 seasons) in their antigenicity. B/Yamagata-lineage isolates resembled B/Bangladesh/3333/2007 and showed antigenicity different from B/Florida/4/2006 (the vaccine strain for 2008/09 season). AH1pdm isolates were all similar to A/California/7/2009pdm (the vaccine strain for 2009) in their antigenicity (see p. 287 of this issue).
Seasonal vaccine strains selected for 2009/10 season: A/Brisbane/59/2007 and A/Uruguay/716/2007 were selected for AH1 and AH3, respectively (the same as for 2008/09 season). For B, selected was B/Brisbane/60/2008 belonging to the Victoria lineage (see p. 287 of this issue).
Vaccine production and immunization rate among the elderly: For 2008/09 season, 53,920,000 doses of the seasonal vaccine were produced, and 49,020,000 doses were used. The vaccination coverage among the elderly (primarily those aged 65 years or older defined by the Preventive Vaccination Law) was 56% in 2008/09 season (55% in 2007/08 season). For 2009/10 season, after having produced 45,040,000 doses of the seasonal vaccine, the production was switched to AH1pdm vaccine and 54,000,000 doses will be produced by March 2010. The immunization of AH1pdm vaccine started on October 19 targeting the medical staff first.
Conclusion: It is increasingly important to grasp the trends of both pandemic (H1N1) 2009 and seasonal influenza. Sentinel surveillance and infectious agent surveillance, both have been playing an important role in this respect, should be strengthened. Virus isolation should be conducted throughout the year, and possible antigenic, genomic and drug-sensitivity changes should be followed. These activities are crucial for detecting possible changes in the properties of the virus including its virulence and also for obtaining appropriate vaccine strains. Vaccine strain for AH1 recommended for 2010 in the Southern Hemisphere is A/California/7/2009pdm in place of the seasonal influenza (WHO, WER 84: 421-432, 2009).
Data obtained for 2009/10 season, preliminary report (http://idsc.nih.go.jp/iasr/influ-e.html): AH1pdm are continuously reported in large numbers. AH3 was isolated/detected from 7 cases in Fukuoka, Hokkaido, Saitama, Shizuoka and Wakayama Prefectures in weeks 36-39 (see p. 297 of this issue), but there have been no reports of influenza B and AH1 since week 29 and week 36 respectively (as of November 10, 2009).