Influenza virus (seasonal + AH1pdm) in Japan,
    from week 19 of 2009 to week 19 of 2010


Isolation/detection of influenza virus in Japan, from week 19 (May 4-10) of 2009
to week 19 (May 10-16) of 2010 (as of May 13, 2010)
Figure 1. Weekly cases of influenza and isolation/detection of influenza viruses from week 36 of
2008 to week 19 of 2010
Figure 2. Isolation/detection of influenza virus by prefecture, week 19 of 2009-week 19 of 2010
  From May 2009, PHIs have carried out rapid detection of pandemic influenza AH1pdm virus by RT-PCR to differenciate from seasonal influenza virus.  In week 19 (May 4-10), the first two influenza AH1pdm virus were isolated.  The number of reports of detection/isolation of AH1pdm sharply increased from week 20 (May11-17), exceeded 500 in week 28 and decreased from week 4 of 2010 and kept a low level after week 12 of 2010 (Fig. 1).  

  From week 19 of 2009 to week 19 of 2010, PHIs in all 47 prefectures (including Tokyo Metropolis) reported isolation/detection of AH1pdm from 31,435 cases.  During the same period, isolation/detection of seasonal AH1 from 66 cases, AH3 from 948 cases and B from 203 cases were reported .  The proportion of AH1pdm to all influenza virus is 96%.

  During 17 weeks from week 19 to week 35 of 2009 (the last week of the 2008/09 season), isolation/detection of AH1pdm from 9,724 cases in all prefectures, AH1 from 66 cases in 24 prefectures, AH3 from 913 cases in 40 prefectures, and B from 103 cases in 21 prefectures were reported (Fig. 2, lower). 

  From week 36 of 2009 (the first week of the 2009/10 season) to week 19 of 2010, isolation/detection of AH1pdm from 21,710 cases in all prefectures were reported.  In addition, isolation/detection of seasonal AH3 (from 15 cases in 10 prefectures during weeks 36-44 and 20 cases in 10 prefectures during weeks 8-19), B/Yamagata lineage (from a case in Osaka Prefecture in week 47, a case in Shiga Prefecture in week 2, 4 cases in 2 prefectures in weeks 9-10, 3 cases in 2 prefectures in weeks 13-14 and 2 cases in Mie Prefecture in week 17), B/Victoria lineage (from a case in Niigata Prefecture in week 50 and 78 cases in 14 prefectures during weeks 3-17) and B/lineage not determined (from 10 cases in 5 prefectures during weeks 5-16) were reported.  However, seasonal AH1 has not been reported since week 36 (Fig. 2, upper).

Infectious Agents Surveillance Report, Infectious Disease Surveillance Center,
National Institute of Infectious Diseases

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