The Topic of This Month Vol. 33, No. 11 (No. 393)

2011/12 influenza season, Japan
(IASR 33: 285-287, November 2012)


The 2011/12 season's influenza epidemic (from week 36/September of 2011 to week 35/August of 2012) was caused mainly by subtype AH3 and to lesser extent by influenza virus type B.  Influenza A(H1N1)pdm09 that dominated in the 2009/10 epidemic and occupied about a half of the influenza virus isolates in 2010/11 season was rare after April 2011.

Incidence of Influenza: Under the National Epidemiological Surveillance of Infectious Diseases (NESID), 5,000 influenza sentinels (3,000 pediatric and 2,000 internal medicine clinics) report diagnosed influenza cases at weekly basis.  In the 2011/12 season, the epidemic index (number of cases/sentinel) became above 1.0 in week 49 nationwide, and the level was maintained for 22 weeks till week 18 of 2012.  The epidemic attained its peak in week 5 of 2012 with the incidence of 42.7 cases/sentinel (Fig. 1), which was the second highest in the past 10 seasons following the peak observed in 2004/05 season (50.1 cases/sentinel) (  The cumulative number of cases per sentinel of this season, 342.5, was also the second highest in the past 10 seasons following 415.4 in 2009/10 season.

At prefecture levels, the epidemic index exceeded 10.0 first in Miyagi and Aichi in week 50 of 2011.  The number of prefectures with the epidemic index exceeding 10.0 increased to 12 in week 2 of 2012 and then to 42 in week 3 resulting in the nationwide influenza epidemic (

The total number of patients who visited medical institutions, which was estimated from sentinel site reports, was about 16,480,000 from week 36 of 2011 to week 18 of 2012 (September 6, 2011-May 6, 2012).  According to the hospitalization surveillance that started in September 2011, total 11,118 patients were hospitalized in the “designated sentinel hospitals” (about 500 hospitals with more than 300 beds), among which 1,487 were clinically severe (

Isolation/detection of influenza virus: In 2011/12 season, the prefectural and municipal public health institutes isolated total 5,457 influenza viruses (as of October 18, 2012, Table 1).  In addition, there were 1,799 influenza virus detections by PCR alone.  Among the total 7,256 isolated/PCR-detected viruses, 5,755 were derived from influenza sentinels and 1,501 from elsewhere (Table 2).

Influenza viruses of the 2011/12 season consisted mainly of subtype AH3 (71%) and type B (28%).  AH1pdm09 were few (0.2%).  Former seasonal AH1 subtype virus has not been isolated since week 36 of 2009.  Influenza B viruses consisted of Victoria and Yamagata lineages, whose isolation/detection ratio was 2:1.  Viruses isolated/detected from overseas travelers were subtype AH3 (25 cases), type B (5 cases) and AH1pdm09 (2 cases) (Table 2).

Nationwide, subtype AH3 was predominant among the isolates from the beginning till week 9 of 2012, well after the epidemic peak, when type B influenza started to exceed subtype A (Fig. 1 and Fig. 2).  In Okinawa Prefecture, however, the epidemic did not fade; the patients further increased from week 26 of 2012.  The AH3 epidemic in this region lasted from June to September of 2012 (IASR 33: 242, 2012).

As for age distribution of the patients, the peak age was invariably 5-9 years for subtype AH3, B/Victoria and B/Yamagata lineages (Fig. 3 and Fig. 4).

Antigenic characteristics of the 2011/12 season isolates and their drug resistance (see p. 288 of this issue): Of the eight AH1pdm09 isolates tested, six were antigenically similar to A/California/7/2009, the vaccine strain for 2009/10-2012/13 seasons.  Subtype AH3 isolates was antigenically similar to A/Victoria/361/2011 (the 2012/13 season vaccine strain), whose antigenicity was slightly changed from that of A/Victoria/210/2009 (the 2010/11-2011/12 season vaccine strain used in Japan).  Antigenicity of Victoria lineage isolates that occupied 2/3 of all the type B isolates was similar to that of B/Brisbane/60/2008 (the 2009/10-2011/12 season vaccine strain), and that of Yamagata lineage that occupied 1/3 of the type B isolates was similar to that of B/Wisconsin/1/2010 (2012/13 season vaccine strain).

None of the nine AH1pdm09 isolates tested had the H275Y mutation implicated in the oseltamivir resistance, while 2.0% of the tested 2010/11 season isolates had the H275Y mutation.  Among 278 subtype AH3 isolates tested, only one had R292K mutation attributable to oseltamivir/peramivir resistance (

Immunological status of Japanese population: According to the data of National Epidemiological Surveillance of Vaccine-Preventable Diseases (see p. 294 of this issue) that was obtained with serum samples collected from July to September in 2011, frequency of anti-A/California/7/2009pdm09 HI antibody positives (titer higher than 1:40) was average 49%.  The antibody positive rate was relatively high for 5-24 years of age (64-78%).  For age groups 0-4 years and 50 years or older, positive rates were 24-38%, which were higher than in 2010 (blood samples collected during July-September).  Antibody positive populations for subtype AH3 and B/Victoria lineage were average 50% and 45%, respectively, and were highest among age group of 15-19 years (68% and 57%, respectively).  Antibody positive rate for B/Yamagata lineage was generally low, 18% in average and highest (38%) in age group 15-19 years.

Influenza vaccine: The quantity of trivalent vaccines produced in 2011/12 season was 28,880,000 vials (calculated for 1mL/vial), of which estimated 25,100,000 vials were used for vaccination.  The vaccination coverage of the elderly (older than 65 years) in compliance with the Preventive Vaccination Law was 53.3% (53.1% in 2010/11 season).

The vaccine strain selected for 2012/13 season for AH1 was A/California/7/2009pdm09 which is the same as for 2010/11 and 2011/12 seasons, whereas the vaccine strains for AH3 and type B were changed to A/Victoria/361/2011 and B/Wisconsin/1/2010 (Yamagata lineage), respectively (see p. 297 of this issue).

Avian influenza virus A(H5N1) and swine-origin A(H3N2) variant: From November 2010 to March 2011, highly pathogenic avian influenza (HPAI) virus subtype A(H5N1) spread among wild birds and domestic fowl in various places in Japan.  Epidemic of the virus among birds has been continuously reported from Indonesia, Vietnam and Egypt before and after 2011 and human cases of A(H5N1) infection, too.  More recently in September 2012, HPAI among birds was reported from China and Nepal (

Since July 2012, United States has reported more than 300 human cases of A(H3N2) variant influenza virus infections that occurred through exposure to pigs.

Act on Special Measures For Pandemic Influenza, etc. Preparedness and Response: In preparation for the case of rapid spread of the novel influenza and other emerging/re-emerging infectious diseases of similar public health concern, and based on the experience of pandemic (H1N1)2009, “Act on Special Measures For Pandemic Influenza etc. Preparedness and Response” was issued on May 11 of 2012 (

Additional comments: Trends of outbreaks should be monitored continuously by sentinel surveillance, school outbreak surveillance, hospitalization surveillance and other possibilities.  The virus isolation should be conducted throughout the year and the antigenic and genetic changes of the epidemic strains should be monitored so as to secure vaccine candidate strains.  Monitoring of the resistance to anti-influenza drugs among isolates should be continued.  These measures are all important for future risk management measures.

Flash reports on the isolation and detection of influenza viruses in 2012/13 season are found in p. 300 of this issue and


Copyright 1998 National Institute of Infectious Diseases, Japan