The Topic of This Month Vol. 32, No. 11 (No. 381)

2010/11 influenza season, Japan
(IASR 32: 314-316, November 2011)

While the previous 2009/10 influenza season experienced an unusual epidemic of A(H1N1)pdm09 in autumn of 2009, the usual epidemic pattern with a single peak in winter returned to the 2010/11 season (starting in week 36 of 2010/September and ending in week 35 of 2011/August) (

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 weekly.   The seasonal epidemic (weekly cases per nationwide sentinel exceeding 1.0) started in week 50 of 2010 and ended in week 21 of 2011 lasting for 24 weeks.   The main epidemic peak was in week 4 of 2011 (31.9 cases/sentinel); the second and the third smaller peaks were observed in weeks 11 and 16 (Fig. 1).   The incidence of influenza of this seasonfs peak was the eighth largest in the past 10 seasons.   However, on account of protracting influenza season, the cumulative number of cases per sentinel (275.00) of this season was the fourth high in the past 10 seasons.   The estimated total number of patients who visited medical facilities (including non-sentinel facilities) in Japan between week 36 of 2010 and week 22 of 2011 was about 13,760,000 (95% confidence interval: 13,430,000-14,100,000) (provisional figures).

The number of cases per sentinel exceeded 10.0 in Okinawa, Fukuoka and Saga Prefectures in the first week of 2011 and in the next week in total 26 of 47 prefectures (

According to “Overview of severe and fatal influenza cases (Tuberculosis and Infectious Diseases Control Division, Ministry of Health, Labour and Welfare)” (, 404 persons were hospitalized with acute pneumonia on account of ventilator needs (190 cases), acute encephalopathy (115 cases) or requirement of intensive care unit use (287 cases) from September 6 of 2010 to March 15 of 2011 (figures with overlaps).   There were 149 deceased cases, among whom 124 had underlying disease(s) and 19 were not hospitalized.

Isolation/detection of influenza virus: Total 7,625 influenza viruses were isolated by the prefectural and municipal public health institutes in 2010/11 season (as of October 18, 2011, Table 1 in p. 316 of this issue).   In addition, there were 4,343 reports of influenza viruses that were detected by PCR alone.   Among the total 11,968 isolated/PCR-detected viruses, 8,615 were derived from influenza sentinels and 3,353 from elsewhere (Table 2 in p. 316 of this issue).

Influenza viruses isolated/detected in 2010/11 season consisted of types AH1pdm09 (52%), AH3 (32%) and B (15%) (Table 2 in p. 316 of this issue).   The seasonal AH1 subtype that prevailed until 2008/09 season disappeared after week 36 of 2009.   Type B isolates were mostly of Victoria lineage and rarely of Yamagata lineage.   Isolation/detection from overseas travelers included AH1pdm09 (37 cases), AH3 (24 cases) and type B (7 cases) (Table 2 in p. 316 of this issue).   Among AH1pdm09 strains analyzed in 2010/11 season, 2.0% had H275Y mutation associated with oseltamivir resistance (1.0% in 2009/10 season) (see p. 317 of this issue and

While in the beginning of 2010/11 season AH3 was predominant, AH1pdm09 became dominant in week 49 of 2010, subsided after a peak in week 3 of 2011 and overtaken by AH3 after week 7.   Later than week 12, type B became dominant over type A (Fig. 1).   The first large epidemic peak in Fig. 1 was contributed mainly by AH1pdm09, the second peak by AH3 and the third peak by type B (see also Fig. 2 in p. 316 of this issue).

As for the age distribution of influenza patients, while AH1pdm09 infection was most frequent among 15-19 year olds in 2008/09 season, 5-9 year olds became dominant in 2009/10 and 2010/11 (Fig 3 & Fig 4 and see also IASR 31: 248-250, 2010).   The proportion of the patients older than 20 years was higher in 2010/11 than 2009/10.   AH3 and type B infections were most frequently found among 5-9 year olds in 2010/11, too.

Antigenic characteristics of 2010/11 isolates (see p. 317 of this issue): The antigenic analysis data presented below are those obtained with the isolated viruses only.   AH1pdm09 isolates were all similar to A/California/7/2009 (2009/10 & 2010/11 vaccine strain).   All the AH3 isolates resembled A/Victoria/210/2009 (2010/11 vaccine strain) and A/Perth/16/2009 (WHO recommended vaccine strain).   Dominant isolates among the type B were Victoria lineage and resembled B/Brisbane/60/2008 (2009/10 & 2010/11 vaccine strain).   Type B isolates of Yamagata lineage, which were isolated in small numbers, resembled B/Wisconsin/1/2010 (2010/11 representative strain) and largely different from B/Florida/4/2006 (2008/09 vaccine strain).

Immunological status of Japanese population: According to the data of National Epidemiological Surveillance of Vaccine-Preventable Diseases (see p. 323 of this issue) that was obtained with serum samples collected from July to September in 2010, frequency of anti-A/California/7/2009pdm09 HI antibody positives (titer higher than 1:40) was average 40%, and the positive frequency among each age group was 65% for 10-14 year olds, 64% for 15-19 year olds, 56% for 5-9 year olds and 54% for 20-24 year olds.   Thus the antibody-positive population was much higher in 2010 samples than in 2009 samples collected during the same period (July-September) of the year (IASR 31: 260-261, 2010) except for age groups of 0-4 years and above 50 years (12-24%).   Antibody positives to AH3, B/Victoria lineage and B/Yamagata lineage were 40%, 33% and 27%, respectively.   Among different age groups, the antibody positive rate to AH3, B/Victoria lineage and B/Yamagata lineage was highest for 15-19 year olds (62%), for 35-39 year olds (61%) and for 20-24 year olds (64%), respectively.

Vaccination for 2010/11 and 2011/12 seasons: The quantity of trivalent vaccines produced in 2010/11 season was 29,280,000 vials (by a calculation of 1mL/vial), of which 24,470,000 vials have been used for vaccination by estimate.   The vaccination coverage of the elderly (older than 65 years) in compliance with the Preventive Vaccination Law was 53% (50% in 2009/10 season).

Vaccine strains selected for 2011/12 season were same as for 2010/11; A/California/7/2009 for AH1pdm09, A/Victoria/210/2009 for AH3 and B/Brisbane/60/2008 belonging to the Victoria lineage for type B (see p. 326 of this issue).   From 2011/12 season, infant vaccine dose was changed; now infants above 6 months and below 3 years require two doses of 0.25 mL and those above 3 years and below 13 years two doses of 0.5 mL, both at a 2-4 week interval.   The dose for the person over 13 years is the same as 2010/11 season (one or two doses of 0.5 mL).

Revision of Preventive Vaccination Law: Preventive Vaccination Law and Law Concerning Special Measures for Compensation of Health Damage Associated with Administration of the Pandemic Influenza Vaccine were partially modified (see p. 331 of this issue).   New immunization scheme that can be implemented with a limited timeframe in the situation like influenza A(H1N1)pdm09 was introduced on October 1, 2011.   Unified reporting system for influenza vaccine adverse events started from 2011/12 season to include routine immunization (Preventive Vaccination Law) and voluntary vaccination (Pharmaceutical Affairs Law); the reported data will be compiled by Pharmaceuticals and Medical Devices Agency and evaluated by the joint evaluation committee of Ministry of Health Labour and Welfare.

Hospital-Based Surveillance: From September 5 of 2011, on admission of influenza patients, the sentinel hospitals are required to report the admission to the health centers together with information on need of using intensive care units, respirators, encephalography and other apparatus for diagnosis of acute encephalopathy (see p. 333 of this issue).

Remarks: Since November 2010, different places in Japan experienced outbreaks of highly pathogenic A(H5N1) influenza among wild birds and domestic fowl (   The human cases of avian influenza A(H5N1) infection are continuously reported from abroad.

So as not to lose the lessons learned from the pandemic of A(H1N1)pdm09 and to be prepared for the next influenza pandemic, “Cabinet Meeting on Preparedness against Pandemic Influenza” was held on September 20, 2011, and it updated the “Pandemic Influenza Action Plan” (see p. 332 of this issue).

Through sentinel surveillance, school outbreak surveillance and hospitalization surveillance, the possible emergence of the influenza has to be closely monitored.   Virus isolation should be conducted throughout the year for monitoring of possible antigenic, genomic and drug-sensitivity changes and for collection of appropriate vaccine candidate strains.

Flash report of influenza virus for 2011/12 season is available on (see also p. 334-337 of this issue).

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