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Eurosurveillance, Volume 14, Issue 24, 18 June 2009
Rapid communications
Epidemiology of influenza A(H1N1)v virus infection in Japan, May - June 2009
  1. Infectious Diseases Surveillance Center, National Institute of Infectious Diseases, Tokyo, Japan

Citation style for this article: Shimada T, Gu Y, Kamiya H, Komiya N, Odaira F, Sunagawa T, Takahashi H, Toyokawa T, Tsuchihashi Y, Yasui Y, Tada Y, Okabe N. Epidemiology of influenza A(H1N1)v virus infection in Japan, May - June 2009. Euro Surveill. 2009;14(24):pii=19244. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19244
Date of submission: 10 June 2009

Between 9 May and 4 June 2009, a total of 401 laboratory-confirmed cases of influenza A(H1N1)v virus were reported in Japan, from 16 of the 47 Japanese prefectures. The two areas most affected were Osaka prefecture and Kobe city where outbreaks in high schools occurred leading to school closures. To date all cases have had symptoms consistent with seasonal influenza and no severe or fatal cases have been reported.


Following the emergence of a new influenza A(H1N1) virus (henceforth: influenza A(H1N1)v virus) and the relevant declarations by the World Health Organization (WHO) [1], the Ministry of Health, Labour and Welfare (MHLW) of Japan launched a case-based surveillance for influenza A(H1N1)v virus infection in addition to the existing sentinel surveillance system for seasonal influenza and imposed entry screening on travelers from affected areas (Canada, Mexico and the United States) starting from 28 April 2009 [2].

The following case definitions of suspected and confirmed cases have been used:

A suspected case of influenza A(H1N1)v virus infection is defined as a person with high fever (>38°C) OR at least two acute respiratory symptoms (nasal obstruction/rhinorrhea, sore throat, cough, fever/feverishness) AND who meets at least one of the following criteria: 

    a) within the last seven days returned from a country or region with an epidemic of influenza A(H1N1)v;
    b) was in close contact (within two meters) with a confirmed case within the past seven days;
    c) handled samples suspected of containing influenza A(H1N1)v virus in a laboratory or other setting within the past seven days;

A confirmed case of influenza A(H1N1)v virus infection is defined as a person with high fever (>38°C) OR at least two acute respiratory symptoms (nasal obstruction/rhinorrhea, sore throat, cough, fever/feverishness) AND influenza A(H1N1)v virus infection that has been laboratory confirmed by real-time PCR and/or viral isolation.

For all travellers from the affected areas who are febrile at the entry, a quarantine officer performs a rapid diagnostic test for influenza. If the result of rapid test is positive for influenza A, a PCR test for influenza A(H1N1)v is done. The Quarantine Law and the Pandemic Influenza Preparedness Action Plan of the Japanese Government request confirmed cases and close contacts of confirmed cases to be hospitalised/isolated for seven days considered to be the infectious period [3,4].

The primers for conventional and real-time RT-PCR for the detection of A(H1N1)v virus were developed by the National Institute of Infectious Diseases and became available on 29 April. All 75 prefectural and municipal public health institutes and quarantine stations in Japan became ready to perform conventional and real-time RT-PCR test by 4 May. Since the first laboratory-confirmed cases were reported on 9 May, the number of cases of influenza A(H1N1)v increased continuously, resulting in a total of 401 laboratory-confirmed cases as of 4 June 2009. This report summarises the epidemiological characteristics of the confirmed cases reported in Japan from May to June.

The first four laboratory-confirmed cases of influenza A(H1N1)v were reported at the Narita International Airport quarantine station on 9 May 2009. The patients were travellers who returned from Canada on 9 May. Although all of them showed mild symptoms, they were hospitalised in an isolation ward of a designated hospital for seven days, in accordance with the Quarantine Law and the Pandemic Influenza Preparedness Action Plan of the Japanese Government [3,4].

The first laboratory-confirmed cases without travel history were detected on 16 May as follows:

A high school in Ibaraki city, in Osaka prefecture near the border with Hyogo prefecture, noticed an increase in the number of absentees due to influenza-like symptoms in the middle of May 2009. On 16 May the school was closed in conformity with the School Health Law [5]. According to this law (enacted in 1958), influenza-like illness/seasonal influenza is one the infectious diseases that can trigger school closure. The number of absentees that leads to school closure is decided by the school authorities. In many cases, 5 to 10 absentees in a class may lead to closing the class; 2-3 closed classes may lead to school closure.

None of the sick high school pupils in Ibaraki had travel history to the countries affected by the new influenza. On 16 May, five teenagers were confirmed with influenza A(H1N1)v virus infection: one from the school in Ibaraki in Osaka prefecture, and four from Kobe City in the neighbouring Hyogo prefecture. Subsequently, outbreaks in three schools were reported during the next few days in these adjacent prefectures. The local governments of Kobe City and Osaka prefecture implemented extensive school closures, deciding to close not only schools with infected students but all schools in both districts, for one to two weeks from 16 May. As a result, over 4,200 schools with around 650,000 children/students were closed. By 19 May, the number of confirmed cases reported in the two districts reached 172. However, after school closures, the number of new confirmed cases decreased (Figure 1). By 4 June a total of 357 cases were reported from the two prefectures.

Figure 1. Confirmed cases of influenza A(H1N1)v virus infection in Japan, by date of onset and cumulative number as of 4 June 2009 (n=392*)



Outside these two prefectures only sporadic cases were reported, the majority of whom had a travel history abroad or an epidemiological link to a traveller from affected areas including Osaka (Figure 2). In all, confirmed cases were reported from 16 of the total of 47 Japanese prefectures.

Figure 2. Geographical distribution of confirmed cases of influenza A(H1N1)v virus infection in Japan as of 4 June 2009 (n=401)



Reflecting the outbreaks in high schools described above, confirmed cases in the age group of 15-19 years accounted for 64% (256) of all cases, followed by 10% (40) of cases in the age group of 10-14 years. Only four cases (1%) were over 60 years of age (Figure 3). Overall, the median age of cases was 16.0 (range 1-69 years). Male cases accounted for 63% (254) and female cases for 37% (147) of all cases. Large outbreaks observed in high schools may have contributed to the difference in gender (as more boys than girls attend the affected schools).

Figure 3. Age distribution of confirmed cases of influenza A(H1N1)v virus infection in Japan as of 4 June 2009 (n=401)



Information on clinical symptoms was available for 217 confirmed cases (Figure 4). The most frequent were fever (206, 95%), cough (128, 59%), and sore throat (85, 39%). Thirteen cases (6%) reported diarrhoea and five cases (2%) had nausea. Antiviral treatment of either oseltamivir or zanamivir was prescribed to about 90% of the 217 confirmed cases with known clinical symptoms.

Figure 4. Clinical symptoms of confirmed cases of influenza A(H1N1)v virus infection in Japan as of 4 June 2009 (n=217)



No cases with pneumonia and/or respiratory failure, requiring ventilatory support, were reported. Other severe symptoms such as multiple organ failure were not reported either. Only three cases required hospitalisation due to underlying medical conditions, although a total of 135 cases were hospitalised for the purpose of isolation based on the Quarantine Law and the Pandemic Influenza Preparedness Action Plan of the Japanese Government [3,4].

Among the confirmed cases, six (including two cases aged over 60 years) had underlying diseases: asthma (3), asbestosis (1), epilepsia (1), myodystrophia (1); and one case was pregnant. As of 4 June 2009, no severe or fatal case had been reported.

The epidemiological characteristics of the patients with influenza A(H1N1)v virus infection have been reported by the investigation teams including members of IDSC/NIID and local government, who conclude that the severity of disease is similar to that of seasonal influenza [6,7].

The next steps include addressing the questions of how to improve the surveillance system to detect, monitor, and control the cases of influenza A (H1N1)v and how to prepare for the more severe cases as the epidemic is expected to expand in the winter season. We need to decide when the case-based surveillance for influenza A(H1N1)v should be ceased and integrated into the sentinel surveillance of seasonal influenza. To evaluate the pathogenicity, planned surveillance systems, such as severe pneumonia surveillance and ILI cluster surveillance, should be launched before the coming winter season. The Pandemic Influenza Preparedness Action Plan of the Japanese Government also needs to be amended so that medical resources would not be wasted by the patients with mild symptoms merely for the purpose of isolation.

Acknowledgement
We thank Dr Yamashita, Dr Morikane, Dr Shigematsu, Dr Taya, Dr Yahata, Ms Otake and Ms Maeda for their review and support.


References

  1. World Health Organization (WHO). Swine influenza - Statement by WHO Director-General, Dr Margaret Chan. 27 April 2009. Available from: http://www.who.int/mediacentre/news/statements/2009/h1n1_20090427/en/index.html
  2. Ministry of Health, Labour, and Welfare (MHLW) of Japan. Official notification [in Japanese]. 29 April 2009. Available from: http://www.mhlw.go.jp/kinkyu/kenkou/influenza/090429-02.html
  3. Ministry of Health, Labour, and Welfare (MHLW) of Japan. Official notification about amendment of the Quarantine Law [in Japanese]. 12 May 2008. Available from: http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou04/pdf/16-04.pdf
  4. Ministry of Health, Labour, and Welfare (MHLW) of Japan. Pandemic Influenza Preparedness Action Plan of the Japanese Government. October 2007. Available from: http://www.mhlw.go.jp/english/topics/influenza/dl/pandemic02.pdf
  5. Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. School Health Law [in Japanese].13 June 1958 (amended on 31 March 2008) Available from: http://law.e-gov.go.jp/htmldata/S33/S33F03501000018.html
  6. Infectious Disease Surveillance Center (IDSC)/National Institute of Infectious Diseases (NIID), Kobe Institute of Health. Interim report on clinical presentation of the novel influenza A (H1N1) cases reported from Kobe City. 21 May 2009. Available from: http://idsc.nih.go.jp/disease/swine_influenza_e/idsc_e2009/clinical_epi_osaka2.html
  7. Infectious Disease Surveillance Center (IDSC)/National Institute of Infectious Diseases (NIID), Osaka Prefecture and Public Health Center of Osaka Prefecture. Interim report on two clusters of the novel influenza A (H1N1) infection in Osaka Prefecture. 19 May 2009. Available from: http://idsc.nih.go.jp/disease/swine_influenza_e/idsc_e2009/clinical_epi_kobe.html


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