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Volume 16, Number 10—October 2010
Letter

Underlying Medical Conditions and Hospitalization for Pandemic (H1N1) 2009, Japan

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To the Editor: Early epidemiologic reports suggested that infection with pandemic (H1N1) 2009 virus most commonly occurred in teenagers and young adults (1). Although this infection appears to have a mild clinical course and the mortality rate has been relatively low in Japan (1.6 deaths/1 million population) (2), the reported number of patients with severe (requiring intubation or admission to an intensive care unit) cases of this disease has been increasing (2). The difference in severity of pandemic (H1N1) 2009 infection may be attributed to differences in underlying medical conditions or to age-related differences in susceptibility. To explore these differences, we investigated the incidence of laboratory-confirmed cases of pandemic (H1N1) 2009 virus infections resulting in hospitalization in Japan and the patients’ age-specific risks for hospitalization associated with underlying medical conditions.

During the outbreak in Japan, hospitals, local public health centers, and local governments were required to report hospitalizations associated with pandemic (H1N1) 2009 virus to the Ministry of Health, Labour and Welfare in Japan. Infection was confirmed by PCR at local public health centers. The data for hospitalized patients were integrated through a nationwide surveillance system, the interim National Epidemiologic Surveillance of Infectious Diseases.

We collected data on patients who were hospitalized in Japan for pandemic (H1N1) 2009 virus infection from July 28 through December 14, 2009. The number of new cases (incidence) increased in the middle of August and peaked at the end of November (2). We excluded from analysis those patients who had been hospitalized for the purpose of infection containment. Medical conditions were defined as diseases that are risk factors for severe illness associated with seasonal influenza: chronic respiratory diseases, chronic cardiovascular diseases, chronic renal diseases, chronic liver diseases, neurologic diseases, hematologic diseases, diabetes, and immunosuppression caused by treatments or illnesses (including malignant neoplasm) and chronic childhood diseases (3,4). Data on the prevalence of the diseases among the population were obtained from the 2005 National Patient Survey and from population estimates obtained by the Ministry of Internal Affairs and Communications in May 2009.

During the surveillance period, 12,702 laboratory-confirmed cases for which the patients required hospitalization were reported; 110 of these patients died (Table). Hospitalization incidence was 10.0 admissions/100,000 persons (overall incidence of hospitalization in the general population in 2005 was 1,462.8 admissions/100,000 persons). Median age of hospitalized patients was 7 years (interquartile range 5–11 years). Of the 10,721 patients for whom type of care was known, 680 (6.4%) were admitted to an intensive care unit. Among 486 hospitalized women 15–44 years of age, only 42 (8.6%) were pregnant; this percentage is small compared with results of studies conducted in other countries (57). None of the pregnant women were in critical condition and none died of pandemic (H1N1) 2009 infection during the surveillance period. The overall lower prevalence of pregnant women in Japan (≈0.67% of the total population) compared with that in other countries (8) might account for the low number of pregnant women hospitalized for pandemic (H1N1) 2009 virus in Japan.

Of all 12,702 patients, 4,420 (34.8%) had underlying medical conditions. In terms of age, 1,848 (32.6%) of 5,670 patients 5–9 years of age and 324 (81.0%) of 400 patients >70 years of age had underlying medical conditions. For those with underlying medical conditions, incidence of hospitalization was relatively higher for children than for adults; rates for hospitalized children were 814.1, 573.7, and 219.0 per 100,000 persons for those 5–9, 10–14, and 15–19 years of age, respectively; whereas, the rate for hospitalized adults 60–69 years of age was 6.7/100,000 persons. In all age categories, the incidence of hospitalization for pandemic (H1N1) 2009 virus infection was notably higher for patients with underlying medical conditions than for those without. The attributable risk for medical conditions associated with hospitalization for pandemic (H1N1) 2009 virus infection was highest among patients 5–9 and 10–14 years of age (744.7 and 549.6, respectively), whereas the risk was lowest (6.5) among patients 60–69 years of age. The relative risk associated with underlying medical conditions was comparatively higher for patients in age groups 15–19, 20–29, and 30–39 years (48.4, 54.9, and 48.4, respectively) than among those >70 years of age (17.9). In contrast to seasonal influenza (4), persons <20 years of age had higher risk for hospitalization for pandemic (H1N1) 2009 than did older age groups.

Considering our findings regarding the attributable risk for hospitalization, interventions that aim to control pandemic (H1N1) 2009 virus infection among children, especially those with underlying medical conditions, can be considered key for minimizing the strain (financial, staffing, space) on the healthcare system. Our findings justify prioritizing the treatment of children and young adults by vaccination and early prescription of antiviral drugs.

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Taro TomizukaComments to Author , Yoshihiro Takayama, Tokuaki Shobayashi, Yasumasa Fukushima, and Yasuhiro Suzuki

Author affiliations: Author affiliations: National Institute of Public Health, Saitama, Japan (T. Tomizuka, Y. Suzuki); Ministry of Health, Labour and Welfare, Tokyo, Japan (T. Tomizuka, Y. Takayama, T. Shobayashi, Y. Fukushima, Y. Suzuki); Tokyo Medical and Dental University, Tokyo (T. Tomizuka)

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References

  1. Shimada  T, Gu  Y, Kamiya  H, Komiya  N, Odaira  F, Sunagawa  T, Epidemiology of influenza A(H1N1) virus infection in Japan. Euro Surveill. 2009;14:pii:19244.
  2. Office of Pandemic Influenza Preparedness and Response, Ministry of Health, Labour and Welfare. Epidemiological information update: pandemic (H1N1) 2009 in Japan (ver. 3) [in Japanese]. 2009 Apr 23 [cited 2010 Jun 29]. http://www.mhlw.go.jp/bunya/kenkou/kekkaku-kansenshou04/pdf/100423-01.pdf
  3. Harper  SA, Bradley  JS, Englund  JA, File  TM, Gravenstein  S, Hayden  FG, Seasonal influenza in adults and children—diagnosis, treatment, chemoprophylaxis, and institutional outbreak management: clinical practice guidelines of the Infectious Diseases Society of America. Clin Infect Dis. 2009;48:100332. DOIPubMed
  4. Thompson  WW, Shay  DK, Weintraub  E, Brammer  L, Bridges  CB, Cox  NJ, Influenza-associated hospitalizations in the United States. JAMA. 2004;292:133340. DOIPubMed
  5. Louie  JK, Acosta  M, Jamieson  DJ, Honein  MA; California Pandemic (H1N1) Working Group. Severe 2009 H1N1 influenza in pregnant and postpartum women in California. N Engl J Med. 2010;362:2735. DOIPubMed
  6. Hewagama  S, Walker  SP, Stuart  RL, Gordon  C, Johnson  PDR, Friedman  ND, 2009 H1N1 influenza A and pregnancy outcomes in Victoria, Australia. Clin Infect Dis. 2010;50:68690. DOIPubMed
  7. Siston  AM, Rasmussen  SA, Honein  MA, Fry  AM, Seib  K, Callaghan  WM, Pandemic 2009 influenza A(H1N1) virus illness among pregnant women in the United States. JAMA. 2010;303:151725. DOIPubMed
  8. Jamieson  DJ, Honein  MA, Rasmussen  SA, Williams  JL, Swerdlow  DL, Biggerstaff  MS, H1N1 2009 influenza virus infection during pregnancy in the USA. Lancet. 2009;374:4518. DOIPubMed

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Cite This Article

DOI: 10.3201/eid1610.091755

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Table of Contents – Volume 16, Number 10—October 2010

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Taro Tomizuka, National Institute of Public Health, 2-3-6 Minami, Wako, Saitama, Japan

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Page created: September 08, 2011
Page updated: September 08, 2011
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