An epidemiological analysis of severe cases of the influenza A (H1N1) 2009 virus infection in Japan.
BACKGROUND
The age distribution of confirmed cases with influenza A (H1N1) 2009 has shifted toward children and young adults, in contrast to interpandemic influenza, because of the age specificities in immunological reactions and transmission characteristics.


OBJECTIVE
Descriptive epidemiological analysis of severe cases in Japan was carried out to characterize the pandemic's impact and clinical features.


METHODS
First, demographic characteristics of hospitalized cases (n = 12,923), severe cases (n = 894) and fatal cases (n = 116) were examined. Second, individual records of the first 120 severe cases, including 23 deaths, were analyzed to examine potential associations of influenza death with demographic variables, medical treatment and underlying conditions. Among severe cases, we compared proportions of specific characteristics of survivors with those of fatal cases to identify predictors of death.


RESULTS
Age distribution of hospitalized cases shifted toward those aged <20 years; this was also the case for deaths without underlying medical conditions. Deaths in adults were mainly seen among those with underlying medical conditions, resulting in an increased risk of death as a function of age. According to individual records, the time from onset to death in Japan appeared rather short compared with that in other countries.


CONCLUSIONS
The age specificity of severe cases and their underlying medical conditions were consistent with other countries. To identify predictors of death in influenza A (H1N1) 2009 patients, more detailed clinical characteristics need to be examined according to different age groups and types of manifestations, which should ideally include mild cases as subjects.
Wada
Koji
K
Department of Preventive Medicine and Public Health, Kitasato University School of Medicine, Kanagawa, Japan. kwada@med.kitasato-u.ac.jp
Nishiura
Hiroshi
H
Kawana
Akihiko
A
D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D002648:Child; D002675:Child, Preschool; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000472:pathogenicity*; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality / Q000503:physiopathology* / Q000821:virology; D007564:Japan; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D012720:Severity of Illness Index; D055815:Young Adult