title: Sex- and age-related differences in morbidity rates of 2009 pandemic influenza A H1N1 virus of swine origin in Japan.
abstract: BACKGROUND
  The objective of the present study was to determine whether the morbidity rates of the 2009 pandemic influenza A H1N1 virus (pdmH1N1) varied by age and/or sex.
  
  
  RESULTS
  Retrospective analysis of 2,024,367 cases of pdmH1N1 was performed using the national surveillance data from influenza sentinel points in Japan. The male-to-female morbidity ratios (M/F ratios) in nineteen age groups were estimated as the primary outcome. The M/F ratios for pdmH1N1 influenza were: >1 in age groups <20 years and ≥80 years (p<0.001); <1 in age groups 20-79 years (p<0.001). This data suggests that males <20 years of age may be more likely to suffer from pdmH1N1 influenza than females in the same age categories. When the infection pattern for pdmH1N1 was compared with that of seasonal influenza outbreaks between 2000 and 2008, the M/F ratio for pdmH1N1 influenza was higher in ages 3-29 years and lower in ages 40-79 years. Because the present study was based on the national surveillance, it was impossible to estimate the morbidity rate for the Japanese population. It is also likely that the data did not capture asymptomatic or mild infections.
  
  
  CONCLUSIONS
  Although exposure to the pdmH1N1 virus is assumed to be similar in both boys and girls, M/F ratios were >1 in those younger than 20 years. The subsequent reversal of the M/F ratio in the adult generation could be due to several possibilities, including: greater immunity among adult males, more asymptomatic infections among males, less reporting of illness by males, or differences in exposure to the virus and probability of visiting a clinic. These results suggest that the infection and virulence patterns of pdmH1N1 are more complex than previously considered.
authors.affiliation: Department of Biostatistics, Oita University Faculty of Medicine, Yufu City, Oita, Japan.
authors.forename: Nobuoki
authors.initials: N
authors.lastname: Eshima
authors.forename: Osamu
authors.initials: O
authors.lastname: Tokumaru
authors.forename: Shohei
authors.initials: S
authors.lastname: Hara
authors.forename: Kira
authors.initials: K
authors.lastname: Bacal
authors.forename: Seigo
authors.initials: S
authors.lastname: Korematsu
authors.forename: Minoru
authors.initials: M
authors.lastname: Tabata
authors.forename: Shigeru
authors.initials: S
authors.lastname: Karukaya
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Yasui
authors.forename: Nobuhiko
authors.initials: N
authors.lastname: Okabe
authors.forename: Toyojiro
authors.initials: T
authors.lastname: Matsuishi
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D004196:Disease Outbreaks; D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000378:metabolism*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology*; D007564:Japan; D008297:Male; D008875:Middle Aged; D009976:Orthomyxoviridae Infections / Q000453:epidemiology / Q000821:virology; D058873:Pandemics; D012189:Retrospective Studies; D012737:Sex Factors; D013552:Swine