title: Severe respiratory disease concurrent with the circulation of H1N1 influenza.
abstract: BACKGROUND
  In the spring of 2009, an outbreak of severe pneumonia was reported in conjunction with the concurrent isolation of a novel swine-origin influenza A (H1N1) virus (S-OIV), widely known as swine flu, in Mexico. Influenza A (H1N1) subtype viruses have rarely predominated since the 1957 pandemic. The analysis of epidemic pneumonia in the absence of routine diagnostic tests can provide information about risk factors for severe disease from this virus and prospects for its control.
  
  
  METHODS
  From March 24 to April 29, 2009, a total of 2155 cases of severe pneumonia, involving 821 hospitalizations and 100 deaths, were reported to the Mexican Ministry of Health. During this period, of the 8817 nasopharyngeal specimens that were submitted to the National Epidemiological Reference Laboratory, 2582 were positive for S-OIV. We compared the age distribution of patients who were reported to have severe pneumonia with that during recent influenza epidemics to document an age shift in rates of death and illness.
  
  
  RESULTS
  During the study period, 87% of deaths and 71% of cases of severe pneumonia involved patients between the ages of 5 and 59 years, as compared with average rates of 17% and 32%, respectively, in that age group during the referent periods. Features of this epidemic were similar to those of past influenza pandemics in that circulation of the new influenza virus was associated with an off-season wave of disease affecting a younger population.
  
  
  CONCLUSIONS
  During the early phase of this influenza pandemic, there was a sudden increase in the rate of severe pneumonia and a shift in the age distribution of patients with such illness, which was reminiscent of past pandemics and suggested relative protection for persons who were exposed to H1N1 strains during childhood before the 1957 pandemic. If resources or vaccine supplies are limited, these findings suggest a rationale for focusing prevention efforts on younger populations.
authors: Division of Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, MD, USA.
authors: Gerardo
authors: G
authors: Chowell
authors: Stefano M
authors: SM
authors: Bertozzi
authors: M Arantxa
authors: MA
authors: Colchero
authors: Hugo
authors: H
authors: Lopez-Gatell
authors: Celia
authors: C
authors: Alpuche-Aranda
authors: Mauricio
authors: M
authors: Hernandez
authors: Mark A
authors: MA
authors: Miller
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000145:classification / Q000302:isolation & purification*; D007251:Influenza, Human / Q000150:complications* / Q000821:virology; D008297:Male; D008800:Mexico / Q000453:epidemiology; D008875:Middle Aged; D009305:Nasopharynx / Q000821:virology; D011024:Pneumonia, Viral / Q000453:epidemiology* / Q000401:mortality / Q000821:virology; D055815:Young Adult