Factors associated with infection by 2009 pandemic H1N1 influenza virus during different phases of the epidemic

Int J Infect Dis. 2011 Oct;15(10):e695-701. doi: 10.1016/j.ijid.2011.05.012. Epub 2011 Jul 20.


Objective: The focus of this study was to ascertain the factors associated with 2009 pandemic influenza H1N1 (pH1N1) infection during different phases of the epidemic.

Methods: In central Taiwan, 306 persons from households with schoolchildren were followed sequentially and serum samples were taken at three sampling time-points starting in the fall of 2008, shortly after influenza vaccination. Participants who seroconverted between two consecutive blood samplings were considered as having serological evidence of infection. A generalized estimation equation (GEE) with a logistic link to account for household correlations was applied to identify factors associated with pH1N1 infections during the pre-epidemic (April-June) and epidemic (September-October) periods.

Results: The results showed that receiving an inactivated seasonal influenza vaccine (ISIV) and having a hemagglutination inhibition assay (HI) titer of 40 or higher resulted in a significantly lower likelihood of pH1N1 infection during the pre-epidemic period only, for both children and adults (adjusted odds ratio (OR) 0.3, 95% confidence interval (CI) 0.12-0.9). Having a previous infection by pH1N1 with a baseline titer of 20 or higher resulted in a significantly lower likelihood of infection by pH1N1 during the epidemic period (adjusted OR 0.06, 95% CI 0.02-0.16).

Conclusions: Our results provide the first serological evidence to suggest a protection effect from receiving an ISIV against pH1N1 infection only when the HI titer reaches 40 or higher during the pre-epidemic period. This study gives an important insight into the control and intervention measures required for preventing infections during future influenza epidemics.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adolescent
  • Adult
  • Aged
  • Antibodies, Viral / blood
  • Child
  • Child, Preschool
  • Female
  • Hemagglutination Inhibition Tests
  • Humans
  • Influenza A Virus, H1N1 Subtype* / immunology
  • Influenza Vaccines / immunology
  • Influenza, Human / epidemiology*
  • Influenza, Human / immunology
  • Male
  • Middle Aged
  • Pandemics*
  • Risk Factors
  • Seroepidemiologic Studies
  • Taiwan / epidemiology
  • Young Adult


  • Antibodies, Viral
  • Influenza Vaccines