title: Worldwide transmission and seasonal variation of pandemic influenza A(H1N1)2009 virus activity during the 2009-2010 pandemic.
abstract: BACKGROUND
  Seasonal influenza activity varies with geography and time of year.
  
  
  OBJECTIVE
  To describe how pandemic influenza A(H1N1)2009 [A(H1N1)pdm09] activity varied during the 2009-2010 pandemic.
  
  
  METHODS
  We analyzed influenza virological data compiled by the World Health Organization from June 2009-August 2010. We calculated weekly proportions of A(H1N1)pdm09-positive specimens out of all A(H1N1)pdm09-positive specimens detected during the study period for each country. We compared parameters of pandemic activity (e.g., peak A[H1N1]pdm09 weekly proportion [peak activity], number of weeks between the 5th and 95th percentiles of A(H1N1)pdm09 cumulative weekly proportion [duration of activity]) between countries in temperate and tropical-subtropical regions. We quantified the proportion of A(H1N1)pdm09 out of all influenza A specimens by country and correlated it with countries' central latitudes.
  
  
  RESULTS
  We analyzed data from 80 countries (47 temperate, 33 tropical-subtropical). The median proportion of cases identified during the peak week was higher in temperate (0·12) than in tropical-subtropical (0·09) regions (P<0·01). The median duration of activity was longer in tropical-subtropical (27 weeks) than in temperate countries (20 weeks) (P < 0·01). In most temperate countries (98%), peak pandemic activity occurred during the fall-winter period. There was a positive correlation between country central latitude and proportion of A(H1N1)pdm09 out of all influenza A specimens (r: 0·76; P<0·01).
  
  
  CONCLUSIONS
  The transmission of A(H1N1)pdm09 exhibited similarities with seasonal influenza transmission in that activity varied between temperate and tropical-subtropical countries and by time of year. Our findings suggest the potential utility of accounting for these factors during future pandemic planning.
authors.affiliation: Influenza Division, Centers for Disease Control and Prevention, Atlanta, GA, USA; Epidemic Intelligence Service, Centers for Disease Control and Prevention, Atlanta, GA, USA.
authors.forename: Aaron D
authors.initials: AD
authors.lastname: Storms
authors.forename: Maria D
authors.initials: MD
authors.lastname: Van Kerkhove
authors.forename: Eduardo
authors.initials: E
authors.lastname: Azziz-Baumgartner
authors.forename: Wing-Kei
authors.initials: WK
authors.lastname: Lee
authors.forename: Marc-Alain
authors.initials: MA
authors.lastname: Widdowson
authors.forename: Neil M
authors.initials: NM
authors.lastname: Ferguson
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Mounts
mesh_terms: D002980:Climate; D005843:Geography; D014943:Global Health; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D058873:Pandemics; D012621:Seasons
keywords: 2009 influenza A (H1N1); pandemic influenza; seasonality
references.citation: Epidemics. 2010 Sep;2(3):132-138
references.pmid: 20847900
references.citation: Epidemiol Rev. 1996;18(1):64-76
references.pmid: 8877331
references.citation: J Infect Dis. 2006 Nov 1;194 Suppl 2:S92-7
references.pmid: 17163395
references.citation: Annu Rev Med. 2000;51:407-21
references.pmid: 10774473
references.citation: Int J Infect Dis. 2010 Dec;14(12):e1099-105
references.pmid: 21036090
references.citation: Curr Opin Infect Dis. 2010 Oct;23(5):415-20
references.pmid: 20644472
references.citation: Lancet Infect Dis. 2004 Nov;4(11):704-8
references.pmid: 15522683
references.citation: Bull World Health Organ. 2012 Apr 1;90(4):306-10
references.pmid: 22511828
references.citation: Epidemiol Infect. 2010 Oct;138(10):1472-81
references.pmid: 20158932
references.citation: J Virol. 2007 Jun;81(11):5429-36
references.pmid: 17182688
references.citation: J Infect Dis. 2012 Sep 15;206(6):838-46
references.pmid: 22829641
references.citation: Influenza Other Respir Viruses. 2013 Nov;7(6):1328-35
references.pmid: 23551904
references.citation: Euro Surveill. 2009 Oct 22;14(42):
references.pmid: 19883548
references.citation: PLoS One. 2007 Dec 12;2(12):e1296
references.pmid: 18074020
references.citation: Euro Surveill. 2009 Oct 22;14(42):
references.pmid: 19883551
references.citation: Environ Health Perspect. 2011 Apr;119(4):439-45
references.pmid: 21097384