title: The influence of climatic conditions on the transmission dynamics of the 2009 A/H1N1 influenza pandemic in Chile.
abstract: BACKGROUND
  The role of demographic factors, climatic conditions, school cycles, and connectivity patterns in shaping the spatio-temporal dynamics of pandemic influenza is not clearly understood. Here we analyzed the spatial, age and temporal evolution of the 2009 A/H1N1 influenza pandemic in Chile, a southern hemisphere country covering a long and narrow strip comprising latitudes 17°S to 56°S.
  
  
  METHODS
  We analyzed the dissemination patterns of the 2009 A/H1N1 pandemic across 15 regions of Chile based on daily hospitalizations for severe acute respiratory disease and laboratory confirmed A/H1N1 influenza infection from 01-May to 31-December, 2009. We explored the association between timing of pandemic onset and peak pandemic activity and several geographical and demographic indicators, school vacations, climatic factors, and international passengers. We also estimated the reproduction number (R) based on the growth rate of the exponential pandemic phase by date of symptoms onset, estimated using maximum likelihood methods.
  
  
  RESULTS
  While earlier pandemic onset was associated with larger population size, there was no association with connectivity, demographic, school or climatic factors. In contrast, there was a latitudinal gradient in peak pandemic timing, representing a 16-39-day lag in disease activity from the southern regions relative to the northernmost region (P < 0.001). Geographical differences in latitude of Chilean regions, maximum temperature and specific humidity explained 68.5% of the variability in peak timing (P = 0.01). In addition, there was a decreasing gradient in reproduction number from south to north Chile (P < 0.0001). The regional mean R estimates were 1.6-2.0, 1.3-1.5, and 1.2-1.3 for southern, central and northern regions, respectively, which were not affected by the winter vacation period.
  
  
  CONCLUSIONS
  There was a lag in the period of most intense 2009 pandemic influenza activity following a South to North traveling pattern across regions of Chile, significantly associated with geographical differences in minimum temperature and specific humidity. The latitudinal gradient in timing of pandemic activity was accompanied by a gradient in reproduction number (P < 0.0001). Intensified surveillance strategies in colder and drier southern regions could lead to earlier detection of pandemic influenza viruses and improved control outcomes.
authors.affiliation: Mathematical, Computational & Modeling Sciences Center, School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, USA. gchowell@asu.edu
authors.forename: Gerardo
authors.initials: G
authors.lastname: Chowell
authors.forename: Sherry
authors.initials: S
authors.lastname: Towers
authors.forename: Cécile
authors.initials: C
authors.lastname: Viboud
authors.forename: Rodrigo
authors.initials: R
authors.lastname: Fuentes
authors.forename: Viviana
authors.initials: V
authors.lastname: Sotomayor
authors.forename: Lone
authors.initials: L
authors.lastname: Simonsen
authors.forename: Mark A
authors.initials: MA
authors.lastname: Miller
authors.forename: Mauricio
authors.initials: M
authors.lastname: Lima
authors.forename: Claudia
authors.initials: C
authors.lastname: Villarroel
authors.forename: Monica
authors.initials: M
authors.lastname: Chiu
authors.forename: Jose E
authors.initials: JE
authors.lastname: Villarroel
authors.forename: Andrea
authors.initials: A
authors.lastname: Olea
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D050936:Basic Reproduction Number; D002648:Child; D002675:Child, Preschool; D002677:Chile / Q000453:epidemiology; D002980:Climate; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission* / Q000821:virology; D008297:Male; D008875:Middle Aged; D013997:Time Factors; D055815:Young Adult