The influence of climatic conditions on the transmission dynamics of the 2009 A/H1N1 influenza pandemic in Chile

Abstract

BackgroundThe 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.MethodsWe 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.ResultsWhile 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.ConclusionsThere 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.

Death Patterns during the 1918 Influenza Pandemic in Chile

“…Experimental studies indicate that influenza transmission is favored by lower temperatures and humidity levels ( 39 ), and we speculate that the higher relative death rate estimated for these areas could be explained by more favorable climate conditions in southern Chile ( 18 , 40 ). Moreover, the southward gradient in severity of the 1918 pandemic aligns with the patterns of the 2009 influenza A(H1N1) pandemic in Chile ( 28 ) and in Brazil ( 23 ). In contrast to patterns in RR for death, we did not find any association between latitude and cumulative absolute pandemic excess mortality rates during 1918–1921.…”

“…We quantified the extent of spatial autocorrelation in estimates of excess deaths and RR in the 24 provinces by using the Moran I statistic and a nearest-neighbor spatial mixing matrix ( 28 ) via randomization tests ( 29 ). This analysis evaluates whether pandemic mortality rates in contiguous provinces are more similar than those obtained for any pair of provinces selected at random.…”

Vigilancia de laboratorio de enfermedad meningocóccica invasora en Chile, 2006-2012

“…El mayor aumento de las tasas de EMI se registró en la RM, lo cual podría estar explicado por la mayor densidad poblacional que presenta la región, con respecto a las otras regiones, facilitando la transmisión persona-persona del agente etiológico 44 . Por otro lado, Santiago, la capital de Chile y de la RM, es el punto de llegada y de salida de inmigrantes y turistas, posibilitando la importación de nuevos agentes infecciosos y su posterior transmisión, como sucedió con el virus influenza pandémico A (H1N1) en el año 2009 45,46 . La segunda región que presentó aumento en sus tasas de EMI fue la Región de Valparaíso.…”

Incidence of influenza-associated hospitalization in rural communities in western and northern India, 2010–2012: A multi-site population-based study

“…These factors have been implicated in the transmission dynamics of respiratory infections and more specifically for influenza. 19,20 Modelling has shown the existence of two types of environmental conditions which Incidence of influenza-associated hospitalization explain the different seasonality of influenza globally e cold-dry as in temperate climates like United States and Europe; and humiderainy as in tropical climates of south-east Asia. 21 Antigenic and genetic analysis of the viruses circulating at Vadu and Ballabgarh suggest that the viruses circulating at both sites were similar making it unlikely that inherent differences in circulating viruses were responsible for differences in clinical features and hospitalization rates between the two sites.…”