Abstract
BackgroundThe seasonal drivers of influenza activity remain debated in tropical settings where epidemics are not clearly phased. Antananarivo is a particularly interesting case study because it is in Madagascar, an island situated in the tropics and with quantifiable connectivity levels to other countries.ObjectivesWe aimed at disentangling the role of environmental forcing and population fluxes on influenza seasonality in Madagascar.MethodsWe compiled weekly counts of laboratory-confirmed influenza-positive specimens for the period 2002 to 2012 collected in Antananarivo, with data available from sub-Saharan countries and countries contributing most foreign travelers to Madagascar. Daily climate indicators were compiled for the study period.ResultsOverall, influenza activity detected in Antananarivo predated that identified in temperate Northern Hemisphere locations. This activity presented poor temporal matching with viral activity in other countries from the African continent or countries highly connected to Madagascar excepted for A(H1N1)pdm09. Influenza detection in Antananarivo was not associated with travel activity and, although it was positively correlated with all climatic variables studied, such association was weak.ConclusionsThe timing of influenza activity in Antananarivo is irregular, is not driven by climate, and does not align with that of countries in geographic proximity or highly connected to Madagascar. This work opens fresh questions regarding the drivers of influenza seasonality globally particularly in mid-latitude and less-connected regions to tailor vaccine strategies locally.
Highlights
The biological mechanisms responsible for the seasonality of influenza and other respiratory infections remain to this day one of the most intriguing topics in infectious disease epidemiology
Respiratory specimens are collected by health worker staffs and shipped to the National Influenza Center (NIC) to be tested for the presence influenza virus by viral isolation and RT–PCR, as previously described.[25]
Influenza B predominated in the middle of the 2009, and A(H1N1)pdm[09] did not become dominant until October 2009.25 In interpandemic years, different subtypes of influenza cocirculated without any noticeable periodicity pattern
Summary
The biological mechanisms responsible for the seasonality of influenza and other respiratory infections remain to this day one of the most intriguing topics in infectious disease epidemiology. Influenza predominates in winter in temperate regions of the world and in the rainy season in the Tropics,[1,2,3] many tropical and subtropical regions experience more complex seasonal patterns including semiannual cycles and year-round activity.[3,4,5] Many putative mechanisms have been put forward, but a unifying theory accounting for the diversity of seasonal patterns observed globally remains elusive.[3,6,7,8] A better understanding of influenza seasonality is important to help tailor surveillance strategies and optimize the local timing of influenza vaccination,[9] especially in developing country settings where influenza disease burden is becoming increasingly recognized.[10,11,12,13,14]. Antananarivo is a interesting case study because it is in Madagascar, an island situated in the tropics and with quantifiable connectivity levels to other countries
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