Influenza seasonality in Madagascar: the mysterious African free‐runner

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.

Seasonal dynamics of influenza in Brazil: the latitude effect

“…Studies of influenza in the tropics started recently. We found reports for: Brazil [10, 11], Vietnam [12], Madagascar [13], Myanmar [14] and China [15]. …”

Timing and periodicity of influenza epidemics

“…It will be interesting to learn whether these could be related to nonlinearities in the absolute humidity response, or whether these represent geographic cross-roads in areas of low seasonal forcing combined with robust coupling to regions subject to pure annual influenza cycles, aligned with winters in the northern (Northern China, Europe and North America) and southern hemispheres (Southern China, Australia). At the end of the spectrum of observed periodicities, a relatively remote subtropical island like Madagascar does not appear to experience influenza cycles entrained on any type of regular periodicity, based on analysis of long-term epidemiological records (19). Future analyses may shed light on the complexity of the interplay among environmental factors, population coupling, and the occasional term-time forcing, in fine-tuning seasonality of epidemic and pandemic influenza globally.…”

Both hemispheric influenza vaccine recommendations would have missed near half of the circulating viruses in Madagascar

“…Our assessment may appear applicable only to Madagascar due to its erratic influenza viral circulation pattern . However, we found that low successful strain match between WHO's recommended vaccination compositions and circulating viruses is a constant, not an exception as based on a review of the data from studies that investigated the proportion of influenza strain matching and which took into account the timing of vaccine delivery .…”

“…Nasopharyngeal and/or throat swabs from patients with influenza‐like illness (ILI) (as defined by the evolving WHO case definition) are analysed as previously described . Between 2002 and 2014, 4413 influenza viruses circulating in Madagascar were detected (for more details, see Alonso et al and Table ). As part of the WHO Global Influenza Surveillance and Response System (GISRS), at least twice a year the NIC of Madagascar shares influenza virus isolates with the WHO Collaborative Centre (WHO CC) of London where genetic and antigenic characterizations (HI assays) are performed for the update to the influenza vaccine recommendations.…”

“…Tropical countries have a much more complex circulation pattern of influenza as they are frequently out of phase with the dynamics predicted for their hemispheric group. Several studies have assessed the best recommended vaccine composition mainly for the Southern Hemisphere . Studies by Mello et al, Waiboci et al, Alonso et al and Mah‐E‐Muneer et al used a method to measure the proportion of circulating influenza viruses in the epidemic period after vaccination that matched with the recommended vaccine compositions in tropical countries.…”