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Several studies that analyzed the burden of influenza during the 2009 pandemic found that excess mortality tended to wane or even disappear in equatorial regions, adding to evidence that suggests that seasonal influenza may have a lower impact in tropical settings. We therefore analyzed with great interest the claims of two studies recently published in this journal of relatively high influenza-associated mortality burden in Bangladesh and hospitalization burden in Rwanda, two countries with mild winters. We detected a poor match between the temporal signature of influenza virus detection and ARI cases. The absence of respiratory syncytial virus (RSV) testing in these studies may have also contributed to distortions, especially in Rwanda where hospitalization rates were disproportionately high among younger children, usually the most affected by RSV. Additionally, many of the deaths attributed to influenza were of patients with severe previously existing conditions. We suggest that the issues discussed here are seriously considered in a putative recommendation of seasonal influenza vaccines in tropical settings.
This is a manuscript in which we discussed the evidence that influenza affects less equatorial regions than areas of higher latitude.