This study represents the first attempt to characterise the viral aetiology of ILIs in Bolivia and the first in South America in the post-influenza A(H1N1)pdm09 period. It was not designed for providing a representative picture of the general Bolivian population: (i) samples originated from the region of Santa Cruz only (this item is further discusses below); (ii) samples sent to CENETROP were not expected to faithfully represent ILIs in the region of Santa Cruz, with obvious recruitment biases such as clinical severity or access to public medical care; (iii) sampling was performed with the objective of obtaining equivalent numbers of cases in all age classes.
The information obtained should therefore be analysed with precautions. Firstly, the great year-to-year variation of infection epidemiological patterns is a reminder that the epidemiology of ILIs should be studied over long periods of time. Each of the three years studied here would have provided, if analysed separately, a partial view of the situation and would have lead to erroneous conclusions (e.g., the absence of hRSV infections in Bolivian children extrapolated from 2011 data). The analysis of a 3-year period is expected to smooth such epidemiological jolts but an extended observation period would have undoubtedly revealed more information about the viruses that circulated at low rates in the period studied (e.g., parechoviruses, parainfluenzaviruses). Secondly, Bolivia includes territories with diverse geographical, climatic and ecological characteristics. Santa Cruz is located in an eastern low-altitude area with a humid tropical climate whilst La Paz is in a western Andean high-altitude area with a colder and dryer climate
. This may explain important epidemiological differences observed between both cities, as exemplified by the case of influenza A(H1N1)pdm09: in 2010, we observed a very limited number of Flu A(H1N1)pdm09 infections in Santa Cruz whilst the PAHO surveillance system reported an intense circulation of the virus in La Paz during the first months of the year (PAHO surveillance website: http://ais.paho.org/phip/viz/ed_flu.asp); in 2011, a large outbreak occurred in Santa Cruz (see Figures
2), whilst only a few cases were observed in La Paz; in 2012, an important outbreak was observed in La Paz, but not in Santa Cruz. This clearly indicates that epidemiological data collected in Santa Cruz cannot be extrapolated easily to the Andean regions of Bolivia. Divergent influenza epidemiological patterns according to geographical distribution were finely documented in a previous Peruvian study
Despite the fact that the local epidemiology of ARIs is modelled by herd immunity and environmental factors, the global picture of ARIs that arises from our study conforms with that observed in different regions of the world. In particular, the choice of the pathogens tested seems to be relevant since (i) the rate of elucidation is close to what is observed in France, using the same molecular assays (authors’ personal data), (ii) peaks of detection match the ILI epidemiological peaks and (iii) in previous studies performed in Brazil, the different viruses identified in the current study were detected, confirming their circulation in the region
[12–16]. The distribution of cases in age groups is also essentially similar with what was reported elsewhere, e.g., in Ecuador
. The most surprising items, which deserve investigations on an extended period of time, were the unusually low frequency of influenza infections in the 0–4 yo age group and the complete RSV eclipse in 2011. However, precise comparison with data collected in South America is most often difficult, e.g., with a molecular study in Ecuador
 and a previous large study in Peru
 (different periods investigated, viruses detected using immunofluorescence tests, parechoviruses, coronaviruses, metapneumoviruses and rhinoviruses not tested).
The genetic characterisation of influenza strains indicates that viruses belonging to distinct evolutionary lineages of Flu A(H1N1)pdm09, Flu A/H3N2 and Flu B have co-circulated in Bolivia. Interestingly, an increasing number of influenza sequences has been made available since 2008 and phylogenetic reconstructions revealed, on the one hand, the existence of South American evolutionary clusters of influenza strains, and on the other hand a complex epidemiological relationship with the rest of the world. Trees suggested the circulation in Bolivia during the study period of viruses originating from Central and North America, Europe, Asia and Australia.