The first cluster was seen near Manaus, one of the most highly populated cities along the Amazon river. The next clusters were identified temporally along the Amazon river, suggesting that the spread of infection was mainly through travel along the river. Subsequent clusters were then seen in less populated areas. Clusters in Peru and Ecuador could have been due to different transmission dynamics, since only the part of the country included in the Amazon region was included for the analysis.
Increased relative risk of infection would be expected in areas of high population density, and for the most part that is what is observed, with clusters centered around the rivers and major cities. However, 2 clusters to the north of the Amazon have the highest relative risk estimates. This could indicate a region of increased vulnerability to the infection.
Almost all identified clusters include a large part of indigenous territories, who are an identified vulnerable population. This could be one of the reasons for occurrence of clusters in areas of low population density.
The lack of access to basic medical services in these regions would allow for a rapid spread of the infection. A look at the availability of hospitals and access to healthcare services shows that, of the 775 hospitals identified in the Amazon region, only 285 have ICU facilities, and most of them are located along the river and in population centers. A vast majority of the indigenous territories lie more than 100km away from hospitals. This combined with lack of transport facilities make access to healthcare very difficult.