Spatial epidemiology of yellow fever: Identification of determinants of the 2016-2018 epidemics and at-risk areas in Brazil

Abstract

Optimise control strategies of infectious diseases, identify factors that favour the circulation of pathogens, and propose risk maps are crucial challenges for global health. Ecological niche modelling, once relying on an adequate framework and environmental descriptors can be a helpful tool for such purposes. Despite the existence of a vaccine, yellow fever (YF) is still a public health issue. Brazil faced massive sylvatic YF outbreaks from the end of 2016 up to mid-2018, but cases in human and non-human primates have been recorded until the beginning of 2020. Here we used both human and monkey confirmed YF cases from two epidemic periods (2016/2017 and 2017/2018) to describe the spatial distribution of the cases and explore how biotic and abiotic factors drive their occurrence. The distribution of YF cases largely overlaps for humans and monkeys, and a contraction of the spatial extent associated with a southward displacement is observed during the second period of the epidemics. More contributive variables to the spatiotemporal heterogeneity of cases were related to biotic factors (mammal richness), abiotic factors (temperature and precipitation), and some human-related variables (population density, human footprint, and human vaccination coverage). Both projections of the most favourable conditions showed similar trends with a contraction of the more at-risk areas. Once extrapolated at a large scale, the Amazon basin remains at lower risk, although surrounding forest regions and notably the NorthWest region, would face a higher risk. Spatial projections of infectious diseases often relied on climatic variables only; here for both models, we instead highlighted the importance of considering local biotic conditions, hosts vulnerability, social and epidemiological factors to run the spatial risk analysis correctly: all YF cases occurring later on, in 2019 and 2020, were observed in the predicted at-risk areas.

Re-emergence of yellow fever in the neotropics — quo vadis?

“…aegypti [35]. Furthermore, several lines of evidence supported sylvatic origin and transmission in this outbreak: (i) distribution of human and NHP YF cases largely overlapped from 2016 to 2018 [62]; and (ii) despite co-circulation of multiple lineages during the outbreaks [60,61], there was no evidence of molecular signatures suggestive of adaptation for increased infectivity in humans [13].…”

“…The new lineage was introduced and circulated undetected into the state of Minas Gerais for almost two years prior to the explosive growth of the outbreak by the end of 2016, at which time it radiated out into the Southeast region of Brazil in at least two independent events and persisted at least up to 2019 [28,60,61]. From 2016 to 2020, the outbreaks reached YFV-free areas in the Southeast and South of Brazil (Figure 1), raising concerns for the re-establishment of sylvatic cycles [62].…”

“…Consequently in 2017, Brazil implemented the use of either single or fractional dosing and expansion of geographic areas subject to mandatory vaccination. These strategies increased the vaccination coverage from 3526 cities and municipalities in 2010 to 4469 in 2018, which led to a significant reduction in subsequent outbreaks [62]. The strengthening of vaccination in Brazil's Southeast region during and immediately after the first wave in 2016/2017 seemed to impact outbreak dynamics with a southward displacement, but only after the second wave of the outbreak in 2017/2018, the number of cases had decreased significantly [62].…”

“…These strategies increased the vaccination coverage from 3526 cities and municipalities in 2010 to 4469 in 2018, which led to a significant reduction in subsequent outbreaks [62]. The strengthening of vaccination in Brazil's Southeast region during and immediately after the first wave in 2016/2017 seemed to impact outbreak dynamics with a southward displacement, but only after the second wave of the outbreak in 2017/2018, the number of cases had decreased significantly [62]. However, despite these immunization campaigns, as of June 2020, the average vaccination coverage stands at 40.14% of the population, with more than 116.2 million unvaccinated people at risk for YFV infection [75].…”

Laboratory and Field Performance of Five Cheap Commercial Light Traps for Capturing Mosquitoes In China

“…Of the more than 50 known mosquito-borne diseases, vaccines are available only for epidemic encephalitis and yellow fever. The Basic way to curb other mosquito-borne diseases is mosquito control [5,6,7,8,9]. During mosquito season or a mosquito-borne disease epidemic, it is necessary not only to implement integrated mosquito control in the external environment, but also to control mosquitoes and prevent bites in residential homes.…”