Abstract
Background Varicella remains a major burden in many tropical regions, where low- to middle-income countries report the highest mortality rates. Understanding when and where varicella incidence increases could help us understand viral transmission and optimize the allocation of resources. Due to the lack of surveillance data, the epidemiology of varicella in the tropics has not been well characterized. Here, we assessed varicella seasonality and analyzed its correlation with climatic variables across Colombia.
Methods We compiled an extensive dataset of weekly varicella reports in children up to the age of ten in 25 municipalities across Colombia. We used generalized additive models to describe the seasonality of varicella in each municipality. Using clustering methods and matrix correlation tests, we then compared the spatial variability in varicella seasonality with that in five meteorological variables across the municipalities. Finally, we developed a mathematical model to examine whether the influence of the climate on transmission rates could result in the observed seasonal patterns of varicella in Colombia and other Central American countries.
Findings Varicella seasonality was markedly bimodal, with a more pronounced peak early in the year in northern municipalities (≈week 15), while later in the year (≈week 40) in southern municipalities, including Bogotá. This spatial gradient was strongly correlated with humidity (Mantel statistic = 0·412, p-value = 0·001) but not temperature (Mantel statistic = 0·077 and p-value = 0·225). Furthermore, a mathematical model that included a small, negative association between humidity and transmission was able to reproduce the observed spatial patterns in Colombia and México. This model also predicted a latitudinal gradient in other tropical countries of Central America, where the seasonality of varicella has not yet been characterized.
Interpretation These results demonstrate a large variability in varicella seasonality across the tropical climates of Colombia. They further suggest that seasonal fluctuations of humidity explain the varicella epidemics calendar in Colombia and other Central American countries. More generally, our results highlight the need to carefully consider the subnational heterogeneity of climates when studying the seasonal epidemiology of varicella and assessing the impact of control measures.
Funding Core funding from the Max Planck Institute for Infection Biology of the Max Planck Society, Berlin, Germany.
Competing Interest Statement
T.K. reports outside the submitted work, having received research grants from the Gemeinsamer Bundesausschuss (GBA Federal Joint Committee, Germany) and the Bundesministerium fur Gesundheit (BMG Federal Ministry of Health, Germany). He further has received personal compensation from Eli Lilly and Company, Teva Pharmaceuticals, TotalEnergies S.E., the BMJ, and Frontiers. M.D.d.C. received postdoctoral funding (2017 to 2019) from Pfizer and consulting fees from G.S.K. L.A.B.G., E.G., D.R., L.J.H., and B.K. confirm that they have no known competing interests.
Funding Statement
This study was funded by the core funding from the Max Planck Institute for Infection Biology of the Max Planck Society, Berlin, Germany.
Author Declarations
I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.
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All the data is freely available on the web pages of the SIVIGILA (http://portalsivigila.ins.gov.co/).
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Data Availability
All data produced are available online at the web pages of the SIVIGILA, IDEAM, NARR, and CHIRPS. Furthermore, all the aggregated data (to the level of municipality-week) and code are stored in Edmond, the open research data repository of the Max Planck Society, to ensure the reproducibility of the results and will be made immediately available following publication.
http://portalsivigila.ins.gov.co/Paginas/Vigilancia-Rutinaria.aspx
https://www.ncei.noaa.gov/products/weather-climate-models/north-american-regional