Spatio-temporal modeling of weekly malaria incidence in children under 5 for early epidemic detection in Mozambique

Transcription

1 Spatio-temporal modeling of weekly malaria incidence in children under 5 for early epidemic detection in Mozambique Katie Colborn, PhD Department of Biostatistics and Informatics University of Colorado Anschutz Medical Campus Colorado School of Public Health October 20, 2017 Katie Colborn, PhD Malaria Early Warning System Mozambique 1/25

2 Outline Mozambique Malaria Early Warning Systems The role of weather in malaria transmission Key interventions for prevention of malaria Recent malaria trends in Mozambique Spatio-temporal model for predicting incidence Exceedance probabilities - modeling uncertainty Practical considerations Katie Colborn, PhD Malaria Early Warning System Mozambique 2/25

3 Malaria in Mozambique Population of nearly 28 million Estimated 8.3 million malaria cases and 15,000 deaths in 2015 Katie Colborn, PhD Malaria Early Warning System Mozambique 3/25

4 Mozambique Katie Colborn, PhD Malaria Early Warning System Mozambique 4/25

5 Malaria Early Warning Systems (MEWS) WHO Roll Back Malaria report in 2001 detailing the need for MEWS for Africa 2012 systematic review by Mabaso and Ndlovu summarized 35 articles on climate-driven malaria epidemic models - many more have been published since then Most MEWS use time-series models with satellite weather data Best models are those that make use of incidence data from continuously monitored populations at relatively small spatial scales (state, district, health facility) Katie Colborn, PhD Malaria Early Warning System Mozambique 5/25

6 Key components of MEWS Continuously monitored population at risk Historic case data on same population Independent variables associated with malaria transmission that can be forecasted into the future or that are associated at lagged time points that provide early enough warning so that individuals can plan a response Katie Colborn, PhD Malaria Early Warning System Mozambique 6/25

7 Role of weather in malaria transmission Temperature and rainfall are important to consider when trying to predict malaria The parasites survive best in ideal weather conditions Mosquitoes survive and are more active in ideal temperatures Rainfall creates new pools for mosquito breeding, but can also wash out existing pools Katie Colborn, PhD Malaria Early Warning System Mozambique 7/25

8 Resources for obtaining weather data Global Land Data Assimilation System (GLDAS) Famine Early Warning System Network (FLDAS) United States Geological Survey (USGS) National Oceanic and Atmospheric Administration (NOAA) Katie Colborn, PhD Malaria Early Warning System Mozambique 8/25

9 A MEWS for Mozambique Project funded by a Grand Challenges Explorations grant from the Bill and Melinda Gates Foundation Partners include: Lancaster University (UK), the Clinton Health Access Initiative, the National Institutes of Health in Mozambique and the National Malaria Control Program in Mozambique Objective 1: to predict the probability that malaria incidence will exceed a policy relevant threshold 8 to 26 weeks in advance in each district in Mozambique Objective 2: Work with key stakeholders in Mozambique to develop a platform for implementing a MEWS Katie Colborn, PhD Malaria Early Warning System Mozambique 9/25

10 Incidence and population data Weekly case reports of children under the age of five from 141 districts reporting in Mozambique between Population data were obtained from WorldPop and the 2007 Mozambique census was used to determine the percent of the population under 5 in each district Katie Colborn, PhD Malaria Early Warning System Mozambique 10/25

11 Malaria incidence by district Katie Colborn, PhD Malaria Early Warning System Mozambique 11/25

12 Malaria in Mozambique Katie Colborn, PhD Malaria Early Warning System Mozambique 12/25

13 Model-based geostatistics Malaria in Mozambique is temporally and spatially correlated We cannot expect to collect all relevant covariates to explain these sources of correlation; therefore, it is important to explicitly account for uncertainty in a prediction model for malaria Model-based geostatistics (Diggle et. al, Applied Statistics, 1998) has been used to incorporate spatial correlation in environmental risk models for various diseases (Diggle et al., Ann Trop Med Parasitology, 2007; Giorgi et al., Spatial and Spatio-temporal Epidemiology, 2016) and to quantify the uncertainty in the predictions from these models (Diggle, et. al, Stat Sci, 2013; Giorgi, 2016) Katie Colborn, PhD Malaria Early Warning System Mozambique 13/25

14 Quantification of uncertainty Exceedance probability (EP): predictive probability that the relative risk exceeds a predefined threshold (or the multiplicative deviation from the expected level of incidence) This MEWS aims to warn when incidence exceeds a level that is of public health concern Taking action is then a context specific decision made by policy makers depending on the available resources Katie Colborn, PhD Malaria Early Warning System Mozambique 14/25

15 Spatio-temporal model Let Y it denote the counts of reported children under 5 years with malaria for week t in the i-th district, for i = 1,..., I = 142 and t = 1,..., T = 313 where t = 1 is the first epi-week of January 2010 and t = 313 is the last available epi-week of We assume that, conditionally on a spatio-temporally random effect, S it, the Y it are mutually independent Poisson variables with mean λ it = m it r it where m it is the population at risk, log{r it } = d it β + S it (1) and d it is a vector of explanatory variables with associated coefficients β. Katie Colborn, PhD Malaria Early Warning System Mozambique 15/25

16 Spatio-temporal model continued Let St = (S it,..., S It ) ; we then model S t using a stationary autoregressive process of the first order, i.e. S t = γs t 1 + W t, γ (0, 1) where W t corresponds to temporal innovation following a multivariate Gaussian distribution with mean zero and covariance matrix Σ, such that [Σ] ij = σ2 D i D j D i { } xi x j exp dx i dx j, D j φ where: D is the area encompassed by the boundaries of district D; σ 2 and φ are two scale parameters for the variance and spatial correlation of the process W t ; x i x j is the Euclidean distance between any two locations x i and x j in district D i and D j, respectively. Katie Colborn, PhD Malaria Early Warning System Mozambique 16/25

17 Three models M1) use of weather variables while ignoring any source of residual extra-poisson variation M2) use of spatio-temporal random effects but no weather variables M3) a spatio-temporal model which combines both weather variables and residual extra-poisson variation Katie Colborn, PhD Malaria Early Warning System Mozambique 17/25

18 Comparison of models For each of the three models, two holdout sets were tested: 1) the last 8 weeks of data and 2) the last 26 weeks of data The remainder of the data were used to train the three models Model performance was compared using RMSE and 95% coverage probability (CP), defined as the proportion of observed malaria cases from the holdout data-set that fell within the 95% prediction intervals Katie Colborn, PhD Malaria Early Warning System Mozambique 18/25

19 Model prediction performance Katie Colborn, PhD Malaria Early Warning System Mozambique 19/25

20 District-level predictions Katie Colborn, PhD Malaria Early Warning System Mozambique 20/25

21 Model predicting 26 weeks in advance Katie Colborn, PhD Malaria Early Warning System Mozambique 21/25

22 Model predicting 8 weeks in advance Katie Colborn, PhD Malaria Early Warning System Mozambique 22/25

23 Exceedance map Katie Colborn, PhD Malaria Early Warning System Mozambique 23/25

24 Practical considerations 8 weeks might not be enough time for the ministry of health to coordinate a response to outbreaks 26 weeks advance warning is ideal for the ministry, but the model does not predict that well that far in advance Any model might require forecasted climate data, which will mean we need to work with a climatologist rather than simply downloading satellite data Katie Colborn, PhD Malaria Early Warning System Mozambique 24/25

25 Thank you Emanuele Giorgi, Lancaster University, UK James Colborn, Clinton Health Access Initiative Eduardo Samo Gudo, INS, Mozambique Baltazar Candrinho, NMCP, Mozambique Andrew Monaghan, NCAR Katie Colborn, PhD Malaria Early Warning System Mozambique 25/25