Description of study sites
In Colombia, Cucuta, the capital of North Santander state (“department”) was selected which has around 750 000 inhabitants, 1176 Km2 in area size and located 320m above sea level at latitude 07°53´00” N, longitude 72° 30´19” W. The climate is warm and characterized by temperatures ranging between 21 and 36 °C; an average annual rainfall of 655 mm and an annual average relative humidity between 70 and 75% (15). While dengue outbreaks have a longer history in Cúcuta, outbreaks of CHIKV and ZIKV were reported after 2014 and 2015, respectively.
In Mexico,137 highly endemic urban municipalities in tropical areas in 28 states of the country with a high incidence of the three arbovirus diseases have been selected.
Public Health Surveillance system
In Colombia, the National Public Health Surveillance System (SIVIGILA) provides systematic and timely information on the dynamics of dengue, Zika and Chikungunya. The data reporting units belong to the public and private health system. The data was obtained from SIVIGILA at the IDS (Departmental Health Institute), with certified authorization
In Mexico, the national surveillance system includes permanent notification of clinical cases of dengue, Chikungunya and Zika, birth defects into the online platform SINAVE (National Epidemiological Surveillance System) as well as entomological data (ovitrap indexes) and response activities (into the online entomological platform). Both platforms are fed on a weekly basis with the information from all health centers (public and private).
Data collection of outbreak and alarm indicators
In Cucuta/Colombia, the study has covered the period from January 1, 2012 to December 31, 2017 including all reported cases of dengue where included using the standardized case-definitions by the Ministry of Health (MoH) and the INS. This study analyses part of the total number of cases of the Chikungunya and Zika outbreak in the urban area of Cúcuta, only those corresponding to the individualized notification of SIVIGILA (16, 17). In Mexico the data collection started also in January 2012, however, the analysis period lasted until December 2018. The data set has been provided by SINAVE and by the entomological online platform from the Secretary of Health.
For the application of EWARS, the temporal unit was defined as the epidemiological ‘week’ (from Sunday to Saturday) and the spatial unit was based on pre-existing administrative units (municipalities or districts). At least three years of surveillance data records were retained for the EWARS analysis including a variable indicating the ‘population size’ of the corresponding district/localities.
Surveillance data on dengue, Zika and chikungunya were recorded as well as the individuals’ place of residence, age, sex, date of onset of symptoms, date of case registration at the health center or hospital and the type of case (‘probable’, ‘confirmed’ by laboratory or by clinical symptoms and ‘hospitalized’ cases).
Weekly information of potential key alarm indicators was collected from each study district/municipality and included a candidate list of epidemiological, entomological and meteorological variables:
- Meteorological: mean outdoor air temperature, rainfall and relative humidity
- Epidemiological: patients’ age, and probable cases)
- Entomological: positive ovitrap, average egg count per trap
Analysis by the EWARS tool:
The general methodological and operational principles of how EWARS processes the calibration and evaluation analysis including the generation of best-fitted prediction algorithm have been illustrated in details elsewhere (9,12,14). In general, and during the retrospective phase of EWARS, the average number of cases of an arbovirus disease – within the expected “normal” or seasonal range illustrated in the endemic channel – are calculated for a fixed time period and with the use of a given z-value (which is the multiplier of the standard deviation of the moving average of weekly case numbers) during the calibration session, which generate a smoothed Endemic Channel (moving average + (Z*SD)). Weekly cases exceeding this Endemic Channel are said to be an outbreak. In this retrospective phase, the algorithm and all parametric coefficients needed for calculating the outbreak probability are computed: these coefficients depend primarily on the sensitivity (i.e. the proportion of correctly predicting an outbreak out of all outbreaks) and positive predictive value, PPV (i.e. the proportion of correct alarms out of all alarms) as direct measures for deciding the best calibrated settings (i.e. those with highest sensitivity and PPV). In the prospective stage of EWARS, an alarm signal (early prediction) is triggered when the ‘outbreak probability’ crosses the proposed ‘alarm threshold’ once prospective weekly information on the relevant alarm indicator(s) are fed into the system. Accordingly, instant numerical and graphical demonstration and interpretation of possible outbreak and its corresponding response plan is illustrated to the user at a given lag week (time between the prediction and an outbreak to occur).
Based on recommendation from previous reports (14), ‘hospitalized’ cases showed to be best outbreak indicator for the prediction of a forthcoming outbreak using EWARS. However, where the proportion of “mild” cases is high and patients are rarely hospitalized (such as in chikungunya and zika), ‘probable’ or ‘confirmed’ cases were used as outbreak indicators. In this study, z-values and alarm thresholds were determined for chikungunya and dengue via both manual and automatized procedures (which can assess the calibration settings via a 1000 iteration process per district). For zika data only the automatized version was employed. On the other hand, due to inadequate dengue records and inconsistent outbreak trend observed in the Colombian data, a data normalization step was performed prior to the application of the EWARS tool.
In Mexico districts with at least three years’ records of at least ≥5 cases per evaluation period were included (N=137). Only districts with no meaningful parameters and outputs from the final results such extreme z-outbreak, thresholds and window sizes or where no convergence was reached or poor calibration match were dropped. Descriptive presentations of the sensitivity, PPV and lag weeks as well as outbreak parameters derived from the evaluation stage were calculated and presented. In the case of Mexico, average values obtained from multiple districts were presents in the final results after rounding off to 1.0.
Descriptive statistics, of both graphics and estimates presentations, was sought in this paper. To give an overview of the pattern of chikungunya and Zika occurrence in Colombia, disease incidences were determined with the cases from the urban area included for this analysis (n cases / 100,000 population).
This study analyzed only secondary data obtained from Colombian and Mexican institutions with the authorization of the Surveillance System (Official Communication 23.02.2018). Ethical endorsement was obtained from the Ethics Committee of the University of Freiburg (N°-145/18) which was approved by local health authorities.