Building an Endemic Channel with epiCo

epiCo’s endemic channel module is a tool to estimate a central tendency line and upper and lower limits for the expected number of cases of a disease based on the historical data provided by the user and a set of customized parameters.

1. What is an endemic channel?

Using an endemic channel is a visual strategy to represent the historical behavior of a disease in a specific region on an epidemic curve that defines the central tendency of the cases during a year and the upper and lower limits where cases are expected to vary. This plot provides three lines known as “\(\color{green}{\text{Safety}}\)”, “\(\color{orange}{\text{Warning}}\)”, and “\(\color{Red}{\text{Epidemic}}\)” bands (Figure 1), which are later used to define the epidemiological situation of the region based on the current (observed) cases.

The broader definition and methodology for the endemic channel were provided by Bortman (1999).

The data needed to build an endemic channel is the weekly or monthly incidence of the disease for previous years in the region of interest. epiCo asks for at least one year of data, but endemic channels are typically estimated with 5 to 7 years of information.

It is natural to presume that more years provide better estimations since the statistical analyses will be more robust, but it is important to have contextual information to ensure that transmission, surveillance, or demographic conditions (among other factors) have not changed during this period of time. Regarding the frequency of the data, weekly incidence may provide more useful information about the disease behavior and the moment in which an epidemiological warning should be raised; however, it is up to the experience of the users, and their context, whether this resolution of the data can be achieved.

Another common decision while building an endemic channel is to ignore previous epidemic years to avoid misrepresenting the traditional transmission dynamics. epiCo does not suggest epidemic years automatically, but its endemic_channel function does provide the option to annotate the outlier years and decide whether to include, ignore, or adjust them. More data-gathering tips and sources are provided later in this vignette.

The central tendency measure (CTM)

To have a statistical summary of the disease behavior, the endemic channel provides a central tendency measure (CTM) that should describe the typical value of the historical data.

Since the nature of the epidemiological data tends not to be normal, the typically used CTM in the endemic channels is the geometric mean. This CTM is known to better represent the expected value of a data set with a skewed distribution and/or is susceptible to outliers (previous outbreaks). The endemic_channel function uses the geometric mean as the default CTM, but the arithmetic mean or median can be chosen according to the user’s experience and context.

It is important to note that as the geometric mean performs a multiplication of the data, when zero cases are reported in a moment of time, the estimation leads to a zero CTM. To avoid this, epiCo’s endemic_channel function performs a shift on the data to sum up one case (the default value) for all observations, and then it subtracts this shift from the final calculation. To avoid random selection of the shift, users can ask the endemic_channel function to find an optimal value based on de la Cruz & Kreft (2019), or they can also ask to use a weighted method as described by Habib (2012).

Finally, the endemic_channel function can perform a Poisson test (unusual behavior method) on the historical data if it is requested by the user after taking into account the pertinence of the test, since it is mostly used for scenarios or diseases with very low incidence records. This method uses the arithmetic mean as CTM.

The upper and lower limits

The upper and lower limits of the endemic channel provide a confidence interval that is used to define the epidemiological bands previously described.

epiCo’s endemic_channel function has the following preset of limits according to the selected CTM:

Arithmetic or geometric mean as CTM: mean +/- t value for a 95% CI multiplied by the variance of the data over the number of observations.
Median as CTM: quantiles 0.25 and 0.75 as lower and upper limits, respectively.
Poisson test (unusual behavior method): limits provided by a two-sided Poisson test with a 0.95% CI

2. Historical data needed to build an endemic channel

This section provides some strategies to obtain, handle, and gather the historical data needed to build an endemic channel. It presumes that the user has either the line list or incidence data for the disease of interest or the need to consult historical data from the Colombian National Surveillance System (SIVIGILA).

Regardless of the user´s case, the goal of this section is to create an incidence object with historical data on the disease. This object must account for the incidence of a single region (no groups are allowed in the endemic_channel function), it must have a weekly or monthly interval (epiweeks are also allowed), and it must represent at least one year of information.

To have a broader understanding of the incidence package, please refer to its vignettes.

Historic incidence from the line list or SIVIGILA data

The endemic channel is more useful when data up to the immediate previous year is available; however, this is not always possible for users outside of the surveillance institutions. As an option, users can access SIVIGILA’s line list data that is typically published until the second semester of the year (i.e., data for 2022 was published in the second semester of 2023).

For this example, the data provided in the data folder of the package will be used. These data present the data for dengue cases for all municipalities in Tolima for the years 2015 to 2021. In this case, we will use the cases from the municipality of Ibagué to calculate the respective endemic channel.

library(epiCo)
library(incidence)

data("epi_data")
data_event <- epi_data

data_ibague <- data_event[data_event$cod_mun_o == 73001, ]

## Building the historic incidence data

incidence_historic <- incidence(data_ibague$fec_not,
  interval = "1 epiweek"
)

knitr::kable(incidence_historic[1:5, ])

dates	weeks	counts
2012-01-01	2012-W01	7
2012-01-08	2012-W02	20
2012-01-15	2012-W03	17
2012-01-22	2012-W04	15
2012-01-29	2012-W05	18

When users have their own line list data, the goal is the same: to clean and handle the data to build the incidence object (named incidence_historic in the previous example). The key message is that a list of dates that account for the event of interest is needed to later be used in the incidence package.

Setting up the data from incidence information

Future versions of epiCo’s endemic_channel function will allow the users to import a table accounting for periods of time (weeks or months) and count of cases, so when they do not have specific information on the dates for each case, the endemic channel can still be built.

3. Using epiCo’s `endemic_channel` function

After gathering the incidence data, users are ready to ask epiCo to build an endemic channel.

The endemic_channel function has the following parameters:

incidence_historic: An incidence object with the case counts of the previous years
observations: A numeric vector with the current observations (default = NULL)
method: A string with the mean calculation method of preference (“median”, “mean”, or “geometric”) or “unusual_behavior” method (Poisson Distribution Test) (default = “geometric”)
geometric_method: A string with the selected method for geometric mean calculation (see epiCo’s geometric_mean function) (default = “shifted”).
outlier_years: A numeric vector with the outlier (epidemic) years (default = NULL).
outliers_handling: A string with the handling decision regarding outlier years:
- ignored = data from outlier years will not be taken into account (default).
- included = data from outlier years will take into account.
- replaced_by_median = data from outlier years will be replaced with the median and taken into account.
- replaced_by_mean = data from outlier years will be replaced with the mean and taken into account.
- replaced_by_geometric_mean = data from outlier years will be replaced with the geometric mean and taken into account.
ci: A numeric value to specify the confidence interval to use with the geometric method (default = 0.95).
plot: A boolean for displaying a plot (default = FALSE).

The output of epiCo’s endemic_channel function is a dataframe with the observation, historical mean, and confidence intervals, and a plot displaying the four epidemiological bands, the current observations (if provided), and the methods and outliers handling set by the user.

Example

The following example uses the previously historic incidence accessed from the SIVIGILA data source to build the 2021 endemic channel for Ibagué City.

observations <- sample(5:40, 52, replace = TRUE)

outlier_years <- c(2016, 2019)

ibague_endemic_chanel <- endemic_channel(
  incidence_historic = incidence_historic,
  observations = observations,
  method = "geometric",
  outlier_years = outlier_years,
  plot = TRUE
)
#> Data after2022-12-25 were not used for the endemic channel calculation.