---
title: "Automated Detection of Seasonal Epidemic Onset and Burden Levels in R"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Automated Detection of Seasonal Epidemic Onset and Burden Levels in R}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```

```{r setup, warning=FALSE, message=FALSE}
library(aedseo)
```

## Introduction

The `aedseo` package performs automated and early detection of seasonal epidemic onsets and estimates
the breakpoints for burden levels from time series data stratified by season.
The seasonal onset (`seasonal_onset()`) estimates growth rates for consecutive time intervals and calculates the sum of cases.
The burden levels (`seasonal_burden_levels()`) use the previous seasons to estimate the burden levels of the current season.
The algorithm allows for surveillance of pathogens, by alarming when the observations have significant growth in
the selected time interval and based on the disease-specific threshold, while also evaluating the burden of current
observations based on previous seasons.

### Seasonal data

To apply the `aedseo` algorithm, data needs to be transformed into a `tsd` object.
If you have your own data, the `to_time_series()` function can be used with the arguments: `observation`, `time`, `time_interval`.
In the following section, the application of the algorithm is shown with simulated data created with the `generate_seasonal_data()`function.
More information about the function can be found in the `vignette("generate_seasonal_wave")`

```{r, include = FALSE}
withr::local_seed(222)
# Construct an 'tsd' object with time series data
tsd_data <- generate_seasonal_data(
  years = 5,
  start_date = as.Date("2020-10-18"),
  trend_rate = 1.002,
  noise_overdispersion = 5,
  relative_epidemic_concentration = 3,
  time_interval = "week"
)

tsd_data <- tsd_data |>
  dplyr::filter(time <= "2025-02-01")

```

In the following figure simulated data (solid circles) are visualised as individual observations.
The solid line connects these points, representing the underlying mean trend over three years of weekly data.
```{r, dpi=300}
plot(tsd_data)
```

### Determining season
Respiratory viruses can circulate in different seasons based on the location.
In the nordic hemisphere they mostly circulate in the fall and winter seasons, hence surveillance is intensified from week 40 to week 20 in the following year.
To include all data, the season in the example is set from week 21 to week 20 in the following year.
In this example burden levels and seasonal onset will be estimated for season *2024/2025*.

### Determining the disease specific threshold
When observations are low there is a risk that randomness will result in significant growth estimates in isolated periods.
To increase the robustness of the method a disease-specific threshold is introduced. It should be set such that subsequent
estimates of growth are likely to be significant as well.
The disease-specific threshold can be determined by examining continuous periods with sustained significant growth,
and determine at what number of observations these events occur.

In this example the disease-specific threshold is determined based on consecutive significant observations from all available previous seasons.
Significant observations are defined as those with a significant positive growth rate.

To capture short-term changes and fluctuations in the data, a rolling window of size $k = 5$ is used to create subsets of the data for model fitting,
and the `quasipoisson` family is used to account for overdispersion.

The disease-specific threshold will be estimated with the four available previous seasons.
The `seasonal_onset()` function can be used for this purpose, without providing the disease-specific threshold.
Then the `consecutive_growth_warnings()` function can be used to create groups with subsequent significant observations.
The data can then be analysed, else you can use plot/autoplot to visualise the sequences of significant observations for each season.

The `sum_of_cases` variable is divided by five to represent an average over a five-week window, defining the 
disease-specific threshold for each time-step.
```{r, dpi=300}
tsd_onset <- seasonal_onset(
  tsd = tsd_data,
  k = 5,
  family = "quasipoisson",
  na_fraction_allowed = 0.4,
  season_start = 21, # Season starts in week 21
  season_end = 20, # Season ends in week 20 the following year
  only_current_season = FALSE
)

consecutive_gr_warn <- consecutive_growth_warnings(
  onset_output = tsd_onset
)

autoplot(
  consecutive_gr_warn,
  k = 5,
  skip_current_season = TRUE
) +
  ggplot2::geom_vline(
    ggplot2::aes(xintercept = 25, linetype = "Threshold"),
    color = "black", linewidth = 0.6
  ) +
  ggplot2::scale_linetype_manual(
    name   = "",
    values = c("Threshold" = "dashed")
  )
```

From the plot above, we observe the length of periods with subsequent significant growth rates (y-axis).
The season with the longest consecutive period of growth is *2021/2022*, lasting 14 weeks.
However, since we are determining a threshold specifically for the *2024/2025* season, it's important to prioritize the 
most recent seasons. The *2023/2024* season shows two periods of significant growth, with the first being the longest 
and coinciding closely in timing with the consecutive growth period observed in *2022/2023*.
We select a disease-specific threshold of 25 to ensure early detection of the seasonal onset while minimizing false positives.

In other words, a season onset is declared when the average observation count over five weeks surpasses 25 and is 
accompanied by a significantly positive growth rate.

Inspect the exact conditions around each detected season start
```{r}
consecutive_gr_warn |>
  dplyr::filter(!is.na(significant_counter)) |>
  dplyr::filter(season != max(consecutive_gr_warn$season)) |>
  dplyr::group_by(season) |>
  dplyr::filter(significant_counter == max(significant_counter)) |>
  dplyr::mutate(disease_threshold = sum_of_cases / 5,
                week = ISOweek::ISOweek(reference_time)) |>
  dplyr::select(season, week, disease_threshold)
```

By inspecting the output from the above code, the disease-specific threshold is established at `25` observations.

## Applying the main algorithm
The primary function of the `aedseo` package is the `combined_seasonal_output()` which integrates the `seasonal_onset()` and `seasonal_burden_levels()`
functions to deliver a comprehensive seasonal analysis.
Detailed information about each function and their respective arguments can be found in the `vignette("seasonal_onset")` and `vignette("burden_levels")`.

```{r}
seasonal_output <- combined_seasonal_output(
  tsd = tsd_data,
  disease_threshold = 25,
  method = "intensity_levels",
  family = "quasipoisson"
)
```

The default function estimates onset and burden levels for the current season. If it is desired to see calculations for all previous seasons, the `only_current_season`
argument should be set to `FALSE`.
*Note: *Burden levels can not be estimated for the first season and needs at least two seasons of data as the estimations are based on data from previous seasons.\\

The `aedseo` package implements S3 methods including the `plot()`, `predict()` and `summary()` functions specifically designed for objects of the `aedseo` package.
`predict()` is only relevant for `tsd_onset` objects.
An example of using the `summary()` S3 method with `tsd_onset` and `tsd_burden_level` objects is shown here.

Seasonal onset output can be extracted by:
```{r}
summary(seasonal_output$onset_output)
```

Seasonal burden output can be extracted by:
```{r}
summary(seasonal_output$burden_output)
```

### Plot the comprehensive seasonal analysis
The `plot()` S3 method for `tsd_combined_seasonal_output` objects allows you to get a complete visualisation of the `combined_seasonal_output()` analysis of the current season.

```{r, dpi=300}
# Adjust y_lower_bound dynamically to remove noisy small values
disease_threshold <- 25
y_lower_bound <- ifelse(disease_threshold < 10, 1, 5)

plot(
  x = seasonal_output,
  y_lower_bound = y_lower_bound,
  time_interval = "3 weeks"
)
```

Using the `intensity_levels` method to define burden levels, the seasonal onset is likely to fall within the `low` or `medium`
category. This is because the `very low` breakpoint is the disease-specific threshold, and season onset is only identified if
the five-week average of the observations exceed this threshold along with a significant positive growth rate.

### Investigate historical estimates
The `historical_summary()` function for `tsd_onset` objects provides historical estimates from all previous seasons.
By utilising this function, it is easy to assess whether current estimates align with previously observed patterns for
a specific pathogen, or if significant changes have occurred. Such changes might result from altered testing practices, 
pathogen mutations, or other factors.

If the analysis indicates notable deviations from past patterns, it is advisable to revisit the method used to define the 
disease-specific threshold, as it might need some adjustment.

```{r, dpi=300}
# Get `tsd_onset` object
tsd_onset <- seasonal_onset(
  tsd = tsd_data,
  disease_threshold = 25,
  family = "quasipoisson",
  season_start = 21,
  season_end = 20,
  only_current_season = FALSE
)

historical_summary(tsd_onset)
```