Getting started with `ggoutbreak`

Background

A collection of tools from COVID-19 with focus on simplicity and a goal to provide a simple pipeline from data to visualisation. The main features are listed below. The documentation is in development. For the time being the best resources are the other vignettes including:

• vignette("covid-timeseries",package="ggoutbreak")
• vignette("incidence-trends",package="ggoutbreak")
• vignette("variant-proportions",package="ggoutbreak")
• vignette("weekly-incidence",package="ggoutbreak")
• vignette("rt-from-incidence",package="ggoutbreak")
• vignette("simulation-test-models",package="ggoutbreak")

Installation

ggoutbreak is hosted on the AI4CI r-universe. Installation from there is as follows:

options(repos = c(
  "ai4ci" = 'https://ai4ci.r-universe.dev/',
  CRAN = 'https://cloud.r-project.org'))

# Download and install ggoutbreak in R
install.packages("ggoutbreak")

You can install the development version of ggoutbreak from GitHub with:

# install.packages("devtools")
devtools::install_github("ai4ci/ggoutbreak")

Features

Simulation

• Poisson (aggregate count) & branching process (line list)
• Time varying parametrisations
• Test harness for estimators, including scoring metrics.

Estimation

• Collection of methods for incidence, growth rate, reproduction number (and wrappers for existing tools)
• Poisson count and binomial/multinomial proportion models

Visualization

• Default ggplot2 visualisations of epidemic time series, including adjustment for population and variable time steps.

Pipeline functions

Simulation

Models:

• Poisson growth rate (count model)
• Poisson reproduction number (count model)
• Branching process model (individual model)
• SEIR ODE (compartment model)

Time varying parameters:

• Rt / growth rate / transmission
• Importation
• Generation time
• Ascertainment
• Delay to observation, e.g. symptoms, admissions, death
• Contact matrices
• Dispersion

Scoring

• mean_quantile_bias - the average of the universal residuals. Lower values are better.
• mean_bias - the bias on the natural scale (which may be interpreted as additive or multiplicative depending on the link)
• pit_was - an unadjusted probability integral transform histogram Wasserstein distance from the uniform (lower values are better).
• unbiased_pit_was - an PIT Wasserstein distance from the uniform, adjusted for estimator bias (lower values are better). This is a measure of calibration.
• directed_pit_was - a PIT Wasserstein distance from the uniform, directed away from the centre, adjusted for estimator bias (values closer to zero are better, positive values indicate overconfidence, and negative values excessively conservative estimates).
• percent_iqr_coverage - the percentage of estimators that include the true value in their IQR. For a perfectly calibrated estimate this should be 0.5. Lower values reflect overconfidence, higher values reflect excessively conservative estimates. This is a measure of calibration but is influenced by bias.
• unbiased_percent_iqr_coverage - the percentage of estimators that include the true value in their IQR once adjusted for bias. This should be 0.5. This is a measure of calibration, and tells you which direction (smaller numbers are over-confident, larger values excessively conservative).
• mean_prediction_interval_width_50 - the prediction interval width is a measure of sharpness (smaller values are sharper). Sharper estimators are superior if they are unbiased and well calibrated.
• mean_crps - the mean value of the continuous rank probability score for each point estimate (lower values are better)
• threshold_misclassification_probability - if a metric has a natural threshold like 1 for $R_t$ then this measures how probable it is that the estimate will propose the epidemic is shrinking when it is growing and vice versa. Lower is better.

Estimation methods

• Rapid simple estimates.
• All methods can operate over multiple sub-group time series.
• Supports daily / weekly / monthly / yearly time series*
• Testing estimators against simulations.

Infectivity profile estimation

• Estimation of generation time or serial interval from published estimates (including uncertainty)
• Resampled from raw interval data (with bootstrapping for uncertainty) from posterior estimates of a fitted gamma distribution

Poisson rate models

• Locfit or GLM fitted time varying quasipoisson model.
• GAM models including adjustment for right censoring.
• Estimates incidence and growth rate
• Normalisation per capita population or to population baseline rate

Binomial proportion models

• Locfit or GLM fitted time varying quasibinomial model.
• Estimates proportion and relative growth rate
• Normalisation against population baseline risk

Multinomial proportion models

• nnet based multinomial proportion models.

$R_t$ estimation methods:

EpiEstim wrapper

Simple adaptor for EpiEstim reference method

Reimplementation of Cori method

• Inputs raw incidence time series
• Reimplementation integrates estimates from a range of time windows
• Handles missing data.
• Less lag and more uncertainty.

Wallinga and Lipsitch growth rates method

• Inputs an exponential growth rate time series estimate (assumed normally distributed)
• Uses MGF of discrete SI distribution
• Monte-Carlo resampling for propagation of uncertainty
• Moderately slow

$R_t$ from modelled incidence

• Inputs a log-normally distributed incidence estimate.
• Propagates uncertainty in infectivity profile and incidence data
• Deterministic & tolerant of missing values.
• Can use incidence model derived from weekly count time series.
• Can deal with negative values in serial interval

Bootstrapped renewal eqaution

• Inputs distributional for of incidence estimate.
• Uses resampling to propagate uncertainty in incidence and infectivity profile
• Non deterministic, can use weekly time-series