pepbp: Simulating an infectious disease outbreak with post-exposure prophylaxis for high-risk contacts

pepbp is a package to simulate an infectious disease outbreak where high-risk contacts of cases receive post-exposure prophylaxis, which reduces their probability of becoming a case.

See Methods for a full description of the underlying branching process model. An installation and quick-start guide for the package is below.

Installation

You can install the package from Github:

devtools::install_github("sophiemeakin/pepbp")

Quick start guide

The main functionality of the package is scenario_sim(). Here is an example to run five iterations for a given scenario:

res <- scenario_sim(
  n_sim = 5,            # number of iterations
  prop_pep = 0.8,       # probability that high-risk contacts (HRCs) receive PEP
  rel_risk = 0.5,       # relative risk of becoming a case after receiving PEP
  hrc_mu = 2,           # average number of HRCs per case
  hrc_disp = 1,         # overdispersion of number of HRCs
  lrc_mu = 5,           # average number of low-risk contacts (LRCs) per case
  lrc_disp = 10,        # overdispersion of number of LRCs
  p_hrc_case = 0.75,    # probability that HRCs become a case (without PEP)
  p_lrc_case = 0.1,     # probability that LRCs become a case
  n_initialcases = 5,   # number of initial cases
  cap_max_gen = 10,     # BP stopping criteria, generations
  cap_max_cases = Inf   # BP stopping criteria, total cases
)

We can then, for example, plot the total cases by generation:

res %>%
  count(iter, generation) %>%
  complete(iter, generation, fill = list(n = 0)) %>%
  group_by(iter) %>%
  mutate(n_cuml = cumsum(n)) %>%
  ggplot(aes(x = generation, y = n_cuml, col = as.factor(iter))) +
  geom_line(show.legend = FALSE) +
  scale_x_continuous(limits = c(0, NA), breaks = seq(0, 10, 2)) +
  scale_y_continuous(limits = c(0, NA), breaks = seq(0, 5000, 200)) +
  scale_color_brewer(palette = "Set2") +
  labs(x = "Generation", y = "Cumulative cases") +
  theme_bw() +
  theme(text = element_text(size = 14))

See vignettes/example-application.Rmd for an application of this model to explore the effect of post-exposure prophylaxis for high-risk contacts on transmission and control of an infectious disease outbreak.

Methods

Input

The branching process model has five inputs:

• The initial number of cases, n.
• The distribution of the number of high- and low-risk contacts per case.
• The probability that high- and low-risk contacts become cases, q_HR and q_LR, respectively.
• The probability that each high-risk contact receives PEP, p.
• The effectiveness of PEP, defined as the relative risk that a high-risk contact becomes a case after receiving PEP, theta.

Model

The outbreak is initialised with n cases. Then, for each case:

1. Generate contacts: Sample the number of high- and low-risk contacts from the respective negative binomial distributions.
2. Assign PEP to high-risk contacts: Each high-risk contact receives PEP with probability p.
3. Determine secondary cases:
   • High-risk contacts who did not receive PEP become cases with probability q_HR.
   • High-risk contacts who did receive PEP become cases with probability theta*q_HR.
   • Low-risk contacts become cases with probability q_LR.
4. Repeat 1 - 3 for new generation of cases.

Output

In the output each row represents a case, for which we report the following:

• The number of high- and low-risk contacts
• The number of high-risk contacts who do and do not receive PEP
• The number secondary cases (high-risk contacts who received PEP; high-risk contacts who did not receive PEP; low-risk contacts)

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Acknowledgements

The structure of {pepbp} is based on the {ringbp} package, originally developed by Joel Hellewell, Sam Abbott, Amy Gimma, Tim Lucas and Sebastian Funk. Thank you!