- Parameterization
- Initialization of the Petri Net
- Equilibrium Conditions and Hazard Functions
- Simulation of Fully Specified SPN Model
- References

This vignette describes how to use the facilities provided by
**MGDrivE2** to run simulations of joint gene drive and
epidemiological (specifically an **SIS** model) dynamics on
metapopulation networks; nodes may contain humans only (such as high
altitude regions), mosquitoes only (such as uninhabited swamp), or
humans and mosquitoes (inhabited regions with breeding mosquito
populations). Infection events (mosquito to human or human to mosquito)
can only occur in nodes where both humans and mosquitoes are allowed to
interact. For more information on how these models are coupled, see the
“MGDrivE2: One Node Epidemiological
Dynamics” vignette.

We start by loading the **MGDrivE2** package, as well as
the **MGDrivE** package for access to inheritance cubes,
**ggplot2** for graphical analysis, and
**Matrix** for sparse matrices used in migration. We will
use the basic cube to simulate Mendelian inheritance for this
example.

```
# simulation functions
library(MGDrivE2)
# inheritance patterns
library(MGDrivE)
# plotting
library(ggplot2)
# sparse migration
library(Matrix)
# basic inheritance pattern
<- MGDrivE::cubeMendelian() cube
```

Before setting up the simulations, we define the parameters used to
calculate equilibrium populations and generate hazards during the
simulation. We define the list of entomological and epidemiological
parameters, `theta`

. For a table of epidemiological parameter
descriptions, see the “MGDrivE2: One Node
Epidemiological Dynamics” vignette.

The chunk of code below defines parameters used to setup the Petri Net and for calculating the equilibria and initial conditions.

Additionally, we specify a total simulation time of 300 days, and to demonstrate additional functionality, we store output every other day.

```
# entomological and epidemiological parameters
<- list(
theta # lifecycle parameters
qE = 1/4,
nE = 2,
qL = 1/3,
nL = 3,
qP = 1/6,
nP = 2,
muE = 0.05,
muL = 0.15,
muP = 0.05,
muF = 0.09,
muM = 0.09,
beta = 16,
nu = 1/(4/24),
# epidemiological parameters
NH = 1000,
X = 0.25,
f = 1/3,
Q = 0.9,
b = 0.55,
c = 0.15,
r = 1/200,
muH = 1/(62*365),
qEIP = 1/11,
nEIP = 2
)
# simulation parameters
<- 250
tmax <- 2 dt
```

We also need to augment the cube with genotype specific transmission
efficiencies; this allows simulations of gene drive systems that confer
pathogen-refractory characteristics to mosquitoes depending on genotype.
The specific parameters we want to attach to the cube are `b`

and `c`

, the mosquito to human and human to mosquito
transmission efficiencies. We assume that transmission from human to
mosquito is not impacted in modified mosquitoes, but mosquito to human
transmission is significantly reduced in modified mosquitoes. For
detailed descriptions of these parameters for modelling malaria
transmission, see Smith & McKenzie (2004) for extensive
discussion.

```
# augment the cube with RM transmission parameters
$c <- setNames(object = rep(x = theta$c, times = cube$genotypesN), nm = cube$genotypesID)
cube$b <- c("AA" = theta$b, "Aa" = 0.35, "aa" = 0) cube
```

Like previous examples, we begin by constructing the Petri Net
(places and transitions). We will make a 3 node metapopulation network;
the first node will contain mosquitoes only, a breeding ground outside a
city, the second node will contain both humans and mosquitoes, a
location at the edge of a city, and the third node will contain only
humans, a location deep in the city and far from mosquito breeding
grounds. We specify the node types in the character vector
`node_list`

; `"m"`

refers to mosquito-only nodes,
`"h"`

to human-only nodes, and `"b"`

refers to
human and mosquito nodes. It is important to note that equilibrium
calculations are done on a node-by-node basis, not over the whole
network (that is, each node’s initial condition is calculated without
taking movement into account). Thus, when we start a simulation with
different levels of disease incidence or population amounts in each
node, the dynamics on the network will still exhibit a burn-in period
before reaching equilibrium.

We specify edges in the movement network separately for humans and
mosquitoes (in `h_move`

and `m_move`

,
respectively). These should be either logical matrices or integer
matrices that can be coerced to logical values. Humans can move back and
forth between the humans-only and both nodes, and mosquitoes between the
mosquitoes-only and both nodes. These matrices are needed to generate
the set of transitions. While we use base `matrix`

objects in
this example, sparse logical matrices from the `Matrix`

library are supported for large networks.

Once the possible movements between nodes are specified, we setup the
places and transitions for the Petri Net, and the summarize those
transitions that apply to each place, the stoichiometry matrix. This is
handled by the `spn_S()`

function, which calls
`spn_Pre()`

and `spn_Post()`

internally for each
half of the calculation.

```
# nodetypes
<- c("m", "b", "h")
node_list <- length(node_list)
num_nodes
# human movement
<- matrix(data = FALSE, nrow = num_nodes, ncol = num_nodes,
h_move dimnames = list(node_list, node_list))
2,3] <- TRUE
h_move[3,2] <- TRUE
h_move[
# mosquito movement
<- matrix(data = FALSE, nrow = num_nodes, ncol = num_nodes,
m_move dimnames = list(node_list, node_list))
1,2] <- TRUE
m_move[2,1] <- TRUE
m_move[
# Places and transitions
<- spn_P_epiSIS_network(node_list = node_list, params = theta, cube = cube)
SPN_P <- spn_T_epiSIS_network(node_list = node_list, spn_P = SPN_P, params = theta,
SPN_T cube = cube, h_move = h_move, m_move = m_move)
# Stoichiometry matrix
<- spn_S(spn_P = SPN_P, spn_T = SPN_T) S
```

Now that we have set up the structural properties of the Petri Net, we can calculate the population equilibrium, and the initial conditions, from parameters defined earlier ( theta ). Remember, these are node-by-node equilibria, not an equilibrium over the entire network. Because each node is independent, we need to set the number of female mosquitoes in the mosquito-only node, and the number of humans in the human-only node, at equilibrium. Additionally, we need to set disease prevalence in the human-only node. We will begin at a slightly lower prevalence than the human/mosquito node, acknowledging the migration between those nodes and the lack of transmission in the human-only node.

```
# SEI mosquitoes and SIS humans equilibrium
# outputs required parameters in the named list "params"
# outputs intial equilibrium for adv users, "init
# outputs properly filled initial markings, "M0"
<- equilibrium_SEI_SIS(params = theta, node_list = node_list,
initialCons NF = 500, phi = 0.5, NH = 1000, pop_ratio_H = 0.15,
log_dd = TRUE, spn_P = SPN_P, cube = cube)
```

Next, we make the movement matrix. Because **MGDrivE2**
is a continuous-time model, unlike **MGDrivE**, the
movement matrix is a stochastic rate matrix (or infinitesimal generator,
in probability theory parlance), meaning that it should have rows that
sum to 0, and the diagonal is the negative sum of off-diagonal elements
in each row. For ease of specification, we separate the stochastic rate
matrix into two parts; a vector of rates (the *positive*
diagonal), and the matrix of renormalized off-diagonal elements (a
probability matrix). We use the function `calc_move_rate()`

to calculate the total movement rate from mortality rate and lifetime
probability of movement (which we assume to be `0.05`

). For
humans, we assume that everyone moves between nodes on average once a
week, or at a rate of `1/7`

.

For both humans and mosquitoes, the vector of movement rates must be
of length equal to the number of nodes in the matrix, and nodes from
which no movement is possible (for mosquitoes, human-only nodes and vice
versa for humans), should have value `NaN`

. If you have made
the logical matrices specifying allowed movement, the following code can
help ascertain which elements should be `NaN`

:
`apply(X = m_move, MARGIN = 1, FUN = Negate(any))`

. Finally
we append the movement objects to the vector of parameters
`initialCons$params`

.

```
# calculate movement rates and movement probabilities
<- calc_move_rate(mu = initialCons$params$muF, P = 0.05)
gam
# set mosquito movement rates/probabilities
# mosquitoes exist in nodes 1 and 2, not 3
<- c(gam, gam, NaN)
mr_mosy <- Matrix::sparseMatrix(i = c(1,2), j = c(2,1), x = 1, dims = dim(m_move))
mp_mosy
# set human movement rates/probabilities
# humans exist in nodes 2 and 3, not 1
<- c(NaN, 1/7, 1/7)
mr_human <- Matrix::sparseMatrix(i = c(2,3), j = c(3,2), x = 1, dims = dim(h_move))
mp_human
# put rates and probs into the parameter list
$params$mosquito_move_rates <- mr_mosy
initialCons$params$mosquito_move_probs <- mp_mosy
initialCons$params$human_move_rates <- mr_human
initialCons$params$human_move_probs <- mp_human initialCons
```

Now that all the necessary parameters have been added to the named
list `initialCons$params`

, we generate the hazard functions,
using the function `spn_hazards()`

. By specifying
`log_dd = TRUE`

, we use logistic density dependence for these
simulations.

```
# approximate hazards for continous approximation
<- spn_hazards(spn_P = SPN_P, spn_T = SPN_T, cube = cube,
approx_hazards params = initialCons$params, type = "SIS",
log_dd = TRUE, exact = FALSE, tol = 1e-8,
verbose = FALSE)
# exact hazards for integer-valued state space
<- spn_hazards(spn_P = SPN_P, spn_T = SPN_T, cube = cube,
exact_hazards params = initialCons$params, type = "SIS",
log_dd = TRUE, exact = TRUE, tol = NaN,
verbose = FALSE)
```

Now that we have generated the Petri Net and associated hazard functions we are ready to proceed with simulations.

Similar to previous simulations, we will release 50 adult females
with homozygous recessive alleles 5 times, every 10 days, starting at
day 20, in node 1 (this allows us to see movement from node 1 to node 2,
before we see an impact on human disease transmission). Remember, it is
critically important that **the event names match a place
name** in the simulation. The simulation function checks this and
will throw an error if the event name does not exist as a place in the
simulation. This format is used in **MGDrivE2** for
consistency with solvers in `deSolve`

.

```
# releases
<- seq(from = 20, length.out = 5, by = 10)
r_times <- 50
r_size <- data.frame("var" = paste0("F_", cube$releaseType, "_", cube$wildType, "_S_1"),
events "time" = r_times,
"value" = r_size,
"method" = "add",
stringsAsFactors = FALSE)
```

As in the “MGDrivE2: One Node Epidemiological
Dynamics” vignette, we will first numerically simulate the
mean-field ODE approximation to the stochastic trajectory, using the
approximate hazards suitable for continuous-state approximation (see
`?spn_hazards()`

). Internally, **MGDrivE2** uses
the high quality numerical solvers in from `deSolve`

to
integrate a mean-field approximation to the stochastic model. We will
then plot several aspects of the population using helper functions
provided by **MGDrivE2**.

```
# run deterministic simulation
<- sim_trajectory_R(x0 = initialCons$M0, tmax = tmax, dt = dt, S = S,
ODE_out hazards = approx_hazards, sampler = "ode", method = "lsoda",
events = events, verbose = FALSE)
# summarize aquatic stages by genotype
<- summarize_eggs_geno(out = ODE_out$state, spn_P = SPN_P)
ODE_e <- summarize_larvae_geno(out = ODE_out$state, spn_P = SPN_P)
ODE_l <- summarize_pupae_geno(out = ODE_out$state, spn_P = SPN_P)
ODE_p
# add stage name
$stage <- "Egg"
ODE_e$stage <- "Larvae"
ODE_l$stage <- "Pupae"
ODE_p
# plot by genotype
ggplot(data = rbind(ODE_e, ODE_l,ODE_p)) +
geom_line(aes(x = time, y = value, color = genotype)) +
facet_grid(stage ~ node, scales = "free_y") +
theme_bw() +
ggtitle("SPN: ODE Solution - Genotypes")
```

Our first plot looks at the aquatic stage distributions, in the 2 nodes with mosquitoes, nodes 1 and 2. At equilibrium, there are significantly more eggs than larvae, and larvae than pupae. Additionally, we see the difference in population size between the nodes, with node 1 carrying around 66% more mosquitoes than node 2. Releases were performed in node 1, and then we see some migration to node 2.

```
# summarize aquatic stages by Erlang stage
<- summarize_eggs_stage(out = ODE_out$state, spn_P = SPN_P)
ODE_e <- summarize_larvae_stage(out = ODE_out$state, spn_P = SPN_P)
ODE_l <- summarize_pupae_stage(out = ODE_out$state, spn_P = SPN_P)
ODE_p
# add stage name
$stage <- "Egg"
ODE_e$stage <- "Larvae"
ODE_l$stage <- "Pupae"
ODE_p
# plot by Erlang stage
ggplot(data = rbind(ODE_e, ODE_l,ODE_p)) +
geom_line(aes(x = time, y = value, color = `Erlang-stage`)) +
facet_grid(stage ~ node, scales = "free_y") +
theme_bw() +
ggtitle("SPN: ODE Solution - Erlang Dwell Stage")
```

We can look at the aquatic stages another way. Time spent in each stage is described by an Erlang distribution, with varying number of states. From the figure, we see that eggs and pupae only have 2 stages, while larvae have 3. We see the effect of releases in the first node, but we do not see the effect on the second node. Since we do not show genotypes here, we cannot see the release of new alleles into the population. Looking at the egg stage, near time = 0, we see a dip in the population of node 1, and a corresponding rise in the population of node 2. This also appeared in the previous plot, and is from the metapopulation coming to equilibrium. This is an artifact of how the equilibrium is calculated.

```
# summarize females/males
<- summarize_females_epi(out = ODE_out$state, spn_P = SPN_P)
ODE_f <- summarize_males(out = ODE_out$state)
ODE_m
# add sex for plotting
$sex <- "Female"
ODE_f$sex <- "Male"
ODE_m$inf <- "S"
ODE_m
# plot adults
ggplot(data = rbind(ODE_f, ODE_m)) +
geom_line(aes(x = time, y = value, color = genotype, linetype = inf)) +
facet_grid(sex ~ node, scales = "fixed") +
theme_bw() +
ggtitle("SPN: ODE Solution - Adult Stages")
```

Looking at the adult population, it is easy to see the difference in population size between nodes 1 and 2. We see modified female mosquito releases in node 1, the propagation of those alleles into the male population of node one, and then migration of those alleles into node 2. Additionally, we see how few mosquitoes in node 2 are actually infected, and a tiny migration of them into node 1. It is also apparent that male mosquitoes are not part of the disease dynamics (and will be left out of further plots). Again, looking closely near time = 0, we see the burn-in of the network prior to reaching equilibrium.

```
# summarize females/humans by genotype
<- summarize_females_epi(out = ODE_out$state, spn_P = SPN_P)
ODE_female <- summarize_humans_epiSIS(out = ODE_out$state)
ODE_humans
# plot
ggplot(data = rbind(ODE_female,ODE_humans) ) +
geom_line(aes(x = time, y = value, color = genotype, linetype = inf)) +
facet_wrap(. ~ node, scales = "free_y") +
theme_bw() +
ggtitle("SPN: ODE Solution")
```

We now breakdown the network by node, showing female mosquitoes and humans, so that we can watch the infection dynamics. First, we look at the humans, where the burn-in near time = 0 is extremely apparent. This was deliberate, as disease incidence is different between the nodes, to remind us of the issues with metapopulation-level equilibrium. As a positive, there is significant reduction in disease incidence in humans. This stems from 2 places: first, as there are no mosquitoes in node 3, there is no disease transmission and disease incidence should fall, and second, release of modified mosquitoes that do not transmit disease aids in incidence reduction. In this setting, more disease incidence reduction is due to lack of transmission in node 3 than modified mosquitoes. However, further simulations could test these differences and determine how much reduction is due to both causes.

As a further example, we run a single stochastic realization of the
same simulation, using the `tau`

sampler with \(\Delta t = 0.1\), approximating 10 jumps
per day. As the adult male mosquitoes do not contribute to infection
dynamics, we will only view the adult female mosquito and human dynamics
here.

```
# delta t
<- 0.1
dt_stoch
# run tau-leaping simulation
<- sim_trajectory_R(x0 = initialCons$M0, tmax = tmax, dt = dt,
PTS_out dt_stoch = dt_stoch, S = S, hazards = exact_hazards,
sampler = "tau", events = events, verbose = FALSE)
# summarize females/humans by genotype
<- summarize_females_epi(out = PTS_out$state, spn_P = SPN_P)
PTS_female <- summarize_humans_epiSIS(out = PTS_out$state)
PTS_humans
# plot
ggplot(data = rbind(PTS_female,PTS_humans) ) +
geom_line(aes(x = time, y = value, color = genotype, linetype = inf)) +
facet_wrap(. ~ node, scales = "free_y") +
theme_bw() +
ggtitle("SPN: Tau-leaping Approximation")
```

Looking at one stochastic realization of our female/human disease
incidence plot, we see quite similar dynamics to the ODE solution.
Dynamics are slightly slower, but otherwise similar, indicating that
incidence reduction is primarily due to recovery in humans. We
hypothesize this (but should still test it) because previous simulations have shown a
reduction in the `a`

allele due to drift, which we can assume
occurs here, but we see similar rates of disease reduction, indicating
that our modified allele is not of major importance for disease
incidence reduction.

- Smith, D. L., & McKenzie, F. E. (2004). Statics and dynamics of malaria infection in Anopheles mosquitoes. Malaria journal, 3(1), 13.