dynamicalFeverScript.R

## Dynamical Fever: computer exercise
## Clinic on Meaningful Modeling of Epidemiological Data &
## Clinic on Dynamical Approaches to Infectious Disease Data
## International Clinics on Infectious Disease Dynamics and Data Program
##
## Juliet R.C. Pulliam, 2012-2015
## Some Rights Reserved
## CC BY-NC 3.0 (http://creativecommons.org/licenses/by-nc/3.0/)

## This exercise uses a simulation model to explore the dynamics and control of an
## imaginary disease in two populations. The goals of this tutorial are to (1) help
## you develop an intuition for how transmission patterns influence the outcome of
## interventions, (2) give you practice in making arguments based on indirect
## evidence, and (3) introduce the concept of a “model world.” Some of the model output
## has been generated previously, and the code for this model and its output is
## available upon request. You may want to look at this code after you have completed
## the exercise, but for now you should focus on the model's output. Begin by loading
## the saved information into your workspace:

rm(list=ls())                   # Clear all variables and functions from the workspace
load("dynamicalFeverData.Rdata")               # Load model functions and output

##
## PART 1: Epidemic dynamics
##
## A disease called Dynamical Fever (DF) causes near-annual outbreaks in dogs and people in
## Daidd County. Typical outbreaks produce similar patterns of cases in the two
## populations, and the outbreaks always occur simultaneously. DF became a reportable
## disease in Daidd County in 2008, and the Department of Health has enlisted your help
## to understand the data they have collected since that time.
##
## In 2007 and 2008 the outbreaks looked pretty similar. First, let's look at how the epidemics
## in dogs and people progressed in 2008:

par(mar=c(5,5,2,1),mfrow=c(2,2)) # Set up plot
plot.cases(data.2008)            # Plot epidemic curve for 2008

## The x-axis is time (week of the year), and the y-axis is the number of new cases observed.
## Overall, there were 759 canine cases and 786 human cases over a period of 16 weeks. In 2009,
## there were 789 canine cases and 804 human cases, and the outbreak lasted 14 weeks. You can
## see that the overall pattern of these outbreaks was similar by comparing the epidemic curves:

plot.cases(data.2009)            # Plot epidemic curve for 2009

## In 2010, there was only one case in dogs and one in humans, but in 2011, another large
## outbreak occurred, with a total of 796 canine cases and 821 human cases over a period of 20 weeks.

plot.cases(data.2010)            # Plot epidemic curve for 2010
plot.cases(data.2011)            # Plot epidemic curve for 2011

## Based on the data from 2008-2011, what have you learned about DF?  What factors might determine the
## differences in epidemic size and duration from year to year? Is it possible to draw any conclusions
## about the natural history of the disease? Why might the epidemic die out each spring?
##
## PART 2: Introduction of a veterinary vaccine
##
## A veterinary vaccine against DF had been approved for use in dogs in mid-2010, but since there was
## not a problem that year, the Health Department did not promote it's use. Following the the disease's
## resurgence in 2011, however, information on the vaccine was sent to veterinarians and pet owners
## in an effort to encourage vaccine uptake. By the beginning of 2012, 40 percent of the dog population
## in Daidd County had been vaccinated.
##
## In 2012, the number of DF cases was substantially lower than in any year other than 2010, with only 34
## cases in dogs and 64 human cases, occurring over a period of 13 weeks:

plot.cases(data.2012)            # Plot epidemic curve for 2012

## Based on this information, opinion among Health Department employees was divided on the success of the
## vaccine. Some employees argued that the vaccine had effectively reduced the number of DF cases, and that
## the reduction in canine cases due to the vaccine might have indirectly protected people from contracting
## the disease as well. Other employees pointed out that there were more cases in 2012 than in 2010 (before
## the vaccine had been approved), and argued that vaccination of only 40 percent of dogs could not explain
## a 20-fold reduction in the number of canine cases (from an average of ~780 cases in previous years with
## outbreaks). This group concluded that the relatively small number of cases in 2012 could be unrelated to
## adoption of the vaccine.
##
## Not liking to rock the boat, the Health Department's Director ultimately decided to continue the vaccine
## information campaign for another year and then reassess the situation. Veterinarians and pet owners were
## encouraged to vaccinate dogs and reminded that the vaccine needs to be renewed annually. The community's
## response to the vaccine was generally positive, with anecdotal evidence suggesting no vaccinated dogs had
## gotten sick in 2012, and by the beginning of 2013, 50 percent of the dog population in Daidd County had
## been vaccinated.
##
## As it turned out, the 2013 data did not clarify things for the Director, and the debate within the department
## only grew more heated, with each side claiming that the 2013 data supported their argument:

plot.cases(data.2013)            # Plot epidemic curve for 2013

## In the first 17 weeks of 2013, 109 dogs and 213 people contracted DF.
##
## What arguments might the groups on each side of the debate make about these data? What additional information
## (other than more years of data) would be useful to help determine to what extent the vaccine is responsible
## for the differences in the outbreaks observed before and after its introduction?
##
## PART 3 - Introduction of a human vaccine
##
## Luckily for the Director of the Health Department, the approval of a human vaccine against DF in mid-2013
## meant that he did not have to take a strong stance on the debate over dog vaccination. Instead, he simply
## redirected funds from the earlier information campaign on dog vaccination to promote human vaccination.
## By the beginning of 2014, 50 percent of the people in Daidd County had been vaccinated, though dog vaccine
## uptake fell to 20 percent.
##
## Although the 2014 season got off to a slow start, the Director was in for a surprise when the outbreak
## picked up later in the season. In the end, the 2014 outbreak was the worst one for years, with 480 canine
## cases and 315 human cases over a period of 29 weeks:

plot.cases(data.2014)            # Plot epidemic curve for 2014

## By mid-June, the Director had had enough and decided step down in order to spend more time with his family
## and their dogs. His former Deputy was quickly appointed to be the Health Department's Acting Director, and
## she worked doggedly over the next few months to ensure that as many people in the community were vaccinated in
## time for the 2015 season as possible. By the beginning of the year, 80 percent of the people in Daidd County
## had been vaccinated. In the meantime, a contamination scare temporarily disrupted the availablility of the
## dog vaccine. The Acting Director had never been convinced that the dog vaccine had caused the reduction in
## dog or human DF cases following its introduction, believing instead that a new, less transmissible strain
## had probably been introduced around the same time as the canine vaccine. She was therefore surprised to
## see so many canine cases in Daidd County in 2015:

plot.cases(data.2015)            # Plot epidemic curve for 2015

## In fact, there were more cases of DF in dogs in 2015 than in any year on record, with 812 canine cases,
## though there were only 149 human cases. As a dog lover, the AD is distraught, and it is at this point that
## she decides she needs outside expertise and brings you in as a consultant.
##
## Having reviewed all of the data available to you, what potential DF transmission patterns could explain all
## of the observed data? What would you advise the AD to do in order to prepare for the 2016 DF season?
##
## PART 4 - Moving forward
##
## Decide on target levels of vaccination for dogs and people in 2016, keeping in mind that it is unlikely that
## you will be able to acheive 100 percent vaccination of either population. Enter these values below, replacing
## XXX and YYY respectively with a number between 0 and 100.

VaxPct.Dogs <- XXX              # Target vaccination level for DOG population
VaxPct.Humans <- YYY            # Target vaccination level for HUMAN population

## We'll now run the model once to see an example of what might happen if these levels of vaccination were
## acheived in 2016:

target.2016 <- run.example(VaxPct.Dogs,VaxPct.Humans)

## Plot the cases through time to see what happened:

plot.cases(target.2016)

## Is this what you expected to happen? You can run the line below as many times as you like to get a feeling for
## whether the outcome above is typical of what would be expected when these levels of vaccination are achieved

plot.cases(run.example(VaxPct.Dogs,VaxPct.Humans))

## The line below will store the results from 1000 runs of the simulation with the target vaccination levels:

target.runs <- replicate(1000,total.cases(run.example(VaxPct.Dogs,VaxPct.Humans)))

## These results can now be plotted to give you a better feeling for the variation in outcomes under an intervention
## acheiving the targetted levels of vaccination in each population:

hist(target.runs["Dogs",], col="dark grey",
     main="Dogs",
     xlab="Number of canine cases",
     ylab="Number of runs")

hist(target.runs["People",], col="dark grey",
     main="People",
     xlab="Number of human cases",
     ylab="Number of runs")

plot(target.runs["Dogs",],target.runs["People",],
     main="For each of 1000 runs",
     xlab="Number of canine cases",
     ylab="Number of human cases")

## Do these plots for your chosen target vaccination levels give you any additional insight into the processes
## underlying DF transmission? If not, try lowering your target vaccination levels for at least one of the
## populations and repeating this section. What is each of these plots showing, and do the results surprise you?
##
## PART 5 - Vaccination outcomes
##
## The following plots show what happens when only one population (dogs or people) is vaccinated, with different
## levels of coverage. The red dots represent the mean number of cases over 100 runs of the simulation, and the
## boxes show the middle 50 percent of runs. The thick black horizontal line is the median outbreak size for
## each intervention.

par(mfcol=c(2,1))
boxplot(CasesByVax1,boxwex=.5,pch=16,cex=.5,names=seq(0,100,10),
        main="Dogs",cex.main=1.5,
        xlab="Percent vaccinated",cex.lab=1.2,
        ylab="Number of cases")
points(1:11,CasesByVax1.mean,col="red",cex=.7,pch=16)
boxplot(CasesByVax2,boxwex=.5,pch=16,cex=.5,names=seq(0,100,10),
        main="People",cex.main=1.5,
        xlab="Percent vaccinated",cex.lab=1.2,
        ylab="Number of cases")
points(1:11,CasesByVax2.mean,col="red",cex=.7,pch=16)

## Why does vaccinating 50 percent of dogs appear to eliminate cases in dogs when vaccinating 50 percent of
## people only reduces the number of human cases by about 50 percent? What do you think vaccinating 50 percent of
## dogs would do to the number of human cases, on average? What about the effect of vaccinating 50 percent of people
## on the number of dog cases?