household_transmission {SIRmcmc}

Estimate parameters from SIR model

Description

Use the Metropolis algorithm to estimate parameters from the SIR compartmental model

Usage

household_transmission(onset_date, household_index, covariate = NULL,
                       followup_time, iterations,
                       delta = c(0.1,0.3,0.4,0.1), plot_chain = TRUE,
                       index = 1, start_date = NULL, prior = unifprior,
                       constant_hazard = FALSE,...)

Arguments

onset_date

A vector of onset dates. If onset_date is a character vector, then it will be coerced to dates using as.Date. NA values are interpretted as remaining susceptible at the end of followup.

household_index

A vector identifying households, which should be the same length as the onset_date argument.

covariate

A vector identifying covariate patterns. If given, then it is interpretted as a factor. A value for epsilon will be given for each level of covariate. If NULL, then epsilon is not estimated.

followup_time

An integer for the followup time in days.

iterations

An integer for the number of iterations of the Metropolis algorithm.

delta

A vector of length 4 for tuning the acceptance rate of the Metropolis algorithm. The order is (1) alpha, (2) beta, (3) gamma, and (4) epsilon.

plot_chain

A boolean of whether to plot the value of the chain vs the iterate.

index

An integer for the index date. Probabilities are conditional on the index date. Any coordinates of onset_date equal to index will have a likelihood of 1. If you want unconditional probabilities, then index should be less than start_date.

start_date

Should be the same format as onset_date. Specifies the start of the followup period.

prior

A function to compute the prior probability of alpha, beta, gamma, and epsilon. Any user written function must take the arguments alpha, beta, gamma, epsilon, and esteps. Builtin functions are unifprior and prior. unifprior is uniform on (0.01,1000).

constant_hazard

If FALSE, then the algorithm computes a time dependent hazard for the cohort and the hazard from the community is proportional to the hazard in the cohort. If TRUE, then the algorithm assumes a constant hazard from the community.

...

Arguments to be passed to the function in the prior argument.

Details

If no covariates are supplied, only the model parameters alpha, beta, and gamma are estimated using a stepwise Metropolis algorithm. The model parameters are drawn from a uniform distribution on (0.1,1). The first step proposes a new alpha using rnorm and the mixing parameter delta[1]. The second and third steps are similar for beta and gamma. If covariates are supplied, the additional parameters, collectively called epsilon, are also estimated.

The algorithm assumes followup begins on start_date and lasts for followup_time days. Any coordinate of onset_date equal to index does not contribute to the likelihood.

Two priors are builtin: prior and unifprior. User defined prior functions must take the arguments alpha, beta, gamma, epslion, and esteps.

Value

An object of the class SIRmcmc.

See Also

prior unifprior

Examples

##A trivial example-------------------------------------------------------
library(graphics)
onset<-sample(c(seq(1,10),rep(Inf,20)),size=500,replace=TRUE)
hh<-sample(seq(1,300),size=500,replace=TRUE)
chain<-household_transmission(onset_date = onset, household_index = hh,
                              followup_time = 10, iterations = 100)
community_attack_rate(SIRmcmc=chain)
secondary_attack_rate(household_size=3,SIRmcmc=chain)

##An example with household transmission---------------------------------
library(graphics)
data(hh.transmission)
set.seed(1)
iterations<-100
T<-30
delta<-c(0.1,0.6,0.8)
index<-0
##Find the MCMC estimates of alpha, beta, and gamma
chain<-household_transmission(
    onset_date=hh.transmission$onset,
    household_index=hh.transmission$household,
    covariate=NULL,
    followup_time=T,
    iterations=iterations,
    delta=delta,
    prior=unifprior,
    index=index
)
#Tabulate true type of transmission
hh_table<-table(
    table(
        is.finite(MCMC_date(hh.transmission$onset)),
        hh.transmission$household)["TRUE",]
)
##Calculate the true SAR
truth_table<-table(hh.transmission$transmission)
truth<-unname(truth_table["Household"]/sum(hh_table[2:3]))
cat("\n\nTrue Value of SAR\n\n")
print(truth)
##Find point and 95% central creditable intervals for MCMC SAR
cat("\n\nMCMC Estimate of SAR\n\n")
secondary_attack_rate(household_size=2,SIRmcmc=chain)
days<-NULL
for(d in c(seq(1:5))){
    days<-c(days,as.character(d))
    a<-sum(table(tapply(X=hh.transmission$onset,INDEX=hh.transmission$household,FUN=diff))[days])
    cat(
        paste0(
            "\n\n",
            d,
            " Day Counting Estimate of SAR\n\n"
        )
    )
    ##Find point and 95% confidence intervals for normal approx to SAR
    print(
        a/sum(hh_table[2:3])+c(p=0,LB=-1,UB=1) *
        qnorm(p=0.975) *
        sqrt(a*(hh_table[2]+hh_table[3]-a)/(hh_table[2]+hh_table[3])^3)
    )
}


##An example with rate ratios----------------------------------------
## Not run: 
    library(graphics)
    data(hh.transmission.epsilon)
    set.seed(1)
    iterations<-100
    T<-30
    delta<-c(0.1,0.1,0.1,0.1)
    index<-0
    ##Find the MCMC estimates of alpha, beta, gamma, and epsilon
    chain<-household_transmission(
        onset_date=hh.transmission.epsilon$onset,
        household_index=hh.transmission.epsilon$household,
        covariate=hh.transmission.epsilon$epsilon,
        followup_time=T,
        iterations=iterations,
        delta=delta,
        prior=unifprior,
        index=index
    )

    ##Find point and 95% central creditable intervals for MCMC SAR
    cat("\n\nMCMC Estimate of SAR\n\n")
    print(secondary_attack_rate(household_size=2,SIRmcmc=chain))
    for(e in c(1,5)){
        hh_table<-table(table(
            is.finite(MCMC_date(hh.transmission.epsilon$onset)),
            hh.transmission.epsilon$household,
            hh.transmission.epsilon$epsilon)["TRUE",,as.character(e)])
        ##Tabulate true type of transmission
        truth_table<-table(
            hh.transmission.epsilon$transmission[which(
                hh.transmission.epsilon$epsilon==e
            )])
        ##Calculate the true SAR
        truth<-unname(truth_table["Household"]/sum(hh_table[2:3]))
        cat("\n\nTrue Value of SAR\n\n")
        print(truth)
        days<-NULL
        for(d in c(seq(1:5))){
            days<-c(days,as.character(d))
            a<-sum(table(tapply(
                X=hh.transmission.epsilon$onset[which(hh.transmission.epsilon$epsilon==e)],
                INDEX=hh.transmission.epsilon$household[which(hh.transmission.epsilon$epsilon==e)],
                FUN=diff))[days])
            ##Find point and 95% confidence intervals for normal approx to SAR
            cat(paste0(
                "\n\n",
                d,
                " Day Counting Estimate of SAR\n\n"
            ))
            print(
                a/sum(hh_table[2:3])+c(p=0,LB=-1,UB=1)
                * qnorm(p=0.975)
                * sqrt(a*(hh_table[2]+hh_table[3]-a)/(hh_table[2]+hh_table[3])^3)
            )
        }
    }

## End(Not run)