Emergency response to a smallpox attack: the case for mass vaccination

Abstract

In the event of a smallpox bioterrorist attack in a large U.S. city, the interim response policy is to isolate symptomatic cases, trace and vaccinate their contacts, quarantine febrile contacts, but vaccinate more broadly if the outbreak cannot be contained by these measures. We embed this traced vaccination policy in a smallpox disease transmission model to estimate the number of cases and deaths that would result from an attack in a large urban area. Comparing the results to mass vaccination from the moment an attack is recognized, we find that mass vaccination results in both far fewer deaths and much faster epidemic eradication over a wide range of disease and intervention policy parameters, including those believed most likely, and that mass vaccination similarly outperforms the existing policy of starting with traced vaccination and switching to mass vaccination only if required.

Fig 1.

Population dynamics of a smallpox attack with the basic reproductive ratio R₀ = 3. The number of infected people, regardless of disease stage, and the number of people, whether susceptible or asymptomatically infected, waiting in the vaccination queue during the aftermath of a smallpox attack. The (A) TV policy and the (B) MV policy are both initiated 5 days after an initial attack of size 1,000 in a population of 10⁷.

Fig 2.

Sensitivity analysis for uncertain model parameters. The number of deaths under TV and MV versus (A) the fraction of the population initially infected, (B) the basic reproductive ratio R₀, and (C) the number of vaccinators per capita, keeping the total population size fixed at 10⁷. The number of deaths under TV versus (D) the fraction of infectees named by an index.

Fig 3.

Breakeven curves for the number of deaths under TV versus MV, as a function of the basic reproductive ratio R₀ and the number of people initially infected I (0). (A) An index names half of his or her contacts, and (B) an index successfully names all contacts. All other parameters are fixed at their base level (Table 1).