(Case) Smallpox Attack | CDC Econ Eval Tutorials

Samuel A. Bozzette, M.D., Ph.D.; Rob Boer, Ph.D.; Vibha Bhatnagar, M.D., M.P.H.; et al; "A Model for Smallpox-Vaccination Policy": N Engl J Med 2003;348:416–25.

The Context

The concern that a rogue nation or a terrorist group might mount an attack with the use of a biologic weapon has motivated multiple studies on the consequences of bioterrorism attacks and optimal response strategies.

We develop the case study based on a published paper to illustrate the outcomes associated with two possible smallpox related alternative scenarios, which are listed below.

Scenario 1: Airport Attack

Scenario 2: Hoax

Methods

The Model: Data and Assumptions

A mathematical model is constructed to describe the smallpox outbreaks in the population of interest, based on the probabilistic development of infection among persons initially infected. The preparedness and response strategies below were modeled.

ST 1	Vaccination of contacts and isolation
ST 2	Postattack vaccination of health-care workers
ST 3	Postattack vaccination of health-care workers and public
ST 4	Previous vaccination of health-care workers
ST 5	Previous vaccination of health-care workers and postattack vaccination of public
ST 6	Previous vaccination of health-care workers and public

The following consecutive stages of smallpox are experienced by each person:

incubation,
fever,
rash,
scab formation, and
recovery.

The duration of each stage, the rate of death, and the rate of transmission are determined probabilistically.

The mean durations of the stages of smallpox infection are derived from the literature. The death rates for the unvaccinated persons, persons vaccinated in the remote past, and persons infected after recent vaccination are determined based on the natural history of smallpox.

The vaccine effectiveness among persons vaccinated once (as would occur with vaccination of contacts) and repeatedly vaccinated, as well as rates of serious complications and mortality caused by complications, are identified from the literature.

Assumptions

The Decision Rule

Results

The Number of Vaccinations for Each Strategy

Control strategy *	Hoax	Airport
	(in thousands)
Vaccination of contacts and isolation	0	680
Postattack vaccination of health-care workers	9	9,700
Postattack vaccination of health-care workers and public	0	177,000
Previous vaccination of health-care workers	9,100	9,600
Previous vaccination of health-care workers and postattack vaccination of public	9,100	177,000
Previous vaccination of health-care workers and public	177,000	177,000

Outcomes

The expected number of deaths for each response strategy is presented in the table below.

Expected Deaths as a Result of Smallpox and Vaccination

Control strategy *		Hoax	Airport
ST 1: Vaccination of contacts and isolation
	Deaths from smallpox	0	2,733
	Deaths from vaccination	0	2
	Total number of deaths	0	2,735
ST 2: Postattack vaccination of health-care workers
	Deaths from smallpox	0	2,731
	Deaths from vaccination	0	26
	Total number of deaths	0	2,757
ST 3: Postattack vaccination of health-care workers and public
	Deaths from smallpox	0	2,631
	Deaths from vaccination	0	482
	Total number of deaths	0	3,113
ST 4: Previous vaccination of health-care workers
	Deaths from smallpox	0	2,192
	Deaths from vaccination	25	26
	Total number of deaths	25	2,218
ST 5: Previous vaccination of health-care workers and postattack vaccination of public
	Deaths from smallpox	0	2,114
	Deaths from vaccination	25	482
	Total number of deaths	25	2,596
ST 6: Previous vaccination of health-care workers and public
	Deaths from smallpox	0	641
	Deaths from vaccination	482	482
	Total number of deaths	482	1,123

* Vaccination of contacts and isolation are part of all control strategies.

From the outcomes in the table above, we can draw the following conclusions:

The strategy of vaccinating and isolating contacts (ST 1) results in
- 0 deaths in the hoax scenario, and
- 2,735 deaths in the airport attack scenario.
Control strategies involving vaccination of the public (ST 3, ST 5, and ST 6) result in considerable number of deaths as a result of vaccination complications (e.g., 482 deaths for the airport attack scenario).
Previous vaccination of health-care workers (ST 4) results in 2,218 deaths in the case of the airport attack scenario.
Adopting ST 5 (a strategy that is broader in scope than ST 4 because it includes postattack vaccination of the public) results in 2,596 deaths, which is higher than the number of deaths for ST 4.
However, if the strategy adopted involved previous vaccination of the public (ST 6), the total number of deaths is 1,123 in the airport attack (despite the increase in the number of deaths from vaccination).
Therefore, the preferable strategies resulting in the lowest numbers of deaths are the previous vaccination of health-care workers only (ST 4) and the broader strategy of previous vaccination of health-care workers and the public (ST 5).
The optimal policy, which minimizes the number of deaths, depends on the probability of attack, as shown in the graph below.

Optimal Policy Varies with the Probability of Attack

This graph shows:
- Previous vaccination of health-care workers is expected to save lives if the probability of attack is above 0.05.
- Previous vaccination of the public is expected to save lives at a probability of >0.23. Therefore, previous vaccination of health-care workers is expected to save lives at a lower threshold probability of an attack.

Sensitivity Analysis

Conclusions

Target population:	U.S. population at large
Study design:	Prospective
Study perspective:	Societal