Biologic Worst Case: Smallpox Terrorism

 Exercise Highlights Biologic Devastation - Comparing Dark Winter and Atlantic Storm

Terrorist attacks using biological agents are potentially deadly beyond imagination. How would we respond to a devastating Smallpox attack?

That is the exactly the scenario tested by the Dark Winter exercise (2001) and Atlantic Storm (2005).


Exercise similarities, important differences


Both Dark Winter and Atlantic Storm focused on government leadership and their ability to manage issues in public health, medical capabilities, diplomacy, domestic response, and critical infrastructure. Both exercises were well developed and planned...they did, however, reach different results. Despite commonalities in scenario and biological agent there are striking differences between the two exercises.

In order to compare the two exercises I studied the documents, video and layout of the scenarios themselves. Of course, some study of the Smallpox virus itself was helpful.



Comparison of Assumptions


Dark Winter focused on the United States as the only target in a “worst-case” scenario while Atlantic Storm targeted the international community with “best-case” circumstances. 

Although both scenarios simulated the use of smallpox as the agent with similar methods of dissemination, there were concerning differences in the projected infection rates, death rates, and person-to-person transmission potential. Smallpox deaths in the Dark Winter scenario were projected at thirty percent while Atlantic Storm used a twenty-five percent. Atlantic Storm also assumed that there was residual immunity among the affected population with 300 million doses of vaccine available. Dark Winter was somewhat less optimistic, assuming a stockpile of 15.4 million doses of vaccine would be available from the Centers for Disease Control and Prevention. Reality is present with vaccine assumptions as the scenario accounted for up to twenty percent of stockpile loss due to contamination or improper use.



Dark Winter hypothesized that 1gram of Smallpox could generate 100 infections when aerosolized resulting in 3000 first generation cases from 30 grams of virus. There is no mention of virus quantity in Atlantic Storm, however, both scenarios disseminate the virus via an aerosolizing device under similar conditions. Dark Winter used 1:10 transmission rate (every one person with smallpox could infect ten others) as compared to Atlantic Storms rate of only a 1:3 ratio. Atlantic Storm also anticipated 1: 0.25 for second to third generation while no mention was made in the Dark Winter scenario of second to third generation transmission. Dark Winter planners integrated herd immunity of twenty percent into the scenario which was not accounted for in Atlantic Storm. The lack of herd immunity in the later exercise may be reflect doubt that any immunity exists among the currently vaccinated population.

I found the following excerpt from the Dark Winter scenario an interesting commentary on person-to-person transmission rate.

“…Given the low level of herd immunity to smallpox and the high likelihood of delayed diagnosis and public health intervention, the authors of this exercise used a 1:10 transmission rate for Dark Winter and judged that an exercise that used a lower rate of transmission would be unreasonably optimistic, might result in false planning assumptions, and, therefore, would be irresponsible. The authors of this exercise believe that a 1:10 transmission rate for a smallpox outbreak prior to public-health intervention may, in fact, be a conservative estimate, given that factors that continue to precipitate the emergence and reemergence of naturally occurring infectious diseases (e.g., the globalization of travel and trade, urban crowding, and deteriorating public health infrastructure) [26, 27] can be expected to exacerbate the transmission rate for smallpox in a bioterrorism event…”

Atlantic Storm best-case scenario planned for adequate disease control, compliance with public health “social distancing” (quarantine/isolation), available vaccine, higher herd immunity, residual protection granted by prior vaccination, and lower transmission rates. The wide range of transmission rates between the two exercises may account for the differences in total number of smallpox cases and deaths. Dark Winters worst-case predicted 1,000,000 deaths with 3,000,000 infections while the Atlantic Storm exercise predicted 660,000 cases and approximately 495,000 deaths.


Learn more about Smallpox from the CDC- click here