Epidemic size, duration and vaccine stockpiling following a large-scale attack with smallpox

On August 16th, 2018, we held Exercise Mataika to test preparedness for a worst-case scenario of a smallpox attack which begins in the Pacific and is followed by a larger scale attack in a highly populous Asian country. The exercise was underpinned by mathematical modelling which aimed to determine the duration and magnitude of the epidemic under different scenarios, the critical threshold for epidemic control, and scenarios where the current stockpile of vaccine is adequate. We constructed a modified SEIR model for smallpox transmission. We found that time to commencing the response, rates of contact tracing and ring vaccination, and rates of case isolation are all influential factors on epidemic size and duration. Ideally, the response should commence in 20 days after the attack, corresponding to 8 days after the first symptoms appear, given an average incubation period of 12 days. Every day of delay worsens the epidemic. The WHO stockpile of vaccine of 34 million doses is adequate if rates of case isolation and ring vaccination are maintained above 60%. If rates of contact tracing, ring vaccination and case isolation fall below 53%, epidemic control is lost. In such a scenario, the epidemic persists for longer than 10 years and over a billion doses of vaccine are needed for epidemic control. There are modifiable factors which can prevent a catastrophic scenario following smallpox re-emergence. These include very rapid response time and high rates of isolation and ring vaccination. Training and capacity building, as well as pre-vaccinated teams, can also assist with rapid response. In low income countries, a smallpox epidemic could overwhelm the health system and far exceed human resource capacity, so low rates of case isolation and contact tracing is a realistic possibility. The consequences of poor epidemic control are catastrophic if rates of case isolation and ring vaccination fall below 53%, the threshold for epidemic control. Global cooperation is also critical, to ensure that vaccine and other resources are directed quickly to affected areas.