Impact of re-emergent smallpox

The challenge

Smallpox virus was eradicated in 1980 but remains a category A bioterrorism agent. Smallpox vaccine is no longer routinely administered, and so due to a reduction in immunity to the virus, and general immunological changes in the population since smallpox was present, the fatality rates of an epidemic today could be catastrophic.

The project

The main determinants of severity of a smallpox epidemic are past vaccination and immunosuppression. Advances in medical care and emergence of HIV since the eradication of smallpox have resulted in unprecedented levels of immunosuppression, thus increasing the population’s vulnerability to epidemics. This project used mathematic modelling to estimate the impact of population immunosuppression and residual vaccine immunity on re-emergent smallpox.

The method

A deterministic SEIR model of smallpox transmission was built for New York City and Sydney, two cities with different past smallpox vaccination policies which impacted immunosuppression and residual vaccine immunity in each population. Sensitivity analyses were conducted on immunosuppression and the reduction of immunity in today’s populations. Should an epidemic re-emerge, age specific rates of infection and death from smallpox were predicted and compared by immunosuppression and past vaccination.

The results

At least 19% of New York’s and 17% of Sydney’s population are immunosuppressed. Should a smallpox epidemic re-emerge, the highest infection rates would be in persons 0–19 years of age, but the highest death rates would be in those >45 years of age. Because of the low level of residual vaccine immunity, immunosuppression was more influential than vaccination on death and infection rates in our model. Despite widespread smallpox vaccination until 1980 in New York, smallpox outbreak severity appeared worse in New York than in Sydney.

The impact

With each passing year, population immunosuppression is a more influential determinant than residual vaccine immunity of the severity of a smallpox epidemic. Immunosuppression is highly prevalent and should be considered in future smallpox outbreak models in order to effectively estimate potential death and infection rates.