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1.4 Is smallpox still a threat?

Since the smallpox virus was declared eradicated ‘in the wild’ in 1980, stocks of virus have been held in secure laboratories in various parts of the world, with the expectation that they would eventually be destroyed. However, since 2001, the perceived threat of bioterrorism has led to debates about whether they should be retained as a vital resource for research into ways to combat a deliberate release of smallpox virus – assuming that samples have been (or could be) obtained by terrorist groups. Mathematical models of how smallpox might spread through a population have been constructed, and the effects of vaccinating people with vaccinia either before or after a theoretical exposure have also been modelled.

Vaccination programmes ended over 20 years ago, so even those individuals who were vaccinated as children are unlikely now to be protected against smallpox (i.e. almost everyone is susceptible), but a decision on whether to reintroduce smallpox vaccination is not straightforward. Serious complications of injecting people with vaccinia can be expected to occur in a minority of individuals – particularly those with immunodeficiency. In the 1960s, smallpox vaccination led to an estimated 1–3 deaths per million doses, but today's population now has very much larger numbers of people whose immune system is suppressed by HIV infection or medical treatment (e.g. for cancer), or following organ transplants. The vaccine contains ‘live’ vaccinia, so even if its recipients are selected to be in good health, they can pass the virus on to others who may be less able to withstand its pathological effects.

So the question of whether the risks of reintroducing smallpox vaccination outweigh the possible benefits depends on estimates of the potential risk of a bioterrorist attack. At the time of writing (2003) the threat is considered to be so low that vaccination has only been reintroduced in the UK and USA for health workers who would be in the ‘front line’ of any response to an outbreak.