Smallpox, big problem?

Smallpox would spread rapidly through an unprotected world.

If smallpox were to return, the disease would spread as fast as it did before vaccination, a new model suggests¹. Swift detection and rapid intervention would be essential to control an outbreak.

The end of vaccination and the loss of natural immunity mean the effects of a smallpox epidemic could be devastating. This is despite the fact that about half the people in the West have already been vaccinated against the virus.

„The predicted scale of an epidemic would increase disproportionately over time, especially in the earlier stages,“ says Steve Leach at the Centre for Applied Microbiology and Research in Porton Down, UK.

The results give a gloomier prognosis than another recent smallpox model by researchers at the US Centers for Disease Control and Prevention in Atlanta². This concluded that the United States has enough vaccine to control a smallpox outbreak.

Gone but not forgotten

Fear of bioterrorism has put smallpox – officially eradicated in 1979 – back at the top of public health agendas. Leach and his colleague Raymond Gani scoured historical records of smallpox outbreaks to estimate how fast the disease would spread in today’s population.

They analysed smallpox epidemics before vaccination, such as in Boston, Massachusetts, in 1721, and after vaccination, such as in London in the twentieth century. They also estimated the degree of natural immunity due to previous exposure to the virus.

For each outbreak they estimated the average number of secondary infections that each smallpox case caused.

They found that each case of smallpox will probably lead to between four and six cases – but as many as 10-12 at the beginning of an outbreak before the disease is detected. Leach and Gani’s numbers could be especially useful to disaster planners in these initial stages.

These figures are similar to transmission rates in the English town of Burford in 1758, where the population was largely susceptible to the disease. „They had not experienced a smallpox outbreak for some considerable time,“ says Leach. About 14% of the population of Burford died from smallpox that year.

It’s good to put firm figures on the rate at which smallpox would spread, says Jaques Valleron, an epidemiologist at the St Antoine Institute for Health Research in Paris.

But, he argues, the quality of disease surveillance and the distribution of vaccine stocks would be far more important to controlling an outbreak than theoretical knowledge. „If there were a biowar with smallpox, the main problems would be largely practical,“ Valleron says.

The recent CDC study, unlike Leach’s, took logistical factors and response rates into account. But the CDC’s rosier picture might result from its having ignored the natural immunity that protected past populations, says Leach. This might lead us to underestimate the measures needed to control an epidemic, he says.

References

1. Gani, R. & Leach, S. Transmission potential of smallpox in contemporary populations. Nature, 414, 748 – 751, (2001).
2. Meltzer, M. I., Damon, I., LeDuc, J. W. & Millar, J. D. Modelling potential responses to smallpox as a bioterrorist weapon. Emerging Infectious Diseases, 7, 959 – 969, (2001).