An epidemic of smallpox could sweep rapidly through the modern world if the virus ever got out, a British study warns. Although smallpox is not as infectious as diseases such as chickenpox or measles, there could be “significant” epidemics.
In the past, the spread of smallpox was held back by vaccination and people’s natural immunity. But since the disease was eradicated in 1979, vaccination has ceased and levels of immunity have dropped dramatically.
While all remaining stocks of smallpox are supposed to be locked up safely in two labs in the US and Russia, the anthrax attacks in the US have fuelled fears that terrorists might get hold of the virus.
So Steve Leach at the Centre for Applied Microbiology and Research at Porton Down near Salisbury decided to estimate the transmission rate of smallpox in Western countries using historical records from Europe and North America.
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Crowded conditions
His team found that before 1900, every infected person in an unvaccinated population infected about five others, on average. In crowded conditions, such as 18th-century London or in hospitals, the figure rose to between 10 and 12. This was the rate during the isolated European outbreaks of the early 1970s.
Today, the secondary infection rate would be between four and six in the community and up to 12 including hospital-acquired infections, they estimate.
Quarantine and vaccination would halt the virus’s spread, but epidemics would still happen if outbreaks were not detected quickly. Although most people recover from smallpox, up to a third die.
Journal reference: Nature (vol 414, p 748)
