Last Updated

09 Dec 2002

Source: Reuters News Service, December 12, 2001.

Scientists Focus on How Smallpox Could Spread

By Patricia Reaney

LONDON (Reuters) - If smallpox were released in a biowarfare attack, the viral disease would spread rapidly with one person infecting from six to 12 others, scientists said Wednesday.

Since the Sept. 11 attacks on the World Trade Center and the Pentagon and deaths from anthrax-laced letters in the United States, fears about the use of biological weapons, including smallpox, have been rekindled.

But estimates of the rate of transmission if an outbreak occurred have varied from 1.5 to more than 20 new infections from each individual with the disease.

Researchers at the Center for Applied Microbiology and Research in England used data from previous outbreaks of smallpox to estimate the transmission rates in a completely susceptible population in the absence of public health intervention.

"Significant epidemics could result, particularly if there were delays in detecting the first cases or in setting up effective public health interventions,'' Raymond Gani and Steve Leach said in a report in the science journal Nature.

Smallpox, which was declared eradicated worldwide more than 20 years ago, is a feared biowarfare agent because it is a highly virulent virus that has killed millions of people over the years.

Gani and Leach used data, including the size of the population affected and case fatality rate, from outbreaks of smallpox in Europe from the 1700s to the 1900s.

During those outbreaks one infected person spread the disease to up to six other people before controls were implemented. In overcrowded and poor conditions in 18th century London the infection rate rose to 10-12 people.

The new estimates are at the higher end of earlier transmission rates because of increased susceptibility.

"Vaccination ceased quite reasonably with the eradication of smallpox. Susceptibility of populations to the virus will therefore have increased subsequently,'' Leach said in an e-mail interview.

People vaccinated in the 1950s and 1960s will have some protection from infection and from the more serious consequences of the disease.

"However, vaccine protection might be expected to decline with time since last vaccination even in vaccinees,'' Leach added.

The United States and Russia have the only authorized research samples of smallpox, which are held under tight security. Scientists fear countries such as North Korea and Iraq might also have stocks of the virus.

Smallpox produces flu-like symptoms and a distinctive and disfiguring rash. It has an incubation period of around 12 days. Symptoms include tiredness, chills, fatigue and fever followed by the pustules that erupt mainly on the face and limbs.