A Penn State professor made new predictions recently about a possible smallpox outbreak based on a study concerning the spread of measles.

"Our understanding of the networks of transmission will allow us to think more carefully about the spread of biowarfare agents," said Ottar N. Bjornstad, assistant professor of entomology and biology.

Bjornstad's findings, published in the Dec. 13 issue of Nature, describe how measles epidemics spread from densely populated areas to more rural areas in waves. The speed of this spread depends in part on the percentage of susceptible individuals in the overall population.

"Today, smallpox would be spread at a horrific rate because the vaccinated population is very small," Bjornstad said, adding that total resistance is not necessary because it is statistically safe for a percentage to be immunized to slow the spread, depending on the contiguousness of the disease.

"You only need to bring the contact rate low enough that, on average, an infected person does not replace themselves before they die," Bjornstad said.

While this percentage is 97 percent for measles, it is only 80 percent for the less-contagious smallpox. Bjornstad said vaccinating 80 percent of the U.S. population is possible with the current infrastructure and number of vaccines.

Because epidemics spread outward from cities, Amy Griffin (graduate-geography) said people in large cities should be immunized first.

"This may not be politically viable," Griffin said. "Everyone would want it immediately."

Bjornstad said the U.S. Centers for Disease Control and Prevention is constantly increasing the supply of vaccines.

"They are trying to get as many (vaccines) as there are people," Bjornstad said. He said a "pulse vaccination," where many are immunized in one day, is the best way to stop disease spread.

Due to the risk of side effects, Bjornstad said mass vaccination strategies cannot be implemented unless there is very clear evidence of an imminent threat. In spite of confirmed cases of anthrax, Bjornstad said a smallpox attack is unlikely.

"Smallpox is much, much harder (than anthrax) to get ahold of and keep alive outside of a human body," Bjornstad said. "I think the probability that we are facing any of these biomultiplying agents is negligible."

He said even though the terrorists could be somewhat successful if they infect themselves and breathe on people in the subway, early warning signs will allow authorities to stop the spread before it becomes an outbreak.

"What we can learn from the flu is that, unless you manage to catch it early and contain it, there is really nothing you can do," Bjornstad said.

Griffin said that after the measles vaccine was introduced in 1967, outbreaks became smaller, more erratic and more difficult to predict. While the disease continued to spread from the cities outward, the total extent was smaller.

Before vaccines, measles spread about 200 miles around cities in two months. After 1967, the disease traveled about 40 to 50 miles in the same amount of time.

Bjornstad and his collaborators, Bryan T. Grenfell and Jens Kappey of the University of Cambridge, reviewed records of measles outbreaks in Great Britain that were recorded weekly beginning in 1944.

Though many countries require the reporting of certain diseases, including the United States, Bjornstad said he chose Great Britain because the data is recorded by city and week.

The CDC data is by state and month, which Bjornstad said is too crude for the study because the recovery time for measles is about two weeks.

Besides smallpox, the childhood illnesses including mumps, rubella, chicken pox, and whopping cough are also spread in a similar fashion to measles.