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By Peter Gorner
Tribune science reporter
Published November 15, 2002

Vaccinating people who come in close contact with smallpox victims may be almost as effective as mass vaccinations in the event of an attack by terrorists, scientists reported Thursday.

Results of a computer model created by researchers at Emory University in Atlanta and published in the journal Science offer the latest hypothesis about the size of a smallpox epidemic, how fast it would spread and how best to contain it.

This study is perhaps the most optimistic of the several published within the last year, differing markedly from computer models that suggested the government's smallpox strategy would not work.

The new study, however, supports the government's plan that targeted vaccinations would be the best response.

The results also reflect the hope that as much as 60 percent of the American public may already have some immunity against smallpox.

"We've simulated what we think might be the most likely attack scenario," said Ira M. Longini Jr., a professor of biostatistics at the Rollins School of Public Health.

"It probably would be a small attack, one to five people infecting themselves and then spreading it across the population.

"If that happened, we found that targeted vaccination is nearly as effective as mass vaccinations."

Increasing the level of immunity in as many individuals as possible before an attack would help, said Elizabeth Halloran, the study's first author.

"We could do that by vaccinating first responders or allowing people to be vaccinated if they so choose. It would make a post-attack effort more effective," she said.

An expert advisory panel assembled by the government last summer recommended against vaccinating the public.

The panel favored selectively vaccinating health care workers and using a "ring vaccination strategy"--immunizing an infected person's immediate contacts--after an outbreak.

That strategy was used to eradicate smallpox globally by 1980 and minimizes exposure to the side effects of the live cowpox virus used in the vaccine, which can be fatal.

The ring vaccination strategy was challenged by Yale University public health professor Edward Kaplan, who co-authored a study published in August in the Proceedings of the National Academy of Sciences suggesting that mass vaccination would be a better alternative to deal with a terrorist attack.

Kaplan's model of an attack on a major city, with an initial 1,000 people infected, projected the ring strategy would allow 367,000 people to be infected and 110,000 to die before eliminating the disease after 350 days.

Mass vaccinations given quickly after an attack would result in 1,830 cases and 560 deaths over 115 days, the study found.

Smallpox experts criticized that model as too nightmarish. They criticized the assumption that a single infected person would come in sufficiently close contact with 50 other people before the case was detected.

Kaplan countered that infectious disease experts were basing their strategy on historical, natural outbreaks, not intentional attacks where bioterrorists were trying to kill as many people as possible.

Creating models for infectious diseases dates to 1760, when mathematician Daniel Bernouli developed a model for smallpox. Since then, modeling has taught science about such things as the importance of core populations in sexually transmitted diseases and the concept of herd immunity created by vaccination policies.

"But they're all over the map when it comes to smallpox," Longini said.

Halloran noted: "This model is the only one to take into account the fact that sometimes the epidemic doesn't take off.

"It was shown that in Europe during the 1950s and 1960s, about 30 percent of the time when somebody got smallpox, nobody else caught it."

It takes some time and close exposure to an infected person to catch smallpox, medical experts say.

"That doesn't mean it can't be spread by casual contact. But the usual pattern is small dynamics, groups of cases every 14 days, close contact," Halloran said.

"I don't think a disease that spreads slowly and requires close contact should cause people to panic."

An earlier computer model by scientists from Porton Down, the British biological and chemical warfare defense laboratories, was designed to estimate the susceptibility of today's population using historical data.

That model showed the disease would spread rapidly with each infected person passing the disease to 6 to 12 others.

The British computers estimated a rate of secondary infections twice as high as a computer model published by the Centers for Disease Control and Prevention in Atlanta.

That model estimated that an outbreak involving 100 people initially exposed in a city the size of Atlanta would find each infected person infecting three others, resulting in 4,200 cases and requiring a year to control.

Such an outbreak would require quarantining at least 1 out of 4 people infected, and vaccinating more than 9 million people, the entire population of Georgia, the CDC said.

All such models are entirely speculative, said Dr. Joseph Barbera, co-director of the George Washington University's Institute for Crisis and Disease Management.

"They don't take into account such modern considerations as air flow in buildings, hand washing and other hygiene improvements, a more informed public and a public health system that theoretically could alert the population and tell them how to protect themselves."

The Emory researchers stressed that mass vaccination remains the most effective defense against any contagious disease.

"But since the current vaccine is relatively dangerous, we'd want to use as few doses as possible," Halloran said.

"Therefore, the strategy we're proposing would be safer to the population, and we still could protect ourselves fairly well."

Roughly 120 million of the 275 million people in the country were born after routine vaccinations stopped and presumably would have no immunity to the smallpox virus.

But if a pool of protection still exists in Americans over 30, the disease might not spread as fast and an epidemic might not be as deadly as some experts fear.

"Studies are under way by immunologists to see how much immunity remains in the population," Halloran said. "We know already that if you've ever been vaccinated, you're less infectious if you do catch smallpox. You won't get as sick and are much less likely to die."