(The Atlanta Journal-Constitution) -- With the White House expected to announce shortly how many Americans will be vaccinated against smallpox to protect against a bioterrorist attack, research by four Atlanta professors says that immunity already present in the population will be a key factor in how fast the disease could spread.

In a paper published today in the journal Science, Dr. M. Elizabeth Halloran and colleagues from Emory University's Rollins School of Public Health say immunity could be left over from childhood vaccinations received by those born before 1972 --- 57 percent of the U.S. population. Or it could be newly created by giving vaccine to health care workers and other first responders likely to come in contact with the first victims.

President Bush is reportedly close to ordering the inoculation of hundreds of thousands of U.S. troops against smallpox but will delay acting on vaccinating civilians due to concerns over possible side effects, The Washington Post said today. Bush's top advisers are said to have endorsed the plan, which was recommended by the Pentagon.

If the U.S. population has some degree of immunity, the authors say, a strategy of "ring vaccination" will be adequate to protect the population once an attack begins.

The Centers for Disease Control and Prevention, which will oversee smallpox vaccinations, prefers that strategy over a mass vaccination campaign. Ring vaccination limits protective shots to those who have been in close contact with victims, and the contacts of those contacts, creating a bull's-eye-shaped firebreak of immunity that the smallpox virus cannot cross.

Ring vaccination, which the CDC and the World Health Organization used to eradicate smallpox in the developing world in the 1970s, tends to use fewer doses of vaccine. That would be important, the authors say, because fewer Americans would be exposed to the risks of serious side effects --- from skin disorders to encephalitis to death --- that come with the smallpox vaccine.

Today's finding is the latest in a series of complex computer models that have examined different strategies to protect against smallpox. Other studies, by scientists at the CDC, Yale University and the Massachusetts Institute of Technology, have supported mass vaccination, or ring vaccination if a quarantine of infected areas is included.

The Emory study uses a model that assumes people interact with specific groups in their daily lives --- households, neighborhoods and day care center clients, for instance --- rather than roaming freely through the entire population. Because smallpox usually spreads through face-to-face contact, the authors say the risk of acquiring the disease would be highest in the groups where victims spend the most time, and less in places where they spend short amounts of time or do not go regularly.

"The idea is to put vaccination where transmission of a disease is occurring," said Ira Longini Jr., a co-author.

In a key feature, the research assumes that Americans older than 30 retain some immunity to smallpox due to the vaccinations they received in childhood. How long that immunity lasts has been hotly debated for the past year; some scientists contend that immunity is lifelong, while others say older Americans are as vulnerable to the disease as those who were never vaccinated.

Copyright 2002 The Atlanta Journal-Constitution.