The October 2001 anthrax attack on the United States could have been much worse, according to researchers at the Bloomberg School of Public Health. A statistical analysis of the attack shows that twice as many people could have contracted the deadly form of inhalational anthrax if they had not received antibiotic treatment. The study, which appears in the March 8 issue of Science, is the first to estimate the number of anthrax cases that may have been prevented by antibiotic prophylaxis and is a valuable tool, the researchers say, for assessing the risks and benefits of anthrax treatments. The analysis also emphasizes the need for rapid detection of disease outbreaks.

To date, the Centers for Disease Control and Prevention have confirmed 11 cases of inhalational anthrax, and of those people, five have died. It is believed that nine of the people contracted inhalational anthrax through contact with contaminated mail and mail-handling facilities in Florida, New Jersey and Washington, D.C. The two remaining anthrax cases were reported in Connecticut and New York, but the origin of their infections has never been determined.

To prevent anthrax infection, public health authorities recommended a 60-day antibiotic regimen for 10,000 people who may have come in contact with the spore-forming bacteria. Approximately 5,000 of these patients were either postal workers in New Jersey or Washington, D.C., or employees of the targeted media company in Florida, and they were the focus of this study.

Using this data, Ron Brookmeyer, professor of biostatistics at the Bloomberg School, and Natalie Blades, a graduate student, created a statistical model to trace the rate of anthrax infection. They calculated that twice as many people could have contracted inhalational anthrax without receiving the 60-day antibiotic treatment.

However, "the numbers need to be put into perspective," Brookmeyer explains. "The antibiotic treatment clearly saved lives, but we are still talking about a very small number of people who might have contracted inhalational anthrax without antibiotics--maybe 17 instead of eight."

After accounting for various incubation periods for anthrax due to sensitivity to the spores, the age of those exposed and other factors, the researchers concluded that no more than 50 people, or less than 1 percent of the thousands who received antibiotic therapy, would have developed anthrax without antibiotic treatment.

"These findings are not meant to suggest that the prophylaxis antibiotic regimen should not be used, but they do suggest the risk of contracting anthrax after completing the 60-day course of antibiotics is very small," Brookmeyer says. "This level of risk should be considered when determining if someone should be treated for exposure beyond the 60-day antibiotic regimen, such as with the anthrax vaccine."

In addition, Brookmeyer says, "This study really underscores the need for better disease surveillance and the need to rapidly detect outbreaks and diagnose people exposed to anthrax and other deadly biological agents. It demonstrates that earlier intervention and treatment can save lives and must be a critical element of any biodefense strategy."