About 70 percent of anthrax cases could be prevented if antibiotics were distributed within 6 days after exposure and patients were to take them for 60 days.

Effective distribution of antibiotics immediately following an attack could prevent about 70 percent of anthrax infections, according to their study, which is published in the December 16, 2004, edition of Nature.

The researchers found that vaccination would be beneficial but acknowledged limitations to its use. To increase the infection prevention rate to 90 percent following an anthrax attack, at least 63 percent of the population would need to be immunized with vaccine before it occurred. At present, that strategy is not considered practical.

Vaccine Following Attack

Yet use of anthrax vaccine following an attack could reduce the amount of time necessary for a course of antibiotic treatment to be effective, the study found.

The Hopkins researchers developed a probability model to predict how many cases of anthrax could be prevented under varying conditions:

• About 70 percent of anthrax cases could be prevented if antibiotics were distributed within 6 days after exposure and patients were to take them for 60 days.

• Prevention rates dropped below 50 percent if antibiotics were delayed more than 10 days. In 2001, some postal workers did not begin taking antibiotics until 9 days after exposure.

The current licensed anthrax vaccine requires six doses over an 18-month period to provide immunity. The researchers looked at what impact a new and improved anthrax vaccine could have and evaluated a range of vaccine characteristics, which could begin to provide immunity ranging between 7 and 28 days.

Vaccine Effectiveness Considered

When vaccine was distributed along with antibiotics after an attack, the researchers calculated that vaccine prevented few additional cases from occurring when compared to just rapid use of antibiotics. However, they found that if the vaccine were 95 percent effective, it could reduce the time that antibiotics would be needed by up to 43 days.

An 80 percent effective vaccine could reduce time on antibiotics by up to 23 days. The vaccine could also prevent cases if an antibiotic-resistant strain of anthrax were used in an attack.

"Our study highlights the need for rapid distribution of antibiotics to minimize casualties from an anthrax outbreak," explained Ron Brookmeyer, PhD, lead author of the study and professor in the Department of Biostatistics at Hopkins' Bloomberg School of Public Health. "But a long course of antibiotics is not ideal, and a new improved anthrax vaccine could be especially helpful in reducing duration of antibiotic treatments."