Study Models Impact of Anthrax Vaccine

Rapidly distributing antibiotics to people exposed to anthrax spores during a bioterrorist attack, could by itself, prevent about 70 percent of anthrax infections from occurring, according to researchers from the Johns Hopkins Bloomberg School of Public Health. To increase the prevention rate to 90 percent, their study found that at least 63 percent of the population would need to be immunized with vaccine before an anthrax attack, which might not be practical. However, the study found that anthrax vaccination given even after an attack could be beneficial in reducing the length of time antibiotic treatment would be needed. These findings could be an important tool for policymakers who must develop effective strategies for containing an anthrax outbreak. The study is published in the December 16, 2004, edition of Nature.

The Hopkins researchers developed a probability model to predict how many cases of anthrax could be prevented under varying conditions. The study found that 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. 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 is 95 percent effective, it could reduce the time that antibiotics would be needed 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 the 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.”

“Public health vaccination policies for containing an anthrax outbreak” was written byRon Brookmeyer, Elizabeth Johnson and Robert Bollinger. Brookmeyer and Johnson are with the Department of Biostatistics at the Johns Hopkins Bloomberg School of Public Health. Bollinger is with the Division of Infectious Diseases at the Johns Hopkins School of Medicine.

The research was funded by the Fogarty International Center and the National Institutes of Allergy and Infectious Diseases.