WASHINGTON (AP) - People exposed to high levels of anthrax may need more than the 60 days of antibiotics now recommended, researchers say.

A team at Johns Hopkins University developed a mathematical analysis of the time needed for anthrax spores to germinate in the lungs and the speed at which they are eliminated by antibiotics.

Their conclusion, published in Monday's online issue of Proceedings of the National Academy of Science, is that in some cases antibiotics should be continued up to four months.

When anthrax in the mail threatened thousands of postal workers and others in 2001 - killing five people - health authorities offered 60 days of antibiotics. After that, exposed people were given the choice of continuing antibiotics, taking a vaccine or ending treatment. Many chose to end treatment.

Last year the Centers for Disease Control and Prevention recommended that in any future outbreak, vaccine be given to exposed people after 60 days of antibiotics.

The new study, led by Ron Brookmeyer, concluded that larger doses of the germs take longer to clear from the lungs and said that, while 60 days of antibiotics is adequate in low exposures, in cases of higher exposure treatment may be needed for up to four months.

"During the 2001 U.S. anthrax attacks, many people failed to take the recommended 60-day course of antibiotics. Full compliance was about 60 per cent or less in some cases. Our model showed that the reason they didn't get sick, even without the antibiotics, was because the anthrax spore exposure levels were very low. If the exposure levels were higher, there would have been more casualties," Brookmeyer said in a statement.

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Proceedings of the National Academy of Science: pnas.org