Trials are set to begin on a new and potentially better anthrax vaccine.
Anthrax spores are highly infectious
Scientists in the United States will test the jab on 480 people as part of a government-funded study.
The vaccine has been developed by VaxGen. It is designed to last longer and have fewer side-effects than the existing vaccine.
However, these are early stage trials. Even if the jab proves effective it will be a number of years before it becomes available.
Millions being spent
The US government has given millions of dollars in recent years to companies trying to develop a new anthrax vaccine.
The current vaccine was developed in the 1950s and is really only used to vaccinate military personnel or people who have contact with animals that harbour anthrax.
Officials insist the vaccine is safe. However, it is controversial. Some soldiers claim it is linked to Gulf War Syndrome.
The main problem with the existing vaccine is that a number of injections are required over a few months before it becomes fully effective. Booster shots are also needed every year.
The US anthrax mail attacks, which claimed the lives of five people in 2001, have spurred many companies to try to develop more effective vaccines and new antidotes.
The US government has drawn up plans to stockpile millions of doses of anthrax vaccine.
Anthrax particles or spores are highly infectious and can kill.
A study, published last year, suggested an anthrax bomb or missile in a city the size of New York could kill as many as 123,000 people even if every victim received treatment.
This latest vaccine will be tested on people in 12 cities across the US. Scientists hope it will provide protection much more quickly than the current vaccine.
"The goal is to develop an anthrax vaccine with better protection, an improved safety profile and a simpler dosing schedule," said Dr Geoffrey Gorse of St Louis University, who is heading the trial.
The UK's Health Protection Agency manufactures another anthrax vaccine at its facilities in Porton Down.
A spokesman said it backed new research into possible alternatives.
"The current US jab requires six inoculations as part of the vaccination schedule," he said.
"The British one requires three to four inoculations. A vaccine that required fewer jabs would be of benefit."