NEW YORK (Reuters Health) Mar 07 - No clear pattern of illnesses are discernible from an analysis of a US database of persons vaccinated with the anthrax vaccine, but researchers cautioned attendees of the American Academy of Allergy, Asthma and Immunology meeting here this week that a much larger sample needs to be collected before the existence of any vaccine-related syndrome can be ruled out.

Investigation of the vaccine's adverse effects remains "an ongoing process," said Dr. Bryan L. Martin, of the Walter Reed Army Medical Center in Washington, DC. "It's a continuum," he said. "This will continue to become better and better data as we use the vaccine more and more."

Controversy continues to surround the military's use of the anthrax vaccine, which some experts believe to be both ineffective and potentially harmful to recipients. In January, the Department of Defense advised that pregnant servicewomen not receive the vaccine due to inconclusive data that it might be linked to an increased risk for birth defects.

In their report, Dr. Martin, Dr. Renata Engler, and colleagues examined data from the Vaccine Adverse Events Reporting System (VAERS), a nationwide database, sponsored by the US Centers for Disease Control and Prevention and the Food and Drug Administration.

More than 2 million doses of anthrax vaccine have been given to more than 520,000 US military personnel since 1998. An independent review panel found 82 cases of severe "adverse events" resulting in hospitalization or prolonged disability that they say might be caused by the anthrax vaccine.

Adverse events ranged from persistent headache, skin rash, tinnitus and prolonged fatigue. Symptoms were "very diverse," Dr. Engler told Reuters Health. "They are clustered together a little bit, but as you can see, they are very small numbers."

She stressed that a larger sample could uncover a pattern suggestive of an anthrax vaccine-related syndrome. "We can't prove or disprove causality at this point in time," she said, adding that researchers "need to be open" to all possibilities. "If you're talking about local reactions--swollen arms or flu-like symptoms for a few days--that's very common, but its also common with other vaccines."

A larger sample--perhaps around 10 million doses--might be needed before numbers become large enough to draw more definite conclusions, Dr. Engler noted.