NEW YORK (Reuters Health) - Among the more than 36,000 U.S. health care workers who've been vaccinated against smallpox since January, six "probable" and 18 "suspected" cases of heart inflammation have been reported, federal health officials said Thursday.

Centers for Disease Control and Prevention (CDC) investigators are also looking into a suspected case of vaccine-related brain swelling in a 38-year-old man.

These latest numbers are part of an update from the CDC's ongoing monitoring of smallpox vaccine side effects among civilians who've been vaccinated since the start of the U.S. program.

The federal government hoped to vaccinate 500,000 health care workers who will be on the front lines in the event of a bioterror attack with the often-deadly smallpox virus. However, the CDC recently revised its target number of vaccinees, saying that if 50,000 workers are immunized, that should be enough for an initial response to a smallpox attack in the U.S.

As of May 9, there were six probable cases of perimyocarditis -- an inflammation of the heart and its surrounding membrane -- among vaccinees, and an additional 18 suspected cases. This includes one case reported between May 3 and 9, the CDC notes in its Morbidity and Mortality Weekly Report.

Due to concerns about reports of heart-related side effects, the CDC warned physicians in March against vaccinating people who have known heart disease or stroke or who have three or more risk factors for heart conditions.

The CDC also reports that a 38-year-old man developed a suspected case of post-vaccine brain swelling in early May.

This side effect, called postvaccinial encephalomyelitis, or PVE, "is a rare adverse event associated with smallpox vaccination," CDC officials write.

However, the patient showed several signs and symptoms "not typical of PVE," and there are other potential causes for the problems, including seizure, which the patient did have.

The man's symptoms, which included intense agitation, confusion and abnormalities on brain scans, may not be related to the smallpox shot, according to the report.

Nonetheless, they encourage state health departments to promptly report any neurological problems following smallpox vaccination.

Between May 3 and 9, one other "serious" adverse event -- a patient who required hospitalization and antibiotic administration -- and 23 "nonserious" events were reported, according to the CDC.

The most common side effects continue to be rash, fever, headache, fatigue and pain, "consistent with mild expected reactions following receipt of smallpox vaccine," the agency says.

SOURCE: Morbidity and Mortality Weekly Report 2003;52:475-477.