In people with weakened immune systems, there is a chance that the live virus in the smallpox vaccine, rather than immunizing them, can result in an infection that gets progressively worse. People who are known to be HIV-positive would not typically be given a smallpox vaccination because of this potentially fatal risk.
More than 438,000 U.S. soldiers were vaccinated against smallpox between December 2002 and October 2003. Of the 10 later identified as HIV-positive, only three were known to be "primary" vaccine recipients, or had never been vaccinated before. Those who had previously received smallpox vaccinations may have benefited from some "leftover" immunity, but the fact that none of the soldiers had full-blown AIDS at the time of vaccination was probably a key factor in preventing a dangerous reaction.
"Fundamentally, it is because their immune systems were not yet impaired or diminished or compromised, because they were presumably early in their infection," said Col. John Grabenstein, PhD, senior author of the study. "This is one of those cases where the distinction between HIV-positivity and AIDS is apparent."
The study raises an important question: should HIV-positive people receive smallpox vaccinations? If they don't, they could be among the first fatalities in the event of a bioterrorism attack. If they do, they could become ill perhaps even lethally ill from the vaccine itself. "In the absence of any cases of smallpox in the world, vaccination programs should take care to exempt HIV-positive people from vaccination," Col. Grabenstein said. But an actual smallpox outbreak might make the vaccination's benefits outweigh the risks, he added
Contact: Diana Olson
Infectious Diseases Society of America