A controversial proposal to study diluted doses of smallpox vaccine for children has been canceled.

The study would have involved 40 children at Cincinnati Children's Hospital Medical Center and the Harbor-UCLA Medical Center. But in late January, the U.S. Food and Drug Administration's Office for Human Research Protections pulled the plug.

Federal and state officials have begun vaccinating emergency and medical first-responders to prepare for the chance of a terrorist using smallpox as a weapon.

But that first wave of vaccinations does not involve children. And by the time children may need the shots, newer, safer vaccines might be ready for testing.

"Bioterrorism preparedness plans have evolved such that, under current plans, the potential to use diluted Dryvax (the brand name for smallpox vaccine) in children will no longer exist," states a Jan. 24 letter from the FDA to the research team.