The trial will test and compare the reactions in humans to the vaccine manufactured by BioPort Corp. of Lansing, Mich., and another being developed by NIH. Bob Bock, an NIH spokesman, said the trial planned for 100 children in first and second grade will not occur until the vaccines are fully tested on 350 adults and shown to be safe for them.

"The results in this study," says an NIH announcement, "will help in the development of improved vaccines for anthrax." The NIH and U.S. Health and Human Services Department are calling for development of the vaccine to protect civilians from terrorist or other attacks.

Critics, however, are appalled.

"This vaccine is totally inappropriate for children, because the [exposure] threat is so remote," said Barbara Loe Fisher, president of the National Vaccine Information Center. "They will likely never be exposed to anthrax either through contamination by animal products or inhalation of weaponized anthrax."

"Children are involved in trials of vaccines that benefit children," she said, "but this vaccine will not do so." Fisher said based on the NIH announcement of "rare severe reactions" to BioPort's vaccine, she fears the parents of children used in the experiments will not be given proper warnings of the vaccine's potential for adverse reactions.

However, Bock said adults and parents or guardians of the children will be given complete information on the two vaccines and their benefits and risks.

"I don't understand how they can do efficacy tests with children at the same time that we are discovering more and more U.S. soldiers who have been harmed by the vaccine," said Steve Robinson, executive director of the National Gulf War Resource Center for service members and veterans. "[NIH officials] want parents to want their children to be vaccinated against the anthrax terrorist attacks that have not happened."

But Bock said that if terrorist attacks occur, both children and adults would be at risk.

BioPort's vaccine, used almost exclusively on soldiers, has already demonstrated an adverse reaction rate 100 times the figure initially stated on the label. Adverse reactions include immune disorders, muscle and joint pain, headaches, rashes, fatigue, nausea, diarrhea, chills and fever. At least half a dozen deaths and a number of birth defects have been attributed to its use.

BioPort officials did not respond to repeated messages seeking comment Wednesday.

Retired Air Force Col. John Richardson, who has independently researched the vaccine extensively, said that in 2003, there were 16,869 federal adverse reaction reports for all vaccines and of those, 1,068 were for the anthrax vaccine. The anthrax vaccine drew more than 6 percent of all vaccine complaints, said Richardson, even though anthrax vaccinations represented less than 1 percent of an estimated 100 million immunizations of all types administered that year.

For last year, he said, there were 15,488 federal adverse reaction reports for all vaccines, and 806 for anthrax, or 5.2 percent of the total.

Complaints to the reporting system - which even federal officials acknowledge typically represent as few as 10 percent of all adverse reactions to vaccines - can be filed by vaccine users, doctors and medical personnel.

The vaccine, aimed at protecting soldiers against anthrax spores fired into the air in combat, has been under attack by service members and their advocates ever since the Pentagon mandated its use in 1998. Aside from the anthrax spore attacks aimed at government officials 3½ years ago, no such attacks are known to have been used during modern warfare or by terrorists.