The bill, sponsored by Senate Majority Leader Bill Frist, R-Tenn., is based on a series of recommendations made by the nonpartisan Advisory Commission on Childhood Vaccines and is backed by more than two dozen children's health groups.

It would, among other things, authorize the Secretary of Health and Human Services (news - web sites) to acquire and maintain a six-month stockpile of vaccines for children and adults to address periodic shortages.

It would also make several changes to the Vaccine Injury Compensation Fund (VICP), a no-fault program that provides up to $250,000 for injuries resulting from side effects from childhood vaccines required for school attendance. It would raise the available award level to $350,000, and it would allow injuries that occurred up to six years ago to be eligible for compensation, up from three years in current law.

The bill would also make it more difficult, however, for families of children injured by vaccines to sue vaccine makers. One key provision of the bill -- a requirement that those alleging harm by vaccine additives such as the mercury-based additive thimerosal first use the compensation program -- was included in a bill last year creating the Department of Homeland Security.

The outcry by parents of autistic children who claim that thimerosal caused or contributed to their children's problems was such that the provision was repealed in a separate bill in February.

It is the question of how to deal with those families that has held up the broader vaccine bill ever since. While extending the compensation program's "reach back" from three years to six years would have allowed most of the families who have sued over thimerosal to seek compensation, it would not have covered everyone.

Sen. Christopher Dodd, D-Conn., who led negotiations for the Democrats on the bill, said that while it has not been determined whether thimerosal actually causes autism or other neurological disorders, he wanted to be sure that if it is proved in the future, families of injured children will be able to get compensation.

"You ought not be excluded from this program because we didn't get the science right," he said.

Democratic aides said the deal that was reached would have allowed families with claims more than six years old to seek compensation through the no-fault program. However, those families, unlike families with more recent claims, would not have been able to go to court if they were unsatisfied with the result of the no-fault process.

They also said that some vaccine makers, who they declined to name publicly, were unhappy with the compromise and that caused the vote on the bill to be canceled.

Republicans, however, said the vote was canceled because not enough senators showed up at the session to produce the required quorum. And Frist denied that a deal had been reached.

"We got very, very close," he told reporters Wednesday afternoon.