WASHINGTON (Reuters)—U.S. Supreme Court justices admitted Tuesday that they were confused by a 1986 law that seeks to make sure that vaccine makers do not exit the business for fear of lawsuits while ensuring that children hurt by vaccines are compensated.

The justices' questions focused not on the facts of the case, but on the convoluted wording of the National Childhood Vaccine Injury Act of 1986.

What is not at issue is that 18-year-old Hannah Bruesewitz is disabled, requiring special care for life.

Nor is the question of whether the diphtheria-tetanus-pertussis, or DTP vaccine, she was injected with as an infant caused her seizures and consequent brain damage, or whether vaccine maker Wyeth, now part of Pfizer, is to blame.

The question is whether Hannah's parents, Russell and Robalee Bruesewitz, can sue.

They tried to take legal action under the vaccine injury law, a no-fault system designed to protect vaccine makers and families alike. It is funded by a tax on vaccines. Parents do not have to prove a vaccine caused their child's injury, but they do usually have to show that the child had an injury that could be caused by the vaccine.

But the court ruled against the Bruesewitz family and they turned to a state court, which said they are not allowed to do that under the 1986 law.

The Supreme Court is not expected to rule until next year, and their questions show the justices struggled to understand the meaning of the law and of the intended lawsuit.

"What they—the language that they used is certainly, to say the least, confusing," said Justice Ruth Bader Ginsburg.

The Bruesewitzs argue that Wyeth knew the vaccine was old-fashioned and dangerous. The vaccine was taken off the market in 1998 and replaced.

Flood of suits

Pfizer, other vaccine makers and doctors groups argue that if the Supreme Court decides families like the Bruesewitzs can sue outside the vaccine court, a flood of lawsuits would follow—many by families who believe that vaccines cause autism.

Justice Sonia Sotomayor wondered what regulations require companies to determine if their vaccine is the safest one possible, or for the Food and Drug Administration to force them to comply.

"Because I don't see why they should stop until they have caused as many injuries as they need to before the FDA says stop," she said.

Justice Anthony Kennedy agreed. "The warning doesn't have to say, "Warning: We could make something better if we wanted to," he said.

But Kathleen Sullivan, representing Pfizer, said the FDA avoids that by working with companies and persuading them to pull risky vaccines off the market voluntarily.

David Frederick, lawyer for the Bruesewitz family, argued that other vaccines were available but Wyeth did not want to spend the time or money to replace its older version.

Ms. Sullivan denied this and said the FDA had not approved a newer version for infants when Hannah got her shot in 1992.

"You are shadowboxing against an infinite number of theories about how there could have been a safer vaccine," she argued.

The American Academy of Pediatrics, American Medical Assn. and other groups said the vaccine court was set up after a slew of lawsuits overwhelmed vaccine makers in the 1980s.

"A genuine threat to the public health emerged as manufacturers abandoned or considered abandoning the vaccine market," they said in a friend-of-the-court brief. They said the U.S. vaccine supply is still tenuous because so few companies make vaccines.