ATLANTA (AP) -- For those convinced that vaccines can cause autism, the sad case of a Georgia girl, daughter of a doctor and lawyer, seems like clear-cut evidence. The government has agreed to pay the girl's family for injury caused by vaccines.

But it turns out it's not that simple -- and maybe not even a first.

The 9-year-old girl, Hannah Poling, had an underlying condition that may have been worsened, triggering her autism-like symptoms.

Her parents believe it was the five simultaneous vaccines she got as a toddler eight years ago that did it. Government scientists say something like a fever or infection could have set off the problem -- but they didn't rule out the vaccines either.

This week, government officials acknowledged they have agreed to pay the Polings from a federal fund that compensates people injured by vaccines. The amount is not yet determined.

While parents and advocates for autistic children say the case is a landmark legal precedent that signals the government is finally conceding potential autism-related risks from childhood vaccines, government officials are saying it's nothing of the kind.

"This does not represent anything other than a very special situation," said Dr. Julie Gerberding, head of the U.S. Centers for Disease Control and Prevention.

Her comments came after the Polings, from Athens, Ga., held a news conference Thursday to talk about their daughter, who accompanied them. They said she was a bright child until she got five shots when she was about 18 months old.

Almost immediately after the vaccinations, she was screaming, feverish and irritable. Then, her behavior gradually changed so she would stare at fans and lights and run in circles.

"It wasn't like a switch being turned off. It was more like a dimmer switch being turned down," said Hannah's father, Jon, a 37-year-old neurologist.

It was heartbreaking, said her 47-year-old mother, Terry, who is trained as both a lawyer a nurse.

"Suddenly my daughter was no longer there," she said.

The family filed a claim with the federal vaccine compensation program in 2002, which the government ultimately decided to concede before any evidentiary hearing.

The case may not be a first, said Gary Golkiewicz, chief special master for the U.S. Court of Federal Claims. He oversees the special "vaccine court" which rules on requests for payments from the vaccine injury fund.

"Years ago, actually, I had a case, before we understood or knew the implications of autism, that the vaccine injured the child's brain caused an encephalopathy," he said. And the symptoms that come with that "fall within the broad rubric of autism."

And there are other somewhat similar cases, Golkiewicz says, that were decided before autism and its symptoms were more clearly defined.

Hannah has a disorder involving her mitochondria, the energy factories of cells. The disorder -- which can be present at birth or acquired later in life -- impairs cells' ability to use nutrients. It often causes problems in brain functioning and can lead to delays in walking and talking.

The Polings were exploring two theories about what happened to Hannah. One is that she was born with the mitochondria disorder and the vaccines caused a stress to the body that worsened the condition. The other was that the ingredient thimerosal caused the mitochondrial dysfunction, Jon Poling said.

CDC officials decline to talk about the Poling case, but they said Hannah's case should not be used to draw conclusions about risks for other children.

Scientists believe that in cases in which a mitochondrial disorder causes a child's brain function to deteriorate, the disorder exists and then is worsened by a fever, infection or other stress on the body.

Scientists don't know if a vaccination -- independent of fever or infection -- can cause such a stress, said Dr. Edwin Trevathan, a pediatric neurologist who heads the CDC's birth defects center.

Others echoed his assessment.

"There are no scientific studies documenting that childhood vaccinations cause or worsen mitochondrial diseases, but there is very little scientific research in this area," said Chuck Mohan, executive director the United Mitochondrial Disease Foundation, a Pittsburgh-based group that raises money for research.

Mohan said there are more than 100 types of mitochondrial disease, and genetic tests can find only a couple dozen.

"Most children with autism do not seem to have a mitochondrial problem, so this association ... is probably relatively rare," said Dr. Edwin Trevathan, a pediatric neurologist who heads the CDC's birth defects center.

Some research suggests they occur in one in 4,000 births, but because some experts believe the rate is closer to one in 2,000, similar to childhood leukemia, because of missed diagnoses. And it is often just as fatal, said Mohan, who lost a daughter to the disease in 1995.

Other federal vaccine advisers sought to portray Hannah Poling as an isolated if not unique case.

She is "not a typical autistic child," said Dr. Paul Offit, chief of infectious diseases at Children's Hospital of Philadelphia and a longtime government vaccine adviser. "It's not a precedent-setting case."

Copyright 2008 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.