WASHINGTON (Reuters) - Campaigners who doubt the safety of vaccines have launched a renewed effort to find a link between diseases such as autism and childhood shots, worrying experts in the field.

Indiana Republican Rep. Dan Burton reopened hearings into the alleged vaccine-autism link on Tuesday, and several groups railed against a decision in Congress earlier this month that made it harder to sue vaccine makers.

Doctors say vaccines may have been the biggest advance in health of the last century, saving millions of lives. But their success has opened the door to questions about safety.

The House Government Reform Committee was scheduled to hold a hearing on vaccine safety on Tuesday. Burton, its chairman, has an autistic grandchild and blames vaccination.

"We are taking a closer look at the science," said Nick Mutton, a spokesman for the committee. "You can't argue with the numbers and the amount of cases. It is becoming an epidemic among our children and something has to be done about that."

Burton has held such hearings for years. Repeated reports have shown no link, including several university-based studies and a 2001 independent Institute of Medicine report saying there was no evidence to show the measles, mumps and rubella, or MMR, vaccine causes autism.

Children are usually diagnosed with autism around age 2, just after they finish their series of vaccines.

FOCUS ON MERCURY

The latest focus is on thimerosol, a mercury-based preservative used in vaccines for decades.

It is no longer used in childhood vaccines -- not because it was shown to be harmful but because U.S. government officials were aware that people believed that mercury may be linked to autism.

"There's always been a minority opinion in the United States that vaccines aren't safe," Dr. Peter Hotez, chair of the department of microbiology at George Washington University and an adviser to the Sabin Vaccine Institute, said in a telephone interview. "There currently is no evidence that there is an association between vaccines and autism but somehow it still lingers."

Hotez said Burton and other vaccine critics are looking in the wrong place. "I have an autistic child and I wouldn't think twice about vaccinating her with the same vaccine series yet again," Hotez said.

"Autism has a strong genetic component. It is inconceivable that something like a vaccine could generate the complex set of neural pathways needed for autism."

He and other vaccine experts were angered by the tone of Burton's inquiries, which they said only raise questions among those who would otherwise have no doubts about vaccine safety.

"I think Dan Burton is not looking after the health of the citizens of his own state of Indiana," Hotez said.

A study published in November in the Lancet medical journal found that children given thimerosol-containing vaccines had safe levels of mercury in their blood as defined by the U.S. Environmental Protection Agency.

The U.S. Centers for Disease Control and Prevention and the Food and Drug Administration along with nongovernmental groups such as the National Foundation for Infectious Diseases, have been pushing public education campaigns aimed at keeping up traditionally strong public support of immunization.