A group of 35 law firms is suing several drug companies over an alleged link between trace amounts of mercury found in certain vaccines and nerve disorders in children. This comes a day after a report saying the link is still unclear.

The class-action suit is being led by an Oregon woman who says her two-year-old son became autistic after getting several vaccinations that contained the preservative thimerosal.

Thimerosal has been used since the 1930s to protect vaccines against contamination from bacteria and fungi, especially in larger vaccine vials that contain several doses.

Thimerosal contains a form of mercury called ethylmercury. The form of mercury that has been linked to nerve damage is methylmercury, found in contaminated fish and seafood.

While the toxic effects of methylmercury are well-known, little is known about ethylmercury.

On Monday, the U.S. National Academy of Sciences released a report saying there is no evidence proving the link between thimerosal and diseases such as autism and attention deficit-hyperactivity disorder.

However, the report also stated it was possible a child's risk of developing such a disorder could rise because of increased exposure to mercury in vaccines.

The report recommended that vaccines containing thimerosal not be used when other vaccines could be used instead.

The law suit, however, seeks to remove all remaining stocks of thimerosal-containing vaccines. The suit also seeks to force drug companies to investigate whether there is a link between thimerosal and nerve disorders in children.

Lawyers filing the lawsuit allege that drug companies were putting profits before the public's safety by using thimerosal in multi-use vials of vaccine instead of producing single-use vials the lawyers say wouldn't need the preservative.

Until recently, several vaccines in the U.S. contained thimerosal. Now, most vaccines are available without it.

According to Health Canada, the only vaccine used in Canada that contains thimerosal is the hepatitis B vaccine. Since last year, thimerosal-free vaccines against hepatitis B have also been available.