The allegations, in yesterday's edition of the Los Angeles Times, are based on a memo sent in 1992 to the head of the drugmaker's vaccine division by consultant vaccinologist Maurice Hilleman. The memo points out the concerns raised in Sweden and elsewhere in the world about thiomersal - a preservative used in vaccines that contains 50 per cent ethyl mercury - and recommends that it should be removed where possible.
However, the newspaper article does not fully reflect the overall conclusion of the memo, which advised that at that time there was no evidence for a risk from thiomersal, and that regulatory actions in Sweden and other countries were based more on a public perception of risk, rather than any direct evidence of toxic effects.
It was not until 1999 that the FDA made its concerns about the effects of the preservative on child development public, and efforts to minimise the use of thiomersal in vaccines began in earnest.
As of 2000, vaccines recommended for use in children and infants have all been available in versions that contain no ('thiomersal-free') or only trace amounts ('thiomersal preservative-free') of the compound, with the exception of some multidose vials intended for use in the developing world. This is because where several doses are taken from the same container, thiomersal offers better protection from contamination than other preservatives such as 2-phenoxy ethanol, according to the World Health Organisation.
Along with other vaccine manufacturer, Merck has been named in more than 4,000 claims filed by parents claiming that their children suffered autism or other neurodevelopmental disorders from mercury in vaccines, with other cases awaiting trial in civil courts.
The plaintiffs cite various scientific studies that they say prove the dangers of thimerosal, despite the publication of a report by the US Institute of Medicine last year which review the clinical and epidemiological evidence and concluded that there was no evidence for a causal link between thiomersal and autism.
Separately, Merck is facing legal attacks over Vioxx (rofecoxib), the COX-2 antagonist painkiller it introduced in 1999. The company is dealing with hundreds of lawsuits, filed after the company withdrew the drug from the market in September 2004, claiming Vioxx drug caused heart problems and that Merck concealed the risks. Merck has denied the allegations.