Persistent concern about the safety of vaccinations prompted the US National Academy of Sciences Institute of Medicine to convene an open public meeting in Cambridge, Massachusetts, on July 16, to review the evidence for an association between thimerosal, a mercury-containing preservative found in multidose vaccine preparations, and neurodevelopmental outcomes, particularly autism, the prevalence of which has apparently increased over the past decade.

Thimerosal, also known as ethylmercury, has been used as a preservative since 1930, and is now hypothesised to carry similar risks as methylmercury, a chemical that is known to be toxic in animals and human beings.

In June, 1999, the American Academy of Pediatrics and the US Public Health Service, acknowledging that mercury exposure from vaccinations given in the first 6 months of life exceeded maximum acceptable levels established in federal guidelines, called for a halt to the manufacture of thimerosal-containing vaccines. Although no longer routinely given in the USA (European and UK regulators have also recommended phasing out their use), they are still the norm in many other parts of the world, especially in developing countries.

Neal Halsey (Johns Hopkins University, Baltimore, Maryland, USA) said that all countries need to stop using these vaccines now, but single-use vaccines, carrying added costs for storage, production, alternative preservatives, and quality control measures, are prohibitively expensive for developing countries.

The meeting was the third in a series of gatherings hosted by the IOM's Immunization Safety Review Committee, an independent expert group established by joint request of the Centers for Disease Control and Prevention and the National Institutes of Health.

In its report, to be issued within 90 days of the meeting, the committee will recommend public health responses for surveillance, research, and policy. It will consider, in addition to the evidence for causality, the biological plausibility of adverse events, and will explore the social and cultural context of the issue.

Lancet July 21, 2001; Volume 358, Number 9277