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Thiomersal vaccines debate continues ahead of UN meeting


Experts are meeting for the fourth time at the end of June to discuss a global treaty on mercury. But such an agreement could hinder vaccination programmes worldwide. Nayanah Siva reports.


In 1997, amid concerns about a possible link between vaccines containing mercury-based chemical thiomersal and autism, the US Food and Drug Administration were called to review the risks of all food and drugs that contained mercury. In 1999, despite the lack of scientific evidence, the US Centers for Disease Control and Prevention (CDC) issued a recommendation to manufacturers that “thiomersal-containing vaccines should be removed as soon as possible”. The CDC said that “there are no data or evidence of any harm caused by the level of exposure that some children may have encountered” but they did recognise “that some children could be exposed to a cumulative level of mercury over the first 6 months of life that exceeds one of the federal guidelines on methyl mercury…”


More than a decade later, regardless of several independent studies and consultations that did not find any association between thiomersal vaccines and autism, the debate still continues. In April, WHO held another consultation into the issue and the UN Environment Programme (UNEP) are working on developing a global legally binding agreement on mercury. The UNEP negotiations, which continue at the end of June, include assessing the safety of vaccines that contain thiomersal and they aim to make a decision by 2013.


Thiomersal has been used in low doses as a preservative in multidose-vial vaccines since the 1930s. But since the public's growing concern about a possible autism—thiomersal link, and the CDC's 1999 recommendation, American companies have been removing the chemical from their vaccines and have been making single-dose vials for years. So far, these changes in vaccine production have only been reported in the USA but there is growing concern that if a global ban of thiomersal is recommended by the UNEP, it will have a deeply negative effect on the developing world.


“If we were to stop using thiomersal in vaccines for any reason then we need an alternative ready”, said Heidi Larson who works on the Project to Support Public Confidence in Immunization, London School of Hygiene and Tropical Medicine, UK. “But we don't have an alternative ready, the only immediate alternative is single-dose vials.” But the logistics of single-dose vials in the developing world is just not viable.


Multidose vials used in the developing world can hold up to ten doses of vaccine, increasing their ease of storage and transport in developing countries. “Most vaccines need to be refrigerated…and multidose vials take up less space per dose in the cold chain during storage and transport…On a per-dose basis, vaccines in multidose vials are also significantly less expensive to purchase from manufacturers. If thiomersal is banned…many immunisation programmes may not be able to reach the same number of children”, said Médecins Sans Frontières in a statement.


Thiomersal-containing vaccines are an essential medicine and they are used in more than 120 countries to immunise at least 64% of the global birth cohort each year. WHO estimates that thiomersal-containing vaccines avert at least 1·4 million child deaths every year and the vaccines are used against fatal diseases, such as diphtheria, tetanus, and hepatitis B.


Implications of a possible ban of thiomersal not only affect the developing world. As Terry Nolan, Chair of the Australian Government's principal vaccine and immunisation advisory committee, says, thiomersal is also used in vaccines for “pandemic influenza because of the need for both rapid production (single-use unit dose presentations take longer to produce and there are resource constraints and availability issues for the syringes in times of surge requirements) and efficient delivery (it is quicker and more efficient for mass immunisation sessions to use multidose vials)”.


One of Larson's main concerns has been the public's confidence in vaccines. “While there were the concerns [about thiomersal] in the US, there were the MMR vaccine concerns here in the UK. And, as people are struggling with their confidence issues around vaccines, in some of their minds all the concerns have merged together reinforcing their anxieties even though the issues are not related…”


In 2004, the US Institute of Medicine did their eighth and final assessment of the safety of immunisations and definitively concluded that there is no link between MMR or thiomersal vaccines and autism. Larson is hopeful that the UNEP will not ban thiomersal because scientific evidence does not show a link between the chemical and any adverse effects. “I hope and I feel reasonably confident that the UNEP will take the advice of WHO and other strong voices that have come forward on this.”




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