Which flu jab would you like this season – the one with mercury, or without? It’s a question you’re unlikely to be asked when the NHS vaccination programme gets underway in October but there actually is a choice. One swine flu vaccine ordered by the government, Panderix, contains thimerosal, a preservative which is 49.6% mercury by weight. The other swine flu jab, Celvapan, is mercury-free.

I found this out by calling the Department of Health on a hunch. When governments order vaccines, and have no intention of telling patients what’s in the mix, they tend to go for the bulk cheap ones. These often contain thimerosal. But the use of mercury in vaccines is far from uncontroversial. A few years ago, class action by American parents of autistic children was brought (unsuccessfully) against the makers of thimerosal. They suspected that mercury, a proven neurotoxin, could have impaired the mental development of their children given its presence in child vaccines. These controversies seldom kick off in Britain because parents are told nothing about the vaccines.

Andy Burnham said last week that his department intends to "target" 500,000 pregnant women when it starts its vaccination programme. Now, NHS24 has plenty to say about pregnant women and mercury. It advises them that mercury "can have a damaging effect on your baby’s developing nervous system" and that pregnant women had best avoid swordfish because it contains "a high level of mercury". By  “high” it means that swordfish is a millionth mercury – one “part per million” is mercury." So why should pregnant women – who are advised to take a swine flu jab – inject themselves with the very stuff that the government considers it is prudent for them to avoid? Why not advise them to take the mercury-free jab? Or at least explain the risks and give them the choice?

The Department of Health tells me “we won’t be advising against pregnant women using it as there is no reason to”. Its standard line (see it in full here) is that “there is no evidence” that mercury (at the levels that exist in thimerosal vaccines) is harmful. This is stating the obvious: if there were evidence of neurological damage – be it Alzheimer’s in the elderly or autism in children – it would be a scandal of Thalidomide proportions. But the Department of Health has for the last few years been phasing thimerosal out of the DTP baby vaccines. Why might it do that, you may ask, if it is so confident that thimerosal was doing no harm?

I suspect that the Department will have to rethink its decision not to tell pregnant women about Panderix (also called Prepandrix) and mercury. The era of the expert patient is upon us. Doctors have to explain themselves, their products, their prescriptions. They can’t really say “the man in Whitehall knows best, you may feel a little prick”. I’d argue they do have a duty to tell an inquisitive patient what’s in the mix. That patient may very well be nervous about mercury, perhaps noting that no-one in the UK government will actually say it is safe (they say there’s no evidence of harm, a crucial difference, and officials are very careful not to go any further).

The use of mercury in medicine is a deeply controversial subject, and one that may divide CoffeeHousers too. In recent years, there have been several studies into thimerosal – all of which have been unable to find evidence of harm. But the overall guidance from most Western governments is quite simple: if you can avoid mercury, do so. The NHS has two swine flu vaccines. It should give everyone, especially pregnant women, the choice of staying mercury free.