Immunisation and the anti-immunisation movement

In fact, a malicious person might prefer to strongly encourage everyone else to accept the inconveniences and tiny risks attendant with immunisation - but not be immunised themselves. If the herd immunity of the population is close to 100 per cent then your individual risk of contracting the infection is close to zero because nobody susceptible can give it to you.

Anti-immunisers, like many people, can struggle with statistics. For instance, some anti-immunisers make much of diseases occurring in immunised people. Yet it is often predictable that the majority of cases of an infection will occur in the immunised. For instance, if 95 per cent of the population receive an immunisation that is 85 per cent effective, then three times as many immunised as unimmunised people will be vulnerable to infection. (After immunisation 15 per cent of the 95 per cent vaccinated are not immune. This is 14.25 per cent of the population, compared with 5 per cent of the population who are unimmunised.)

There is evidence, however, that committed anti-immunisers cannot be swayed by statistics - regardless of whether the statistics are lies, damned lies or self-evident truths. One study suggested opposition to vaccines increased when opponents received information comparing the tiny risks of vaccines to the much greater risks of diseases.

For the most part, I think committed anti-immunisation activists are railing against the success and therefore power of scientific rationalism. This is a straightforward social power struggle between different world views where immunisations happen to be a prominent current battleground. This clash traces back beyond the tribal shaman being irritated about the respect accorded to the inventor of the wheel.

Curiously, the world view that distrusts conventional immunisations often accepts homeopathy. Homeopathy is based on a poorly-validated spiritual view that tiny doses of a toxic substance can be good for your health. Conventional live-virus vaccinations are ironic proof that, at least in one circumstance, homeopathy can work.

Utilitarianism - weighing the costs and benefits of action against the costs and benefits of inaction - is not everybody’s favourite ethic. Nonetheless, a utilitarian analysis underlies many decisions in modern medicine. For example, one hopes the risks of treating a disease are always less than the risks of not treating it; and, of course, that the cost and health risks of immunising are outweighed by the cost and health risks of not immunising.

All vaccinations need individual cost-benefit assessments. It is more important to prevent a dire disease like smallpox than a usually minor disease like chickenpox. It is also more important to prevent common diseases, for instance polio in the 1950s, rather than very uncommon ones, for instance polio now or swine flu in 1976. The current rarity of many vaccine-preventable diseases provides a rational reason to closely assess the risks of vaccines.

In the immunisation debate, as in all disputes, a common human failing must be discarded if one hopes to reach a rational conclusion. Anti-immunisers might argue that humanity almost routinely replaces old facts with new facts and beliefs - but the basic efficacy of immunisation has now resisted refutation for centuries. Beliefs do not change facts but facts should change beliefs.