Vaccination is based on the tried and trusted scientific principle of cause and effect: the vaccine (the cause) offers protection (the effect). It's a straight-line thought, and one that does not admit any confounding complexities, such as the overall health of the person, the immediate environment, hygiene factors, and the like. In fact it's these complexities that Louis Pasteur referred to as 'the field', which he regarded as more significant than a single inoculation.
This might explain why MMR-vaccinated children sometimes get measles, and why some unvaccinated children never get measles. This problem was highlighted in a study in 2000 in Guinea Bissau in West Africa, which discovered that the DPT (diptheria, pertussis, and tetanus) vaccination programme was responsible for the deaths of some of the children. Was the vaccine to blame? Was it one too many insults to an already compromised immune system?
Researchers recently decided to see if the DTP shot was having the same worrying effects in another developing country. They visited a community in Matlab, Bangladesh, and analysed the data from 1986 to 2001 - and concluded that the vaccine was actually saving lives.
The research team admits that it may be a conclusion too far, and that other factors could be responsible for the reduced mortality rates.
But then, you can always prove - or disprove - any cause and effect in a complex system.