While it is not possible to establish that a vaccine is absolutely safe, or that a particular vaccine reaction never occurs, the weight of research has now vindicated MMR as the cause of increasing trends in autism diagnoses.

It also shows that triple vaccines are no more dangerous than singles - maybe less so.

Even so, the threat to children’s health in the wake of the MMR scare is real. Immunisations are vital because of the phenomenon known as “herd immunity.” This occurs when enough people are immune to a particular disease. Even the few who are not immunised are protected because the disease can’t gain a foothold in the community.

If herd immunity is allowed to drop below a certain level - usually at vaccination rates of 92 per cent either through choice or inaction - then people who have not been immunised are at risk.

With MMR vaccination rates dropping below the required level to maintain herd immunity, experts fear that measles will re-emerge as a threat to children’s health.

“Vaccinations against measles have been available for a number of years, so it is possible that people have forgotten that it can be a very serious disease,” says a spokesperson for the Public Health Laboratory Service.

“Not only is measles highly infectious, but its complications can be serious, and occasionally fatal.”

The UK’s comparative success in achieving herd immunity against childhood infections lies behind anxieties about “population mixing”. An influx of immigrants, for example, may pose a health threat to a local population, since many will not have had the benefit of a comprehensive immunisation programme.

Similarly, decades of migration to “new towns”, caused by industrial expansion, may be to blame for outbreaks of some diseases. If the migrant population has different infectious exposures, the result may be a mixing of immune and non-immune children and outbreaks of diseases that have o’t been seen there for many years.

Jill Wyatt is a writer on health matters