Measles targets and herd immunity

Petre Jones

doi:10.3399/bjgp13X670570

Measles epidemics occur in populations with sufficient numbers of susceptible individuals for each infected person to meet and pass on the virus to one or more other susceptible individuals. Herd immunity suggests that if enough people are rendered immune such meetings will be rare enough for the virus not to be passed on and the whole population to be protected. If immunisation rates for the population were high enough we should therefore prevent epidemics such as the current one in south Wales. So if general practice can and does deliver high measles immunisation rates from ages 2–5 years, why do we currently have a problem?

Unfortunately both the model of herd immunity and our immunisation rate markers are flawed. Human communities do not consist of randomly moving particles bumping into each other by chance, they cluster in social groupings. Equally just because 90–95% of 5-year-olds are immunised over a 10-year period does not mean that 95% of people under 15 have been protected. If unimmunised people move into a community the coverage will drop. Moreover if the newcomers cluster together they will form a subgroup at high risk of an outbreak.

In our small inner-city practice in an area of high turnover and immigration, despite a consistently high immunisation rate, we have discovered an important group of susceptible teenagers we were not previously aware of among whom a measles outbreak remains. Despite immunisation rates consistently over 90%, of our 256 10–16-year-olds, 51 were not fully immunised. The autism fallacy accounts for five people not taking up immunisation, but 46 were new entrants to the UK after the age of 5 years old. In our area therefore, despite high immunisation rates, we are at high risk of an epidemic. Herd immunity does not apply.

We cannot afford to rest on our laurels. Having reached high immunisation rates overall is good but not good enough especially in areas of high population turnover and immigration. Measles susceptibility is an ever-present danger.

Do we need to routinely do one-third MMR at age 10–12 years old, or immunise all new entrants along with TB screening?