9.2 Vaccination programmes

In relation to any given infectious disease, a population can be thought of as comprising three groups: those who are sick with the disease, those who are healthy but susceptible to a disease, and those who are immune to infection. This latter group – those immune to infection – may comprise those who have previously been infected but have now recovered and those who have been immunised through public health programmes. Immunisation (here used interchangeably with the term vaccination) therefore forms one of the major public health tools for infectious disease control.

Data on immunisation for use in GIS may come from a variety of sources:

• routinely collected data through national health information systems, geocoded using patients’ postal (zip) codes
• sample surveys, such as the National Immunisation Survey in the USA (see references)
• indirectly via serological surveys, which test for the presence of disease anti-bodies in blood samples taken from a representative subset of the population (e.g. see Vyse et al, 2002)

Since immunisation rates can vary spatially, GIS has several potential uses in helping plan immunisation:
 
Understanding factors affecting immunisation uptake:

An important aspect of immunisation is the uptake rate – the percentage of a given population cohort who have been immunised in a specified time period. One possible use of GIS is in understanding the factors influencing immunisation uptake rates at population level. GIS can be used to relate immunisation data to census data sets or to spatial measures of healthcare accessibility, such as distance to the nearest health facility. For example, Wright et al (2006) analysed immunisation uptake rates for the Measles-Mumps-Rubella (MMR) vaccine in England in relation to census and other governmental socio-economic data.

Targeting:

Closely related is the concept of using GIS for planning immunisation targeting. By better understanding how healthcare access affects immunisation uptake using GIS, for example, it is possible to estimate how many additional children could be immunised if health service delivery strategies change. Such analyses can help in estimating how many additional children might be reached through a new immunisation outreach programme and where best to send the outreach team to maximise its impact. For successful targeting, data are often needed at the small area level (e.g. for areas with populations of 2,000 or less) to identify and target ‘pockets’ of poor immunisation coverage, which may be quite localised.
 

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A second key issue in identifying target areas where immunisation coverage needs to be increased is identifying areas where rates have reached dangerously low levels. A key concept in this regard is herd immunity. Within any given population, there are always likely to be individuals who are not immunised, either through choice or because ill health prevents them from being immunised. However, the epidemiological evidence suggests that provided a sufficient proportion of the population as a whole is immunised, then infectious disease will not be able to take hold within the general population. This is because if enough individuals are immunised, there will be insufficient contact between infected and susceptible individuals for the disease to take hold. Depending on the specific disease, the threshold at which herd immunity occurs will vary. For highly infectious measles, for example, herd immunity is believed to take place if 92-95% of a population is immune to the disease. If a lower percentage of the population is immune to measles, then there is a risk of a measles outbreak occurring. For other diseases, such as mumps, herd immunity occurs at a lower threshold. Understanding herd immunity is therefore important in knowing if immunisation rates are becoming dangerously low.
 

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