Immunisation can confer immunity to specific infectious diseases and be either passive (through injected antibodies, such as the monoclonal palivizumab against respiratory syncytial virus (RSV) infection given to premature infants) or active (through administration of a vaccine). Immunisation invokes antibody and/or cel-mediated immunity and can lead to both short- and longer-term protection in the person who is vaccinated. Immunisation has also been used to eradicate a dis ease such as occurred in the smallpox eradication programme and is currently being targeted in the polio eradication programme. As well as direct effects of vaccination a number of indirect effects can occur – such as protection of individuals who are vaccinated through altering disease transmission leading to ‘herd immunity’; or reduction of antibiotic resistance through selective reduction of pneumococcal serogroups that are associated with antibiotic resistance. The UK immunisation schedule is described in detail and regularly updated in the UK government publication ‘Immunisation against infectious dis ease’ (Green Book).

A similar measure to the incidence rate is the cumulative incidence or risk, which is the number of new cases as a proportion of the total people at risk at the beginning of the exposure time. If in the example above the same 1000 people were observed for a year (i.e. with no one joining or leaving the group) then the one-year risk is 10% (100/1000). The time-period should always be specified for risks. These rates and proportions are used to describe how diseases (and risk factors) vary according to time, person and place. Temporal variation may occur seasonally; for example, malaria occurs in the wet sea son but not the dry, or as longer-term ‘secular’ trends, e.g. malaria may re-emerge due to drug resistance. Person comparisons include age, sex, socio-economic status, employment, and lifestyle characteristics. Place comparisons include the local environment (e.g. urban versus rural) and international comparisons.