## Vaccine efficacy

Vaccine efficacy (VE) is calculated by: (AR in unvaccinated — AR in vaccinated)

where AR is the attack rate (discussed in Section 2.2.1). The VE indicates the maximum achievable level, but poor vaccination technique or storage can reduce this. Also, the more people who are vaccinated, the greater the number of apparent vaccine failures. If the above equation is rewritten to express the percentage of cases vaccinated (PCV) in terms of the percentage of the population vaccinated (PPV) and VE, then:

AR in unvaccinated x 100%

By knowing two of these variables, the third can be calculated. Figure 3.3 shows three curves generated from the equation, each for a different VE. These curves predict the theoretical proportion of cases with a vaccine history. For example, if a measles epidemic is observed in a population with homogeneous measles exposure where 90% of the individuals are vaccinated (PPV=90%) with a 90% effective vaccine (VE=90%), the expected percentage of measles cases with a history of being vaccinated would be 47% (PCV=47%: Example A). However, if only 50% were vaccinated, then 9% of the cases would have been found to be vaccinated (Example B). This is not to say that there is anything wrong with the vaccination programme, but explains why there may appear to be an unexpected number of vaccinated population amongst the cases.

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