ANNEX734
VACCINE EFFECTIVENESS (73-74)
1. Theory
designs (8, 27, 33). Many of the methods are not possible in disease-free populations, as they require cases of disease.
Vaccine effectiveness (VE) refers to vaccine protection achieved in the field within a vaccination programme. This may differ from vaccine efficacy, which refers to protection under ideal conditions. Vaccine effectiveness can vary unpredictably and should be monitored, particularly when there are outbreaks occurring within a vaccination programme. For human medicine, evaluation of vaccine effectiveness is a key step in the assessment of vaccines after they have been licensed. Vaccine protection in the field may differ from protection achieved under ideal conditions owing to poor adherence to cold chain and shelf life requirements. In addition, different batches of vaccine may have different potencies, and individual immune responses to vaccination will vary. Vaccine effectiveness is typically calculated by comparing incidence of disease or infection in vaccinated animals with incidence in unvaccinated animals that were exposed to a similar level of virus using the equation: VE = (RU –RV)/RU
2.1 Outbreak selection – Select a large farm or village that has vaccinated within the last six months but subsequently experienced an outbreak of FMD (several adjacent villages/farms affected by the same outbreak may be assessed in the same investigation). – VE is investigated as soon as the outbreak has finished (the tail-end of an outbreak may be adequate). – There must be good records of which animals were vaccinated. Small-holdings may remember details adequately.
(equation 1)
where RU is the incidence risk or rate in the unvaccinated population, and RV is the incidence in those vaccinated.
– Farmers must be aware of which animals developed FMD. – There must be no recent history of exposure to FMD prior to the outbreak (in the previous three years).
The equation can be reformulated as: VE = 1 – RV/RU
2. Retrospective cohort vaccine effectiveness study
(equation 2)
– Additional vaccination performed during the outbreak will complicate the investigation.
and it is normally given as a percentage. The data needed to calculate VE are often collected in field studies (27). Several different designs are possible. One simple design based on investigation of outbreaks is described in detail below. Readers are referred to other texts for details of other
2.2 Sampling and data collection (templates are included) – Details of local livestock management, vaccination and FMD history are gathered (Table 9). Foot and mouth disease vaccination and post-vaccination monitoring. Guidelines
