5.5.12 What sample size do I need for monitoring ORV campaigns?

Regarding sample size for monitoring ORV campaigns (biomarker detection, herd immunity) WHO and EFSA recommend a minimum of 4 foxes/100 km² to be investigated annually. However, experience has shown that depending on the topographical features of the vaccination area, infrastructure and logistsics this sample size can be difficult to achieve.

Appropriate sample sizes can also easily be calculated using statistical procedures. As the fox density is often unknown, sample sizes should be calculated on the basis of infinite populations. It is worth noting that sample sizes have no spatial basis in wildlife, so the same sample sizes have to be applied for different-sized vaccination areas. Ideally, the sample size should be homogenously distributed within the vaccination area and time to avoid spatial and temporal clustering, otherwise the information could be biased and may not reflect the real situation.

If homogenous sampling cannot be guaranteed, appropriate reference areas can be defined within the overall vaccination area, and the results from each of these reference areas can then be compared and extrapolated to the entire vaccination area. If monitoring data (bait-uptake, seroconversion) are being evaluated, the age structure of the sample size (juvenile, adult) should be taken into account. The establishment of an epidemiological unit devoted to ORV is strongly recommended, as well as the help of a statistician.


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Version 1 - Last updated November 2012