at herd level

These figures can be changed (following the procedures described in Annex 2). For example, if more precision is required, then the allowable error can be lowered to 5%, which in turn will increase the size of the samples to be collected. The main constraint will be the resources available within countries to implement such surveys.

The way in which the epi-units are selected (either using a PPS or SRS procedure) will affect the way in which the proportion of positive animals (and its confidence interval) can be estimated (see Annex 2 for details on this issue).

3.5.2 Post-vaccination monitoring to assess population immunity at herd level

In countries where the main purpose of the vaccination is to reduce the incidence of clinical FMD (epi-setting 1, usually corresponding to stage 2 of the PCP-FMD), this approach may not be recommended, as it is expected that a significant proportion of animals will show immunity due to previous exposure to field virus.

It may, however, be useful to assess immunity at herd level where the expectation for being seropositive is mainly (if not exclusively) due to the administration of the vaccine, and so this methodology is appropriate for countries under scenario B (likely to be in stage 3 of the PCP-FMD or even higher), clearly aiming for eradication, and also for countries in scenarios C and D.

The purpose is to estimate the proportion of ‘epi-units not-adequately vaccinated’ (NAVEU) (see example III.a in Annex 2), which implies that an epi-unit may be defined as ‘adequately vaccinated’ when a given proportion of animals with a specific level of antibodies is found.

In order to estimate the sample size within each sampled epi-unit, it is necessary to establish a threshold proportion below which the epi-unit is considered not protected:

a) suggested values for expected proportion of NAVEUs – 20%;

b) allowable standard error – 10%;

c) level of confidence – 95%;

d) target threshold value to define a single epi-unit as NAVEU is when the proportion of animals with a specific antibody’s titre is: (i) less than 60% in the 6–12 months age group, AND (ii) less than 70% among the 12–24 months age group; e) probability of detecting 0 animals with a level of antibodies equal to or above a specific titre 0.05 (in each of the two age groups).

– Based on the above target values, 62 epi-units and three individual animals 6–12 months old and two individual animals 12–24 months old are needed in each epi-unit selected .

– Increase sample size to 70 epi-units in order to compensate for animals previously exposed to field virus or problems with sample analysis.

– Select the number of epi-units by SRS.

– In each selected epi-unit, animals are selected by SRS or systematic random sampling.

– Collect blood samples according to the procedur e established (at the time of vaccination and/or at any point in time).

– Analyse samples to: - Determine the titres of SP antibodies against homologous vaccine strains. It is also advisable to measure SP antibody titres against field strain(s) to measure protection against circulating virus. Determine the presence of NSPs (exclude any epi-unit in which test results indicate likelihood of infection). - An individual epi-unit is classified as NAVEU if either among the three sampled 6- to 12-month-old animals or among the two sampled 12- to 24-monthold animals no SP positives are found. – Calculate the proportion of NAVEU and its confidence interval: - use equations 3 and 4 (illustrated in Annex 2).

It is important to further highlight that the sampling is restricted only to those age groups eligible for vaccination, and the categorisation of the individual herd as NAVEU will be based on the findings in those selected age groups. Therefore, this approach does not provide information about the overall level of protection within herds.

Sample size, as estimated in the above example, can be changed and adjusted to local conditions and depending on which age group(s) is considered to be the best source of information. The main advantage of this approach is that it will require a number of samples that is significantly less than that required by the methodology described in section 3.5.1, and, in addition, the design and analysis of the study is greatly simplified.