4.3 Investigating outbreaks in vaccinated animals

Although the two concepts are related, they should be viewed as distinct because they differ in the approach used for their estimation: (i) vaccine efficacy is estimated through an RCT; while (ii) vaccine effectiveness is estimated through field observational studies or sometimes field trials under normal programme conditions.

In order to avoid confusion between efficacy and effectiveness; the acronym VE shall refer (in this document) to vaccine effectiveness, with equation 1 reformulated as:

VE = 1 – (RV/RU) (equation 2)

and it is normally given as a percentage.

Along the PCP-FMD, stages 2 and 3 are those where control measures are applied while the disease/infection is still present. It is possible that, in stage 2, vaccination may be the only measure applied (a country may not find it feasible to achieve freedom from FMD and may wish to balance the economic cost of the disease with the cost of vaccination), whereas once stage 3 is entered, a decision to move towards freedom from disease has been taken and a more aggressive policy will be adopted with the clear aim of eradication. It is when countries are in either stage 2 or 3 of the PCPFMD (when FMD outbreaks are still expected to occur) that vaccine effectiveness should be measured to ensure that under field conditions the vaccine used is conferring the expected protection.

Thorough investigation of outbreaks that occur in vaccinated animals, where protection would have been expected, is an important aspect of monitoring the performance of vaccination. Findings should be considered in the context of the wider monitoring programme described in Chapters 2 and 3, so as to decide whether the breakdown may have a specific and local cause or be part of a wider problem with the vaccination programme. A systematic approach is recommended in order to check off all the steps where

Outbreak in vaccinated animals

Introduction of new virus with poor match to vaccine strains Vaccine contaminated with live FMD virus Check timing of vaccination in relation to active circulation of field virus

Failure in vaccination programme in presence of active virus circulation

Conduct field investigation and laboratory confirmation Conduct innocuity testing in remaining vaccine batch See Figure 7

Report to manufacturer if confirmed

Fig. 6 Disease outbreak investigation – Considerations and contributory factors

Contributing factors 1. Host factors: a) Age of vaccinates (young animals received one vaccine dose) b) Health condition (stress, malnutrition, infection) c) Time of last vaccination 2. Vaccine characteristic: a) Low potency b) Unstable c) Past recommended shelf life 3. Vaccine application: a) Vaccination schedule (elapse in interval of vaccinations) b) Low vaccination coverage c) Breach in maintaining cold chain 4. Serological test used for PVM analysis: a) Low test specificity, false positive b) Misinterpretation of lab results c) Untrained laboratory staff 5. Overwhelming challenge due to lack of other effective FMD control measures