ANNEX 1
MONITORING VACCINE COVERAGE (51-58)
1. Introduction The coverage necessary to stop the FMDV from spreading within a herd will depend upon the number of cases that one case generates on average over the course of its infectious period, in an otherwise uninfected, naïve population (the basic reproductive ratio, R0). The value of R0 will depend on the nature of the contact structures within the herd and will be greatest when large numbers of highly susceptible animals have regular contact opportunities. In a fully susceptible herd of housed cattle, an R0 of considerably greater than 10 is possible (42). Similar considerations on proximity and contact networks apply to the spread of FMDV infection between herds, but spread will usually be less efficient, giving rise to lower estimates for the herd-toherd R0 at the start of outbreaks (values of 2–5 have been reported for outbreaks in the United Kingdom and Peru [12, 18, 23]). However, higher values have been reported where conditions favour extremely rapid spread (18). Within herds, the figure of 80% vaccination coverage is commonly cited as a target for control of FMD (4), the denominator for coverage being, in this case, the total number of susceptible animals within herds (i.e. those both eligible and ineligible for vaccination). A vaccination coverage of 80% should reduce an Rv (Rv being the reproduction ratio in vaccinated animals) of 5 to less than 1 and thereby halt the spread of the FMDV among vaccinated animals; it should always be very clear what this 80% is referring to. However, in many cases, vaccination does not fully block transmission and if a 75% probability of achieving this is assumed, then, with 80% coverage, an outbreak will be brought under control only where the R0 is already less than 2.5 (Table 1). At the herd level, it should be possible to vaccinate a high proportion of herds (> 80%) but it will be difficult to achieve 100% effectiveness. However, contact between units (and thus R0) will be reduced by effective biosecurity and this will reduce dependency on unobtainable levels of vaccine protection. Conversely, effective vaccine protection will often block transmission if biosecurity is suboptimal.
Table 1. The relationship between rate of transmission in the population and vaccination coverage needed to halt virus spread (f × h = 1 – 1/R0) Proportion of animals Initial rate of spread (R0)
that must be vaccinated (f), assuming vaccination is 100% effective (h)
Proportion of animals that must be vaccinated (f), assuming vaccination is 75% effective (h)
2.5
60%
80%
4
75%
100%
5
80%
Impossible*
6.7
85%
Impossible
10
90%
Impossible
20
95%
Impossible
* Impossible to eliminate infection even by vaccinating the whole population
It should be noted that, if regional coverage is monitored, within that region there will be areas of high coverage and areas of low coverage, possibly those hardest to access or where farmers are least motivated to vaccinate their animals. Thus, at the aggregated regional level, it may appear that coverage is sufficient to control transmission, but islands of low coverage within the region may allow reservoirs of continued virus circulation to persist. It is also important to bear in mind that not all vaccinated animals will develop a protective level of immune response. As an example, if vaccine coverage = f = 0.9 and the proportion of animals with a protective level of specific antibodies = h = 0.95, then the overall proportion of animals with a protective level of specific antibodies will be p = 0.90 × 0.95 = 0.855 or 85.5%. Furthermore, an immune response may be due to infection rather than vaccination or may reflect earlier rounds of vaccination than that being measured. In young animals, immunity may also reflect passive uptake of antibodies from maternal colostrum. In Chapter 2, reference was made to vaccination coverage and some of the methods used to estimate it. Regardless of the method, a reliable estimation of vaccine coverage requires that the target population to be vaccinated is known. This information, in addition to being essential Foot and mouth disease vaccination and post-vaccination monitoring. Guidelines
