Abstract:
Foot and mouth disease (FMD) is a highly transmissible viral disease of cloven-hoofed animals, which is endemic
in many developing countries. Vaccination is the main tool for FMD control in resource limited endemic
countries like Ethiopia. Vaccine quality, which is often questionable in developing countries, is a critical element
for effective disease control. The present study was aimed at evaluating the field effectiveness of a trivalent FMD
vaccine (containing serotypes O, A and SAT 2), produced and widely used in Ethiopia, in terms of preventing
clinical infection and severe disease. A randomized controlled field trial design was employed in the study in
which the attack rate of clinical FMD infection in vaccinated cattle was compared with the attack rate in unvaccinated controls in cattle population of 16 villages in Gondar Zuria district, Northwest Ethiopia. The vaccine
was administered as a single dose course in the face of an impending FMD outbreak and the trial groups were
monitored for clinical infection until the end of the outbreak. The attack rate of clinical FMD 20 days post
vaccination in the vaccinated cattle (34 %) was significantly lower than the attack rate in the unvaccinated
controls (49 %) (p < 0.001). However, the effectiveness of the vaccine was only 31 % (95 %CI: 20–40 %). This
vaccine effectiveness increased to 52 % ((95 %CI: 33–66 %) 42 days post vaccination. The proportion of severely
affected cattle in the vaccinated group (5.7 %) was significantly lower than in the unvaccinated group (9.4 %)
(p < 0.001), resulting in 39 % (95 %CI: 18–55 %) vaccine effectiveness against severe disease. Generally, the
observed level of vaccine effectiveness was lower than the internationally recommended 75 % plus expected
percentage of protection for a standard potency 3PD50/dose FMD vaccine. Moreover, the level of effectiveness
was insufficient to provide herd immunity to control the disease at the population level. Nevertheless, given the
significant difference in the incidence of clinical disease and severity between vaccinated and unvaccinated
cattle, it might still be worth using the current vaccine to reduce production losses associated with the disease
provided it is cost effective and affordable for the farmers. Factors that cause low effectiveness of the vaccine
need to be identified and addressed for effective control of the disease at population level.
