Abstract:
BACKGROUND : Vaccination is considered to be the best approach to control Rift Valley
fever (RVF) in animals and consequently in humans. This study assessed the efficacy and
safety of the RVF virus (RVFV) Clone 13 vaccine under field conditions.
METHODOLOGY : A vaccine trial was conducted in sheep (230), goats (230), and cattle
(140) in Ngorongoro district, Tanzania. Half of each of the animal species were vaccinated
and the other half received the placebo. Animals were clinicallymonitored and bled before
vaccination and at days 15, 30, 60, 180 and 360 (+/– 10) post-vaccination to measure
Immunoglobulin M (IgM) and IgG antibody responses to RVFV. Survival analysis was
conducted using cox-proportional hazard regression model to measure the time until
an event of interest had occurred and to compare the cumulative proportion of events
over time.
RESULTS : Of 600 animals included in the study, 120 animals were lost during the study,
leaving a total of 480 (243 in the vaccinated group and 237 in the control group) for
complete follow-up sampling. There was no adverse reaction reported at the injection site
of the vaccine/placebo in all animals. Abortions, deaths, or body temperature variations
were not associated with vaccination (p > 0.05). By day 15 post-inoculation, the IgG
seroconversion in vaccinated goats, cattle and sheep was 27.0% (n = 115), 20.0% (n
= 70) and 10.4% (n = 115), respectively. By day 30 post-inoculation, it was 75.0% (n
= 113), 74.1% (n = 112) and 57.1% (n = 70) in vaccinated sheep, goats and cattle,
respectively. By day 60 post-inoculation, IgG seroconversion in sheep, goats and cattle
was 88.1% (n = 109), 84.3% (n = 108) and 64.60% (n = 65), respectively. By day 180, the IgG seroconversion in sheep, goats and cattle was 88.0% (n = 108), 83.8% (n =
105) and 66.1% (n = 62), respectively. By day 360, the IgG seroconversion in sheep,
goats and cattle was 87.2% (n = 94), 85.6% (n = 90) and 66.1% (n = 59), respectively.
Only five animals from the vaccinated group were RVFV IgM positive, which included four
sheep and a goat.
CONCLUSION : RVFV Clone 13 vaccine was well tolerated by sheep, goats, and cattle.
The vaccine induced detectable, but variable levels of IgG responses, and of different
duration. The vaccine is considered safe, with high immunogenicity in sheep and goats
and moderate in cattle.
