Dynamics of serological status.

Rabies control remains challenging in low and middle-income countries, mostly due to lack of financial resources, rapid turnover of dog populations and poor accessibility to dogs. Rabies is endemic in Cambodia, where no national rabies vaccination program is implemented. The objective of this study was to assess the short and long-term vaccination-induced immunity in Cambodian dogs under field conditions, and to propose optimized vaccination strategies. A cohort of 351 dogs was followed at regular time points following primary vaccination only (PV) or PV plus single booster (BV). Fluorescent antibody virus neutralization test (FAVNT) was implemented to determine the neutralizing antibody titer against rabies and an individual titer ≥0·5 IU/mL indicated protection. Bayesian modeling was used to evaluate the individual duration of protection against rabies and the efficacy of two different vaccination strategies. Overall, 61% of dogs had a protective immunity one year after PV. In dogs receiving a BV, this protective immunity remained for up to one year after the BV in 95% of dogs. According to the best Bayesian model, a PV conferred a protective immunity in 82% of dogs (95% CI: 75–91%) for a mean duration of 4.7 years, and BV induced a lifelong protective immunity. Annual PV of dogs less than one year old and systematic BV solely of dogs vaccinated the year before would allow to achieve the 70% World Health Organization recommended threshold to control rabies circulation in a dog population in three to five years of implementation depending on dog population dynamics. This vaccination strategy would save up to about a third of vaccine doses, reducing cost and time efforts of mass dog vaccination campaigns. These results can contribute to optimize rabies control measures in Cambodia moving towards the global goal of ending human death from dog-mediated rabies by 2030.</div

Estimated parameter values obtained using the selected model for three scenarios of the probability of non-lethal infection upon exposure to RABV.

Estimated parameter values obtained using the selected model for three scenarios of the probability of non-lethal infection upon exposure to RABV.</p

Evolution of the proportion of protected dogs in two Cambodian provinces.

For the dog population of the Cambodian provinces of Kandal and Battambang, the proportion of protected dogs were modeled for two vaccination strategies: (A) annual primary vaccination of animals <1 year old and, (B) annual primary vaccination of animals <1 year old and systematic booster vaccination of dogs that have been vaccinated the year before. Plain lines indicate the mean value and grey areas show the 95% confidence intervals. Horizontal dashed lines mark the threshold of 70% vaccination rate recommended by WOAH to control rabies circulation in a dog population, and the vertical dashed lines indicate the year at which this threshold is achieved after implementation of the strategy.</p

Estimation of vaccine doses for different vaccination strategies in Kandal and Battambang province.

Estimation of vaccine doses for different vaccination strategies in Kandal and Battambang province.</p

Probabilistic model of the evolution of dog rabies and serological status during the study period.

The whole dog population is separated in 4 states (indicated by squares): S susceptible dog; V1 dog with protective immunity induced by primary vaccination; V2 dog with protective immunity induced by primary and subsequent booster vaccination; R dog with protective immunity induced by a non-lethal infection. The serological results (by FAVNT) are indicated by circles: N negative FAVNT (P positive FAVNT (≥0·5 IU/mL). The parameters describing the model dynamics are: λ average force of infection; π probability of non-lethal infection; q probability of acquiring protective immunity after vaccination; ρ1 rate of loss of protective immunity induced by primary vaccination; ρ2 rate of loss of protective immunity induced by primary and subsequent booster vaccination; p sensitivity of the serological test for dogs in V1 or V2 status. (*) Sensitivity is assumed perfect for animals in the R state. (**) Specificity is assumed perfect. (***) Dogs in the V1 state are assumed immuno-competent (since they have acquired a protective immunity after primary vaccination), therefore, all are assumed to enter the V2 state after the booster vaccination.</p

Bayes’ factor based comparison of 15 alternative models under three scenarios of non-lethal infection probability upon exposure to RABV.

Bayes’ factor based comparison of 15 alternative models under three scenarios of non-lethal infection probability upon exposure to RABV.</p

Vaccination and follow-up schedule for study groups (created by authors using the licensed program BioRender (http://www.biorender.com).

Dogs without documented previous rabies vaccination (grey) received their primary vaccination (T0, grey). For group 1 follow-up blood samples were taken seven months (T7), around one year (T12-14) and 1.5 years (T18) after primary vaccination. Due to logistic restrains the follow-up sequence differs (blue) within this group between dogs from Kandal (n = 16) and dogs from Battambang (n = 205). From dogs of group 2, a blood sample (T12) was collected one year after their primary immunization to document the immune response of this primary immunization. Afterwards these dogs received a booster vaccination (green), and additional samples were collected (green) more than one year (T26) and three years (T34) after the booster vaccination to monitor its effect on the immune response.</p

R and Stan codes, and anonymized raw data.

“dogs_model” was used to estimate parameters and compare different vaccination strategies, using the following stan programs: "dogs_H0.stan" file to test the null hypothesis of an absence of link between the model parameters and individual covariables, and “dogs_H1_pSe.stan”, “dogs_H1_pSV.stan”, “dogs_H1_rBS.stan”, “dogs_H1_rVS.stan”, “dogs_H1_rFOI.stan” and “dogs_H1_rFOI_cage.stan” files to test alternative assumptions. (ZIP)</p

Evaluation of vaccination strategies–Calculations.

Evaluation of vaccination strategies–Calculations.</p

Studies on the immunogenicity of rabies field primary vaccinations.

Studies on the immunogenicity of rabies field primary vaccinations.</p