Predicting vaccine effectiveness in livestock populations: A theoretical framework applied to PRRS virus infections in pigs

Abstract

Vaccines remain one of the main tools to control infectious diseases in domestic livestock. Although a plethora of veterinary vaccines are on the market and routinely applied to protect animals against infection with particular pathogens, the disease in question often continues to persist, sometimes at high prevalence. The limited effectiveness of certain vaccines in the field leaves open questions regarding the required properties that an effective vaccine should have, as well as the most efficient vaccination strategy for achieving the intended goal of vaccination programmes. To date a systematic approach for studying the combined effects of different types of vaccines and vaccination strategies is lacking. In this paper, we develop a theoretical framework for modelling the epidemiological consequences of vaccination with imperfect vaccines of various types, administered using different strategies to herds with different replacement rates and heterogeneity in vaccine responsiveness. Applying the model to the Porcine Reproductive and Respiratory Syndrome (PRRS), which despite routine vaccination remains one of the most significant endemic swine diseases worldwide, we then examine the influence of these diverse factors alone and in combination, on within-herd virus transmission. We derive threshold conditions for preventing infection invasion in the case of imperfect vaccines inducing limited sterilizing immunity. The model developed in this study has practical implications for the development of vaccines and vaccination programmes in livestock populations not only for PRRS, but also for other viral infections primarily transmitted by direct contact.

Modeling the transmission and vaccination strategy for porcine reproductive and respiratory syndrome virus

“…According to our modeling, a vaccine efficiency of 80% would be necessary to completely prevent new transmission events. Our simulations reinforce the need for improving the current vaccine technologies (Bitsouni et al, 2019), meanwhile non-therapeutic interventions such as enhancement of on-farm biosecurity will continue to be essential in reducing PRRSV spread (Silva et al, 2019).…”

“…Existing disease spread models have explored the dynamics of between-farm PRRSV transmission (Thakur, Revie et al, 2015; Thakur, Sanchez et al, 2015); a limited number of these studies have investigated the deployment of control strategies on PRRSV outbreak incidence (Bitsouni et al, 2019). To our knowledge no modeling framework has accounted for the spatio-temporal spread dynamics of PRRSV in an epidemiological model.…”

Pathogen transmission from vaccinated hosts can cause dose-dependent reduction in virulence

“…Furthermore, high variation in vaccine responsiveness may render a significant proportion of vaccinated animals effectively nonimmunized [52]. The results of our study suggest that partial vaccination or high prevalence of vaccine non-responders may impose less risk with respect to disease invasion and persistence than anticipated from existing theory [4,53,54]. Prediction of the coverage required for herd immunity would benefit from an understanding of the downstream effects of vaccinationinduced changes in exposure dose and their effects on individuals' infectiousness and survival in any particular system.…”

Modeling Economic Effects of Vaccination Against Porcine Reproductive and Respiratory Syndrome: Impact of Vaccination Effectiveness, Vaccine Price, and Vaccination Coverage

“…Outcomes are reported as reduction of viremia in terms of duration and/or magnitude, of clinical signs or of pathological lesions (11,14). Moreover, vaccine effectiveness might also be considered by epidemiological parameters describing disease dynamics and a reduction in virus transmission (15,16).…”