New insights about vaccine effectiveness: Impact of attenuated PRRS-strain vaccination on heterologous strain transmission

Abstract

Vaccination is the main tool for controlling infectious diseases in livestock. Yet current vaccines only provide partial protection raising concerns about vaccine effectiveness in the field.Two successive transmission trials were performed involving 52 pigs to evaluate the effectiveness of a Porcine Reproductive and Respiratory Syndrome (PRRS) vaccinal strain candidate against horizontal transmission of a virulent heterologous strain. PRRS virus, above the specified limit of detection, was observed in serum and nasal secretions for all but one pig (the exception only tested positive for serum), indicating that vaccination did not protect pigs from becoming infected and shedding the heterologous strain. However, vaccination delayed the onset of viraemia, reduced the duration of shedding and significantly decreased viral load throughout infection. Serum antibody profiles indicated that 4 out of 13 (31%) vaccinates in one trial had no serological response (NSR).A Bayesian epidemiological model was fitted to the data to assess the impact of vaccination, and presence of NSRs, on PRRS virus transmission dynamics.Despite little evidence for reduction in the transmission rate, vaccinated animals were on average slower to become infectious, experienced a shorter infectious period and recovered faster. The overall PRRSV transmission potential, represented by the reproductive ratio R0 was lower for the vaccinated animals, although there was substantial overlap in the credibility intervals for both groups. Model selection suggests that transmission parameters of vaccinated pigs with NSR were more similar to those of unvaccinated animals. The presence of NSRs in a population, however, seemed to only marginally affect the transmission dynamics.The results suggest that even when vaccination can't prevent infection, it can still have beneficial impacts on the transmission dynamics and contribute to reducing a herd's R0. However, biosecurity and other measures need to be considered to decrease contact rates and lower R0 below 1.

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

“…The limited effectiveness of vaccines in field conditions (Charerntantanakul, 2012; Roca et al, 2012), highlight the need to estimate the level of protection required to successfully reduce PRRSV between-farm transmission. As describe elsewhere (Linhares et al, 2012), suggested that vaccination can reduce PRRSV shedding rates, a more recent experiment with a European type subtype 1 strain, indicated that the current vaccines have a poor efficacy to reduce between animal transmission (Chase-Topping et al, 2020). Therefore, remain under debate the impact MLV vaccines on the concentration of PRRSV in the environment and the reduction of virus shedding.…”

“…In this study, we assumed that all sow farms have been vaccinated or exposed to PRRSV previously, thus all sites have the same probability to report a PRRSV outbreak. The latent period is not explicitly modeled, since at animal level we expect that nasal shedding would be present at day 7 post-inoculation (Pileri and Mateu, 2016; Chase-Topping et al, 2020), thus latent is embedded within the weekly time-step. To model the local transmission, we modified a gravity model to account for the effect of vegetation coverage surrounding each farm, previous showed to modulate between-farm transmission dynamics (Jara et al, 2020), see equations 2-4.…”

“…Currently, the industry relies heavily on vaccination and biosecurity practices as control and prevention strategies (Silva et al, 2019), with the vaccination of weaned pigs and gilts with modified-live vaccines (MLV) being among the most common (Kroll et al, 2018; Garcia-Morante et al, 2020; Rawal et al, 2020). Several studies have evaluated the efficacy of vaccines in increasing the immunity level and reducing shedding magnitude and duration (Pileri et al, 2015; Chase-Topping et al, 2020); however the absence of studies quantifying the effect of vaccination programs on between-farm PRRSV transmission, has limited our capacity to investigate the effectiveness of current vaccination strategies and the development of new, or the enhancement of existing PRRSV vaccination protocols (Andraud and Rose, 2020).…”

Porcine Reproductive and Respiratory Syndrome Modified Live Virus Vaccine: A “Leaky” Vaccine with Debatable Efficacy and Safety

“…For example, in two early studies, PRRSV-1 MLV vaccination could significantly reduce the R0 value (2.78 to 0.53 and 5.42 to 0.30, respectively), when inoculated with PRRSV-1 field strains with 93.4% or 92.7% of nucleotide similarity with the MLV, respectively [84,86]. In another study, an estimate of R0 for the vaccinated contact group was approximately 5.0, one half of that observed for the unvaccinated contact group (mode R0 = 10) [87]. Given the pig ages, MLV and inoculated virus were diverse, different models resulted in different R0 values.…”

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 ).…”