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Vaccination: many parameters to take into account

Vaccination is one of the main strategies for controlling infectious diseases. The protection provided can take various forms: protection against infection, against transmission of the pathogen if infected, or even only against symptoms; permanent or temporary protection; total or partial protection depending on the effectiveness of the vaccine; vaccine that is easy to package and administer, etc. Constraints may also be social (adherence to vaccination, accessibility of the vaccine) or financial. The latter is particularly important for livestock that are raised for production and profitability. It is in a farmer's interest to vaccinate animals if their welfare is at stake and if the cost of vaccination is less than the cost of the production and welfare losses expected without vaccination. Knowing when it is appropriate to vaccinate is a difficult question, as the conditions required may vary according to the characteristics of the farm and over time depending on the exposure to the risk of infection.

Return on investment after vaccination against BVD according to the farming system and the level of risk of virus introduction from the neighbourhood. The bars represent 80% of the values obtained for each scenario repeated 1000 times, with the median in the middle. The results are variable, as many events are random.

Bovine viral diarrhoea, a disease that causes significant economic losses

Within the BIOEPAR Joint Research Unit (INRAE-Oniris), and in partnership with GDS-Bourgogne-Franche-Comté (Burgundy-Franche-Comté health protection group) and the Institut de l'Élevage (French livestock institute), scientists from the DYNAMO team have contributed to answering this question in the case of an endemic bovine disease: bovine viral diarrhoea (or BVD). Despite numerous regional or national programmes set up to control this disease, the BVD virus is still circulating, particularly in France, and causes significant economic losses for farmers, especially following its introduction into an uninfected herd. This virus is transmitted by contact between animals in the same herd or with those of neighbouring herds, but can also be introduced into the population by the purchase of an infected animal. The animals to be vaccinated as a priority are breeding females, as infection during pregnancy can result in the birth of persistently infected calves (PI), which excrete the virus heavily throughout their lives and die prematurely. The period of greatest risk for virus introduction into a herd will therefore be when pregnant cows are grazing, exposed to the risk of transmission from neighbours. The way in which the herd is managed influences the risk of virus introduction and circulation, depending on the timing of breeding and grazing periods, and whether or not there is contact between cows (at risk of infection) and calves (among which most IPFs are found).

The model simulates the spread and allows the economic benefit of vaccination to be quantified

A simulation model of the spread of BVD virus in a suckler herd was developed to assess the economic benefit of vaccination. It takes into account the characteristics of the infection (transmission rate, abortions, IPF mortality, etc), the vaccination (probability of protection against infection, against symptoms, etc) and the farming system (calving and grazing periods, number of females for breeding, weaning date, etc). The influence of the rearing system was tested by considering three systems representative of a large proportion of French suckler farms, grouped into three basins. Surprisingly, even with similar rearing systems (suckler herd with a calving season on pasture), vaccination is economically interesting in the Charolais basin, but not in the Limousin and Midi-Pyrénées Languedoc-Roussillon basins (figure). This result was observed regardless of the level of risk of virus introduction from the neighbourhood. Work is in progress to determine which characteristics of the farming system influence these observations. In view of the progress made in eradicating BVD in France and the evolution of surveillance towards serological screening in several regions, it is important to take into account the general decline in prevalence and the risk of interference of vaccination with screening.

This work highlights the importance of taking into account the specificities of each farming system when deciding whether or not to vaccinate. The DYNAMO team has developed advisory tools based on these research models.

Related publication : Arnoux, S., Bidan, F., Damman, A., Petit, E., Assié, S., Ezanno, P., 2021. To Vaccinate or Not: Impact of Bovine Viral Diarrhoea in French Cow-Calf Herds. Vaccines 9, 1137. https://doi.org/10.3390/vaccines9101137

Scientific partners : GDS-Bourgogne-Franche-Comté - Institut de l’Élevage

Funding :  This work was funded by the Agence Nationale de la Recherche (ANR), Programme Investissements d'Avenir, project ANR-10-BINF-07 (MIHMES), the Fonds Européen de Développement Régional (FEDER Pays-de-la-Loire), the European Union Horizon 2020 research and innovation programme under the grant agreement No.101000494 (DECIDE), and INRAE. This work was carried out using the HPC resources of GENCI-[CINES/IDRIS/TGCC] (grant 2021-[A0100312468]).