Modified live vaccines vs. killed vaccines


Vaccination

The aim of BVD control is to promote animal health, increase the profitability of the cattle industry and, in certain areas, to eradicate the disease.

BVD control programmes require a co-ordinated, systematic approach with determination of herd status, identification and removal of persistently infected animals and protection of the herd through vaccination and biosecurity measures.

Vaccines are primarily developed to prevent disease as a result of infection. The decision to use a vaccine is usually based on a risk-cost analysis i.e. if the risk that a herd will get infected is high, and the expected economic loss associated with the disease is also high, then the recommendation will be to vaccinate.

Considering the high prevalence of BVDV, a disease which causes high economic losses, vaccination of cattle against BVD is certainly indicated, provided efficacious and safe vaccines are available.

Vaccination provides artificially acquired active immunity. The goal of vaccination is to prime both humoral and cellular immune responses against pathogens without simultaneously causing disease.

Categories of BVD vaccine types available on the market are as follows:
 

Modified live vaccines

Live vaccines contain viable virus which is attenuated so that while it can still replicate, it does not cause disease. Modified live vaccines (MLVs) are convenient; some vaccines require only a one shot primary course to provide relatively long-lasting protection. The immune response is broad, providing both humoral (antibody-mediated) and, more importantly in terms of an animal’s immune protection, cell-mediated immunity1. BVDV strains exhibit wide antigenic diversity, live vaccines have demonstrated good cross-reactivity against heterologous BVD strains.

Conventional MLVs undergo multiple passages through cell cultures, in order to gradually alter the viral population resulting in attenuation (reduced virulence). There have been reports of MLVs in the USA reverting to virulence and causing the production of PIs following administration to pregnant cattle. A few modern MLVs are not attenuated by this traditional method; they are attenuated by unique, innovative methods in terms of vaccine development. In Europe, the safety and efficacy of modern live vaccines is rigorously assessed by the European Medicines Agency prior to granting marketing authorisation.
 

Killed vaccines

Inactivated (killed) vaccines contain virus which has been inactivated, usually by chemical treatment. One advantage to using a killed vaccine over a live vaccine is that the former can be safer. However, inactivated vaccines do not result in infection and therefore the immune response produced is not as complete as that which is stimulated by a live vaccine (particularly affected is the T cell-mediated immune response, active against virus-infected cells)2. Furthermore the immune response, and in particular the foetal protection, tends to be of shorter duration.

An adjuvant is typically needed in inactivated vaccines to increase the animal’s immune response to vaccination. Adjuvants have been associated with the adverse consequences of certain killed vaccines2.


 

References

1: Endsley et al (2003) Maternal antibody blocks humoral but not T cell responses to BVDV. Biologicals 31: 123-125.
2: Bastian et al. (2011) Vaccine 29(32): 5267–5275