A study in Science found different vaccines used to control an infectious disease in chickens have been able to recombine to generate new virus strains. The resulting new viruses have been responsible for significant outbreaks of disease and death in farmed chickens.
Professor Paul Farrell, Virologist, Imperial College London, said:
“The authors argue that new pathogenic strains have been created by recombination of the attenuated vaccine strains of this virus. Their conclusion is that recombination between the vaccine strains has occurred. This is quite possible but a bit surprising since it would imply that both vaccines have gone into the same animal, which would be required for recombination to occur.
“To my mind, the paper does not take into account sufficiently the possibility that recombination may have occurred between the vaccine strains and normal virus strains circulating in Australia at that time. It seems much more likely that the vaccine would encounter the normal circulating virus. The normal virus(es) present in Australia are not included in their Fig 1A.
“Having said that, it is obviously the case that live attenuated vaccines have the potential for recombination. The use of vaccines is always a risk-benefit calculation. There can be a variety of side effects from vaccines; many are well known. For example, we stopped immunising people against smallpox because the adverse reactions began to exceed the benefit as the disease was eradicated.
“Vaccines have been one of the great success stories of medicine and the type of important technicality raised in this article should not be allowed to detract from the enormous health benefit generally provided by vaccines.”
Dr Mike Skinner, Virologist, Imperial College London, said:
“Evolution of novel variants of existing viruses is seen as a particular issue with poultry vaccination, where vaccines are routinely used as they are cheaper than antibiotics (and the latter are not permitted due to issues of residues in food and antibiotic resistance in humans). The vaccines are often used almost as a therapeutic to protect the particular flock from the ever present challenge of virus diseases (for the 6 week life of a broiler or the 2 years of a layer) rather than in a concerted way to try to eradicate the virus from the region. As a consequence, and just as with antibiotics, the global industry regularly sees variant viruses (sometimes recombinants or reassortants, sometimes just ‘simple’ mutants) resistant to existing vaccination, and then has to derive new vaccines specific for the variants.”
Prof Peter Openshaw, Director of the Centre for Respiratory Infection (CRI) at the National Heart and Lung Institute, Imperial College London, said:
“This is a highly plausible study, showing the importance of close monitoring and investigation of outbreaks of respiratory disease in livestock. Viruses are highly adaptable and will exploit every opportunity for self-improvement. We need to be constantly on the lookout for novel outbreaks and to intensify our study of the benefits and risks of well-intended vaccination programmes.”
‘Attenuated Vaccines Can Recombine to Form Virulent Field Viruses’ by Lee et al., published in Science on Thursday 12th July.