Vaccines Combine to Form Infectious Virus
By on July 14th, 2012

Two vaccines that are used to prevent diseases in poultry have found to have combined to produce a new infectious strain of the virus that they were designed to protect against.

Weak Viruses Used as Vaccines

Infectious laryngotracheitis is a respiratory disease in poultry that can kill as many as 1 in 5 infected birds. To combat this disease, farmers in Australia have been using what are called live-attenuated vaccines. This means that the vaccines are a weak form of the virus which cannot cause infections but can still be recognized by the chicken’s immune system so that the chicken is protected from future infections.

In 2006, a new European vaccine which contained a different strain of the virus was introduced into the country. A year later, a new, more infectious, strain of the virus became dominant in the Australian poultry industry. Scientists at the University of Melbourne have sequenced these new viral strains and found that they have been formed by a combination of the European and Australian strains used in the two vaccines.

How two viral vaccines could combine to form a new deadly strain. Image Credit: Australian Science Media Centre

How two viral vaccines could combine to form a new deadly strain. Image Credit: Australian Science Media Centre

When Vaccines Become the Enemy

When a virus infects our body, it enters the cells in our body and uses the machinery inside the host cell to produce more copies of itself. In this case, two viral strains must have entered the same cell, leading to a process called DNA recombination between the DNA sequences of the two viruses. Thus, a new strain with a new genome was formed.

“While recombination has been recognised as a potential risk associated with live virus vaccines for many years, the likelihood of it happening in viruses like this in the field has been thought to be so low that it was considered to be very unlikely to lead to significant problems,” said Professor Glenn Browning, who was a part of this research.

The herpesvirus causing the disease is not infectious in humans. However, live herpesvirus vaccines for other diseases are widely used in humans and other species. This science-fiction-like incidence of viral recombination is definitely something that should make scientists and health-care professionals sit up and examine the possible repercussions of a poorly implemented vaccine.

Author: Shweta Ramdas
Beginning life as a grad student studying human genetics.

Shweta Ramdas has written and can be contacted at

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