Vector vaccines use an altered virus or bacterium (the 'vector', or carrier) to introduce microbial DNA from disease-causing organisms to cells of the body so that the animal develops immunity.

When the vector replicates in the vaccinated bird, it will complete its normal life cycle and at the same time will express immunogenic proteins encoded by the inserted foreign gene or genes.

Vectors selected generally contain large genomes so they can 'accept' foreign genes relatively easily and in a stable manner.

For example, DNA viruses such as fowl poxvirus (FPV) and herpes virus of turkeys (HVT) are very suitable viruses as vectors. Fowl adenoviruses (FAdV) are also suitable but have not been exploited commercially in the poultry vaccine field.

On the RNA virus side, avian viruses with large genomes such as Newcastle disease virus are attractive alternatives but as will be discussed, have some significant disadvantages under certain circumstances.

Recombinant vector vaccines often closely mimic a natural infection and therefore can do a good job of stimulating the immune system.

Field experiences with recombinant vaccines

Although as a whole most recombinant vaccines have rendered good results in the field, some have performed better than others on a relative basis. Following are some examples of field and laboratory performance of recombinant vaccines.

Infectious bursal disease (IBD) recombinant vaccines

IBD recombinant vaccines are perhaps the prime example of a recombinant product that has performed well and very consistently.

Poultry operations that are used to registering premature bursal atrophy consistently (at less than 24 days of age) suddenly realise that with the use of recombinant IBD vaccines the integrity of the bursa of Fabricius improves very substantially and economic performance improves in most cases, albeit not in every circumstance.

Overall, recombinant vaccine use in broilers consistently provides good results in terms of IBD control and economic performance.

In commercial layers the use of recombinant vaccines at the hatchery has replaced in many cases (but not in all) the need for multiple vaccinations against IBD in the field.

In some isolated circumstances where very virulent IBD virus is a problem, there might be a need to combine the use of recombinant IBD vaccines with field application of conventional live attenuated virus vaccines.

Infectious laryngotracheitis (ILT) recombinant vaccines

Where legal and available, ILT recombinant vaccines have become popular for broiler vaccinations.

Fowl pox- and HVT-vectored vaccines are commonly used in boilers because these vaccines can be applied in the hatchery and generally there is no need for field boosting with other products.

In addition, live attenuated vaccines are no longer authorised for use in some countries and thus, the only resource that industry can use is recombinant vaccines.

Although the use of these products has become very popular, protection is still not as efficient as what is accomplished with live attenuated vaccines.

It is also important to underline the fact that most outbreaks of ILT occur after 30 days of age and that many operations around the world commercialise their broilers at 38-42 days of age.

Therefore, if ILT occurs and it happens after 30 days of age, in most cases mortality will not reach high levels and the affected broilers will be marketed without major losses, whether the recombinant vaccines protected satisfactorily or not.

Avian influenza vaccines

Nobody expects recombinant vaccines against avian influenza to be fully protective in situations where field challenge is overwhelming, but anybody should understand that the combined use of recombinant fowl pox and avian influenza virus, with killed vaccines or other types of vaccines, can provide very attractive economic results.

As of today, no vaccine against avian influenza is considered to be fully protective but the mixed use of recombinant vaccines along with killed vaccines usually provides satisfactory results.

Newcastle disease vaccine

There are recombinant vaccines where the vector is Newcastle disease virus and the insert is an avian influenza virus protein. There are also recombinant vaccines where the inserted gene makes a protein from Newcastle disease virus and the vector is turkey herpes virus or fowl pox virus.

Of these, the most commonly used recombinant vaccines are HVT/NDV. These products are used under two very different philosophies:

1. Recombinant Newcastle disease virus vaccine for areas where infectious bronchitis is the priority. In countries like the US where infectious bronchitis is more significant than Newcastle disease, these vaccines are used to provide a low level of protection against Newcastle disease without interfering with infectious bronchitis vaccines.
2. Recombinant Newcastle disease virus vaccine for areas where Newcastle disease virus is the priority. In countries where Newcastle disease is the priority, these vaccines are used to prime birds against Newcastle disease, but the programme is always (or should always be) complemented with conventional live and/or killed vaccines.

Other recombinant vaccines are also used in the field. In general, it is important to consider that for some recombinant vaccines, early protection is not very robust and it might take 3-4 weeks for the recombinant vaccine to reach its full potential.

On the other hand, recombinant vaccines present multiple advantages that conventional live attenuated vaccines don’t and vice versa.