Background 6077 views update:Mar 9, 2016

Vaccination against AI

The avian influenza threat has changed recently, disrupting trade, inducing high levels of mortality and causing enormous economic losses. This has changed the discussion on whether to vaccinate or not. Dr Yannick Gardin of Ceva: “There are many dogmas surrounding the control of Avian flu. Countries who decided to vaccinate were viewed as the bad countries, this label is no longer appropriate.”

Until now, 2015 has seen 309 individual outbreaks of Avian Influenza reported to the World Organisation for Animal Health. This is a staggering 147% increase on the outbreaks reported in 2014. In the US alone, during the last six months, 48 million domestic poultry birds have been culled in 15 states. It has been the worst epidemic of HPAI ever in the US and, despite the outbreak seemingly being under control, there are fears that AI could reappear at any time.

Considering the suddenness and the gravity of this epizootic, many questions have been raised regarding actions to be implemented to control the disease, including interest in using vaccination as a tool to complement measures put in place. It is still believed, however, that biosecurity and sanitary police are sufficient to control the situation and that vaccination is not necessary. It has even been said that vaccination would make things worse by helping the disease become endemic, as happened in some countries in Asia and the Middle East, with a special focus on China, Indonesia, Vietnam and Egypt.

To some extent, the view that vaccination against AI should be banned has actually become a dogma.

New situation

But the situation has changed. The biology of the AI viruses affecting the US is different. The structure of the poultry industry is different. The available technical and financial means are different and we have learned lessons from the past. This all means that today we know better why AI vaccination in Asia and the Middle East was ineffective or poorly effective in eradicating the disease. And, last but not least, a vector vaccine which has recently been developed in the US and introduced on the market has shown properties and potential that make it very different from the old vaccines that are still in use in many vaccinating countries, especially with respect to efficacy and the possibility of monitoring the disease situation in the presence of vaccination. Times have changed. The decision to vaccinate is a tough one. Important information and facts have to be considered before casually discarding the vaccination option or blindly applying it.

Pro’s and cons of vaccine categories

As of today, only 2 categories of AI vaccines are commercially available for poultry, the ‘classical’ killed (inactivated) adjuvanted AI vaccines and the recombinant vector AI vaccines. All of these vaccines, killed or live recombinant vectors show significant differences when it comes to production, route of administration, safety, quality of induced immunity, as well as capacity to overcome the Maternally Derived Antibodies (MDA) against AIV present in the day-old chicks or day-old poults when the breeders have been vaccinated. For recombinant vectors, interference from MDA directed against the vector also needs to be considered. Another important point, and probably the most critical one to consider in case of vaccine prevention against AI, is the capacity of the vaccine to cover the continuous antigenic variations of the virus. A summary of the advantages, drawbacks and limitations of the various types of AI vaccines is presented in Table 1.

In a country or a territory hit by AI, vaccination is primarily a tool to prevent clinical and economic losses due to infection with AIV. It also serves to complement sanitary and biosecurity practices to achieve eradication and recover the AI free status. If it is the sole means used to combat the disease, vaccination cannot lead to eradication. This is why vaccination against AI needs to be organised, co-ordinated and accompanied with disease monitoring and eradication plans. In a country free of AI, vaccination can also be used to lower the risk of being hit and slow the spread of the virus once a farm is affected.

Recombinant vector vaccines

The recombinant vector AI vaccines are the new kid on the block, changing perspectives on using vaccination in AI prevention and control. These vaccines are made from a live at-tenuated virus or bacteria (the ‘vector’), inside the genome of which a gene (the ‘insert’), encoding for the major immunogenic part of AIV (the HA), has been inserted. When it replicates in the birds, the vector expresses the HA, which creates immunity (protection). The HA gene is the only insert that has been used so far, and the different types of recombinant Avian Influenza vector vaccines are abbreviated as rFP-HA, rNDV-HA and rHVT-HA. A number can be used to indicate the subtype of the donor AIV. rHVT-HA5 indicates that the insert comes from an AIV of the H5 subtype.

It is evident that, as of today, the most attractive vaccine solution is the rHVT-HA5. Compared with killed vaccines, it can be used in the hatchery on one-day-old chicks, even in the presence of specific MDA, and shows a long duration of immunity. Its efficacy seems not to be affected by the antigenic variations of the field virus, which is the major weakness of the killed vaccines. rHVT-HA5 has a proven record of efficacy against significant variations of HPAIVs of the H5 serotype and does not require booster vaccinations. Compared with other vectors, it circumvents MDA (when the rFP-HA is neutralised by MDA against AIV and the rNDV-HA by MDA against the vector NDV).

One of the important advantages of vaccination in the hatchery, is to eliminate the well-identified risk of vaccination crews transmitting AI as they go from farm to farm. [Photo: Henk Riswick]

Risks of AI vaccination

Even when an efficacious vaccine is used, the risks identified with AI vaccination are still the same as the ones already 
identified for other vaccinations against the Newcastle disease, Infectious bronchitis, Marek’s disease or any other diseases.

Vaccination is a powerful aid in controlling infectious diseases but can never be a stand-alone solution. Most of the time, vaccines protect against the disease, i.e. the consequences of infection, not against the infection itself, which means that the field virus can still infect, replicate and be re-excreted. There are only very few veterinary vaccines that have the capacity to totally block the pathogenic agent, which is scientifically called ‘sterile immunity’. Even if limited or reduced, shedding the challenging infectious agent is always possible. Thanks to vaccination, the risk is lower but not totally eliminated. Biosecurity programmes always need to be present and strong, bearing in mind that the best protection against any kind of challenge is simply to avoid it in the first place.

Poor quality of vaccine application

At the level of a flock, there are always birds that are missed during the vaccination process. This is generally acceptable in the case of an epizootic disease like Avian Influenza (or Newcastle Disease) if the percentage of missed birds remains low (less than 3-4%) and if the challenge pressure is not constantly applied to every chicken (as it is in the case of the Infectious Bursal Disease or Marek’s disease).

Vaccination at the hatchery is much easier to control and, consequently, far more reliable than any other type of vaccination, particularly vaccination at the farm, which is why it is preferred (18). But this does not mean that big mistakes cannot be made. There are flocks in which as many as 50% of improperly vaccinated chickens can be detected, so monitoring and assessing the vaccination is necessary.

When it comes to using a vaccine like Vectormune AI, it is particularly important to bear in mind that this vaccine does not spread, so if not vaccinated, a chicken will never be protected by the vaccine given to its hatch mates. So all the critical steps of the vaccination process, including storage, distribution, reconstitution and injection, need to be regularly monitored. One of the important advantages of vaccination in the hatchery is it eliminates the well-identified risk of vaccination crews transmitting AI as they go from farm to farm.

The factors of a good immune response

Vaccination stimulates the immune system to produce an immune (protective) response. If the immune system is compromised at the time of vaccination or later, then protection is also compromised. This is why vaccination works so well at some farms and not so well at others. The quality of day-old chickens or day-old poults, housing, feed, the environment, farming, ventilation, as well as control of intercurrent infections, are also critical factors for the success of vaccination.

Vaccination old style

The fact that Avian Influenza is still endemic in countries where AI vaccination has been used is often presented as an argument against vaccination. In fact, this situation is the result of many factors, including the following main ones: In all countries, the AI vaccines used were mostly, if not exclusively, the classical inactivated types. For this reason, they were unable to ensure good vaccine coverage for the flocks by injecting vaccines at the hatchery. And, if hatchery vaccination was in place, interference by MDA neutralised their action. They couldn’t answer the key problem of the field virus evolving or maintain a steady level of efficacy, whatever the challenging virus. Vaccination was implemented more or less as a stand-alone solution, without comprehensive monitoring of the infection. Also, dedicated technical and financial means were generally poor, so that no real co-ordinated action plan and, in particular, no compensation system for farmers affected by the disease were introduced, which meant that the farmers often tried to save the surviving birds by bringing them to the market and, by doing so, helped the disease to spread.

The future of AI vaccination

Avian Influenza has dramatically changed in a few years. It is now induced by more types of virus and is present in more countries. These more recent viruses are also better “adapted” to wild waterfowl populations. They do not behave like HPAIV in wild waterfowl, which means they can be carried over much longer distances. This unfortunately is helping the spread of the disease. It has also changed our view of it and how to control it. The risk is now much higher and it is the right time to forget about the old dogmas and adapt to this new situation.

Fortunately, a new vaccine has been developed that can answer most of the objections we had against vaccination. Many experiments have been conducted with it and it has demonstrated a strong capacity to protect against a wide variety of different H5 type HPAIV and to overcome the presence of MDA and be used reliably at the hatchery. These features did not exist for the classical vaccines used up to now. It is believed that, due to this new rHVT-HA5 vaccine, vaccination can no longer be neglected, but needs to be considered as a real tool to protect the poultry industry against clinical and economic losses without impairing the implementation of a truly efficacious disease monitoring system aimed at eradication. Recent experiments have demonstrated the efficacy of this vaccine against circulating HPAIVs of the H5 serotype.

It is also believed that more funds should be dedicated to AI vaccine research. We cannot only continue attempts to understand the rain and the ways to control it. It is now time to work more intensively on umbrellas.

References available upon request.

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