Vaccinations are an important tool for maintaining and improving animal health. They are a prophylactic measure to prevent infection with pathogens.

In addition, regular vaccinations help to reduce the spread of zoonotic pathogens. They also remain an important part of animal disease control. For example, vaccination against Newcastle disease is mandatory in broiler chicken farming.

(Further information on Newcastle disease and strategies for combating it can be found here: tierseucheninfo.niedersachsen.de/anzeigepflichtige_tierseuchen/gefluegelseuchen/newcastle_krankheit/newcastle-disease-21656.html)

Looking ahead to the coming years, it can be assumed that vaccination programmes will become increasingly important. The use of antibiotics to treat infectious diseases is being discussed in relation to the production of residue-free food and is also being criticised in connection with the development of highly resistant germs.¹

The European Union is therefore calling for the targeted implementation of vaccination strategies in order to reduce the number of therapeutic treatments.¹

However, it is generally true that vaccinations alone are not sufficient to ensure animal health and thus animal welfare. Vaccinations should rather be seen as part of an overall concept in which hygiene and management also play an important role. Vaccination strategies must therefore be tailored to the specific situation of each individual farm and should be developed in close consultation with the veterinarian responsible for the herd.

Table 1 lists the diseases for which vaccination is usually carried out to protect animals from infection. Newcastle disease is the only disease for which vaccination is mandatory. 

* The information provided here is based on practical experience. As the timing of vaccination and the form of administration also depend in part on the vaccine used, you should always consult with the veterinarian responsible for your livestock. 

** Maternal antibodies are antibodies produced by the hen and absorbed by the chick in the egg before hatching. They provide protection against infection for a certain period of time after hatching. However, maternal antibodies are broken down over time, so the animal must produce new antibodies to be protected against infection. This antibody production is stimulated with the help of the vaccine. 

The aim of any vaccination is to build up vaccine protection in as many animals in a herd as possible. However, it should not be assumed that 100% of the herd will be protected against infection after a single vaccination. In every herd, there will always be animals that do not show an immune response after proper vaccination. These animals are referred to as "non-responders".

Multiple vaccinations can help to reduce the proportion of non-responders. However, broiler chickens are usually only vaccinated once due to the relatively short rearing period. Depending on the farm situation, multiple vaccinations may also be advisable.

Even when vaccination is successful, there are various situations that can lead to disease in the flock despite vaccination. In this case, we refer to a so-called vaccine breakthrough. The different scenarios are explained below:

The basic principle of vaccination is based on administering antigens of a pathogen to the animals in order to stimulate an immune response. After administration, the animal's body produces antibodies against the corresponding antigens, among other things.

However, pathogens can mutate in the field. If this adaptation of the field pathogen is accompanied by a change in the antigen structure, a vaccine breakthrough can occur despite vaccination. In this case, the vaccine antibodies can no longer bind to the surface of the field pathogen. They are therefore not neutralised and the animals fall ill despite vaccination.³

Vaccine breakthrough can also occur if a flock comes into contact with a field pathogen before the development of robust immunity is complete.³

In unfavourable cases, broiler chickens can fall ill even after successful immunisation. Stress caused by both infectious and non-infectious factors leads to a general weakening of the immune system.³ In this case, the neutralisation of infectious agents may be insufficient or fail to occur, resulting in the animals still showing symptoms. In addition to vaccination, stress avoidance is therefore an important factor in ensuring that animal health is maintained.

Depending on the properties of the vaccine used, it is administered in different ways:

• Live vaccines ⇒ Live vaccines contain pathogens that are capable of reproducing. However, these pathogens have had their disease-causing properties greatly reduced, so that the animals do not become ill after vaccination. Live vaccines are usually administered via drinking water or as a spray.² An exception to this is the Marek's disease vaccine, which is administered either under the skin or into the muscle (needle vaccination).
• Inactivated vaccines ⇒ Vaccines that contain inactivated pathogens that are unable to reproduce. They usually have to be administered by needle vaccination.² However, inactivated vaccines are not used in broiler chickens.
• In ovo vaccinations ⇒ In ovo vaccinations involve administering the vaccine directly into the egg around the 18th/19th day of incubation. This form of vaccine administration is becoming increasingly important in the rearing of slow-growing genetics. In addition to needle vaccination, Marek's vaccination can also be carried out as an in ovo vaccination before hatching.   

During the actual vaccination, regular inspections of the stalls should be carried out. This encourages resting animals to get up and visit the drinking troughs.⁴ Increasing the light intensity can also help to encourage the animals to drink water. Vaccine uptake should be monitored by observing animal behaviour and water consumption.

After vaccination, the water pipes should be flushed thoroughly again. This removes any vaccine residues from the pipes that could promote the formation of biofilms.

In order to achieve good vaccination success, the following factors must always be taken into account:

• The herd should be healthy at the time of vaccination.
• The veterinarian responsible for the herd must certify that the animals are fit for vaccination before the vaccination is carried out.
• Vaccines must be stored in a cool place (at 2–8°C) and protected from light before use. An exception to this is the vaccine against Marek's disease, which must be stored using liquid nitrogen.
• Care must be taken to ensure that the expiry date of the vaccine is not exceeded.
• Vaccinations must be carried out in accordance with the enclosed instructions for use.
• To avoid contamination of the vaccine, a container (vaccination bucket) should be available for mixing the vaccine, which is only used for carrying out vaccinations.

Furthermore, any stress factors should be avoided as far as possible after administration of the vaccine. Only in this way will the broilers have the opportunity to develop stable immunity and be sufficiently protected against disease during the rest of the fattening period.⁴

If vaccination is not carried out properly, there is a risk that the broiler chickens will only develop incomplete immunity. The following sources of error should therefore be³ ruled out in advance:

• Improper storage, e.g. the vaccine must not be allowed to freeze. Care must be taken to ensure that the cool bags containing the vaccine are not placed too close to the back wall of the refrigerator. 
• Inadequate cooling during transport of the vaccine
• Failure to observe the expiry date
• Incorrect and/or unclean vaccine preparation in stock
• Incorrect dosage of the vaccine
• Insufficient drinking water quality – the vaccine can be damaged by various factors: It must therefore be ensured that the physical and chemical parameters of the drinking water correspond to the target values and that the water does not contain any contaminants or algae. To ensure good water quality, the use of distilled water is recommended.
• The distances between the supply tank and the drinking troughs must not be too long.
• The drinking water must be free of cleaning, disinfectant or medicinal residues. Therefore, these should not be used two days before and immediately after vaccination.

In addition to animal hygiene and herd management, farm-specific vaccination programmes contribute to maintaining animal health and thus also to a higher level of animal welfare in broiler farming. However, the guidelines regarding storage and administration must be strictly observed. Only with correct handling can the animals build up sufficient immunity and thus achieve good vaccination success.