Chickens are susceptible to many different diseases, especially when they are young. Instead of focusing on treating diseases if chickens get sick, it is much more effective to prevent diseases in the first place. One way to do that is to vaccinate a flock against common infectious diseases.
A vaccine works by helping a bird to build up a defense against specific diseases that they may be exposed to during their life. Vaccines help to prepare the immune system by encouraging it to create antibodies in advance. That way if the bird ever faces the actual disease, they will be protected. But vaccines only work if they are given correctly using the proper technique and the right dosage.
Vaccines can be delivered in many different ways. Some vaccines are given to the whole flock at once, like spray vaccines. Spray vaccinations may be ingested or inhaled by the birds. Typically, a coarse spray has larger droplets of liquid that coat the birds, and is ingested when the birds preen their feathers. Fine sprays are inhaled by the birds and travel deep into their lungs. One challenge with spray vaccinations is making sure the amount of vaccine is distributed consistently on the birds so they are equally protected. The person delivering the vaccine would walk through the flock several times, spraying the vaccine in a fanning motion, to try and get the most even coverage (see image below).
Vaccinations can also be delivered to each bird individually. That means every single chicken has to be picked up, handled, and vaccinated in order for them to be protected. This technique is time consuming and is only used when it is the best option for keeping the flock healthy. Just like people can receive vaccines in different parts of the body, like their muscle or blood vessel, so can chickens! Some methods used to vaccinate individual chickens include using an eye dropper, puncturing the wing web (bottom right), or injecting into the leg (bottom left) or breast muscle.
Vaccines only work if they are given properly, so it is crucial to conduct routine checks for accuracy. For example, if a bird was given an eye drop vaccine with blue coloured dye in it, her tongue will turn blue for a few minutes afterwards. So looking at a bird's tongue is an easy way to see if they got the vaccine -- cool eh! (below left photo). Even doing simple checks like making sure you're using the right vaccine, and is hasn't expired, are important. Finally, looking at the placement of the vaccine injection (on the wing, for example) can tell you if it is being delivered correctly. Doing this kind of evaluation in real time can give the vaccine crew crucial feedback so they can adjust their technique.
Vaccinations are just one part of a rigorous Biosecurity program that is designed to help protect a flock from infectious disease.