A vaccine dose does not depend on body weight: the mechanism of action of vaccines is different to that of antibiotics and, as a result, the dose does not depend on the body weight of the target animal. When considering vaccines against coccidiosis in poultry, the dose is made up of a suspension of sporulated oocysts of different species of Eimeria.
In this suspension, the oocysts are not evenly distributed unless it is mixed thoroughly. If, in addition to this, the dose is reduced, the chance that the chicks will receive all the oocysts of every species decreases exponentially.
A vaccine does not have to be distributed throughout the body and the vaccine components (antigen and adjuvant) do not act directly on the pathogen. In general, the activity of vaccines starts with a rapid and local innate response depending on the route of administration.
Usually vaccines against coccidiosis in poultry are administered orally, thus the first innate immune response takes place in the gut. As a result of this first phase, antigen-presenting cells are activated (dendritic cells and/or macrophages) that are responsible for processing the antigen and carrying it to Peyer patches where they activate specific T lymphocytes, thereby starting the acquired immune response. As Eimeria is an intracellular parasite, the acquired immunity needed is of the cellular type. In this second phase, effector T cells are generated that produce cytokines ensuring a concentration in the tissues that is independent of the age/weight of the animal for the same dose of vaccine.
Does the same thing happen with human vaccines?
In human medicine, the vaccines that are used in children (paediatric vaccines) and in adults are the same and are used in the same dose. It would not occur to anyone to reduce a vaccine dose for children because of the risk of inadequate protection that this would entail. There are examples of toxoids (tetanus), inactivated vaccines (rabies) and live vaccines (varicella) where the dose is the same for children as for adults.
How is a vaccine dose determined?
The dose that is indicated in the leaflet for each vaccine is the one that has been shown to be effective during the different research phases in the target animal species and with the minimum age that can be vaccinated. On the basis of these studies, the health authorities in each country recommend the specified dose for each product. For this reason, the dose should be administered irrespective of the age of the animal.
There are some older products that have different doses for different age groups, but this would not be allowed nowadays.
What harm can be done by a dose that is lower than the recommended one?
Administration of a vaccine dose that is lower than the recommended one can result in a lower immune response, compromising the protection of the individual against the pathogen. Specifically, with vaccines against coccidiosis in poultry, if a full dose is not administered, there is a very strong possibility that there will be an uneven administration of both oocysts and Eimeria species amongst the vaccinated animals with a consequent lack of onset of immunity in some of the chicks.