Vaccines and vaccination principles
How vaccines work, booster responses, live, inactivated, subunit and vector vaccines, DIVA, cold chain, schedules, herd immunity and efficacy sums.
On this page
Vaccination is among the most cost-effective ways to protect livestock and poultry from many infectious diseases. It made the global eradication of rinderpest possible, keeps Newcastle disease out of most well-managed commercial poultry flocks, protects dogs and people from rabies, and is central to controlling foot-and-mouth disease and PPR. But vaccines only work well if they are the right type, stored and given correctly, at the right age and interval, to enough animals in the population.
Vaccine types, immune responses and vaccination programmes are core topics in veterinary immunology, preventive medicine and livestock management.
How vaccines work
A vaccine presents the immune system with an antigen, part or all of a pathogen made harmless, so that the body develops memory. When the real pathogen arrives, the immune system responds faster and more strongly, preventing or reducing disease.
- Humoral immunity: B cells produce antibodies (IgM first, then IgG, and IgA at mucosal surfaces).
- Cell-mediated immunity: T cells kill infected cells and coordinate the response, essential against viruses and intracellular bacteria.
The primary response after the first dose is slow and modest; a booster some weeks later produces a much stronger secondary (anamnestic) response. This is why many vaccines need two doses at the start and periodic boosters.
Passive immunity: colostrum
Vaccination gives active immunity, made by the animal itself. Newborn calves, lambs, kids and piglets are born with almost no antibodies, because the placenta of these species does not let them pass, so they depend on passive immunity from colostrum. A calf needs good-quality colostrum, often about 10 per cent of its body weight, within the first few hours of life, because the gut's ability to absorb antibodies falls sharply over the first day. Vaccinating dams before birth raises colostral antibodies against diseases such as calf scours and clostridial infections.
Types of vaccine
| Type | Description | Examples | Notes |
|---|---|---|---|
| Live attenuated | weakened pathogen that replicates without causing disease | PPR, rinderpest (historic), many Newcastle disease strains, brucellosis S19 | strong, long immunity, often one dose; need cold chain; rare reversion risk |
| Inactivated (killed) | killed whole pathogen, usually with an adjuvant | foot-and-mouth disease, rabies, clostridial vaccines | safer, but need boosters; adjuvants enhance response |
| Toxoid | inactivated toxin | tetanus, enterotoxaemia (clostridial) | protects against toxin, not the bacterium |
| Subunit and recombinant | purified proteins or virus-like particles | some swine and poultry vaccines | very safe; may need adjuvants |
| Vector vaccines | harmless virus carrying genes of the target pathogen | turkey herpesvirus-vectored vaccines in poultry | can protect against two diseases |
| Nucleic acid (DNA, mRNA) | genetic instructions to make the antigen | emerging in veterinary use | rapid development |
DIVA (Differentiating Infected from Vaccinated Animals) or marker vaccines allow tests to tell vaccinated animals from infected ones, important for disease-free trade status and eradication campaigns.
Why vaccines fail
- Wrong strain or serotype, as with foot-and-mouth disease.
- Broken cold chain: most vaccines must be kept at 2 to 8 °C; live vaccines are damaged by heat and sunlight, and some by freezing.
- Maternal antibody interference: antibodies from colostrum or egg yolk can neutralise vaccines given to very young animals.
- Wrong route, dose or schedule, such as missing the booster.
- Poor immune status: malnutrition, parasites, stress, immunosuppressive diseases.
- Already infected or exposed before immunity develops.
- Chlorinated water or disinfectants inactivating live vaccines given in drinking water.
Vaccination programmes
- Schedules set the age, interval and boosters, adapted to local disease risk and maternal antibody levels.
- Routes: injection (intramuscular, subcutaneous), intranasal, eye drop, drinking water, spray, in ovo (into eggs) and wing web for poultry.
- Herd immunity: when enough animals are immune, transmission stops, protecting even unvaccinated individuals; coverage needed is at least 1 − 1 ÷ R0 of the population immune.
- Records of batch numbers, dates and animals vaccinated, and monitoring of response by serology.
- Combining vaccination with biosecurity, good nutrition and parasite control, since vaccines rarely give complete protection.
Measuring vaccine efficacy and planning doses
Efficacy. During an outbreak, 40 per cent of unvaccinated calves and 8 per cent of vaccinated calves fall ill.
Vaccine efficacy = (attack rate unvaccinated − attack rate vaccinated) ÷ attack rate unvaccinated
- = (40 − 8) ÷ 40 = 0.80, or 80 per cent.
Doses. A district plans to vaccinate 2,000 goats against PPR. Vaccine comes in 50-dose vials, and once opened a vial must be used within a few hours, so field wastage is expected to be about 15 per cent.
- Doses needed = 2,000 ÷ (1 − 0.15) ≈ 2,353 doses.
- Vials = 2,353 ÷ 50 ≈ 48 vials, kept in cool boxes with ice packs throughout.
If wastage could be cut to 5 per cent by grouping animals at vaccination points, only about 2,105 doses (43 vials) would be needed.
Key points
- Vaccines create immune memory; boosters trigger faster, stronger secondary responses.
- Types: live attenuated, inactivated, toxoid, subunit and recombinant, vector and nucleic acid; DIVA vaccines allow infected animals to be distinguished.
- Failures come from wrong strains, broken cold chain (2–8 °C), maternal antibodies, wrong schedule, poor health and prior infection.
- Herd immunity requires at least 1 − 1 ÷ R0 immune.
- Efficacy = (ARU − ARV) ÷ ARU; plan doses allowing for wastage.
What the exams ask
- School-leaving and entrance papers: what vaccines are and vaccination schedules for common livestock and poultry diseases.
- Degree and research-fellowship entrances: immune responses, vaccine types and their merits, adjuvants, DIVA, causes of vaccine failure, and routes.
- Veterinary and professional examinations: designing vaccination programmes, cold chain management, efficacy evaluation and herd immunity.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do young animals need several doses?
Maternal antibodies from colostrum or yolk may block early doses, and their level varies between animals, so repeated doses make sure each animal is vaccinated once those antibodies fall.
Can a vaccinated animal still get sick?
Yes, if the vaccine strain does not match, immunity has waned, the dose was spoiled, or the challenge is overwhelming. Vaccination reduces risk and severity but is not perfect.
Why must vaccines be kept cool?
Heat inactivates many vaccines, especially live ones, so a vaccine left in the sun can become useless without any visible change.
Sources
- Tizard, I. R. Veterinary Immunology, 10th edition. Elsevier, 2018.
- Tizard, I. R. Vaccines for Veterinarians. Elsevier, 2021.
- World Organisation for Animal Health. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. WOAH, updated annually.
- Roth, J. A. "Veterinary vaccines and their importance to animal health and public health." Procedia in Vaccinology 5, 127–136 (2011).
- Meeusen, E. N. T., Walker, J., Peters, A., Pastoret, P.-P. and Jungersen, G. "Current status of veterinary vaccines." Clinical Microbiology Reviews 20, 489–510 (2007).
More in animal science and veterinary
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.