There are two main types of vaccines:
1. Killed vaccines
2. Live or modified attenuated live vaccines.
Killed vaccines
Killed vaccines have no ability to cause disease and contain the right protein (antigen) to get a protective immune response. In most cases, two doses (usually at least 3–4 weeks apart) are needed for these vaccines to be effective.
The first dose is referred to as the primary or sensitising dose, and produces a low antibody response over 7–21 days. The second, or booster dose produces a more rapid, stronger and longer lasting antibody response so the animal has lots of antibodies in their blood stream ready to nullify the antigens from either the organism or its toxin, hence preventing disease.
For vaccines that require two doses, the timing between doses is important. Most of the information on vaccines comes from the companies that develop them, and their goal is to identify the best protocol to get protection as soon as possible. Recommendations on vaccine packs usually refer to the minimum time between doses.
In reality, if animals have received the first dose some time in their life, and almost certainly if in the last six months, a second dose will still act as a booster and result in a strong, prolonged antibody response. However, the animal is not fully immune in the period between the first and second doses, and so animals are at risk of succumbing to a disease if only one dose is given.
Once the booster dose has been given, antibodies will reach their maximum levels within several days, and animals will be immune to the disease (in general).
Over time, the antibody levels in the blood fall, and at some point for most diseases these vaccine-induced antibodies fall below the protective level (see Figure 1). Therefore, while some antibodies are still present, there are not enough to immediately stop disease, and some animals may then get the disease.
For this reason, most vaccines require additional (usually annual) boosters. Clostridial vaccines are good examples of vaccines that require two initial doses and then a follow-up annual booster vaccination to maintain the highest level of immunity.
In contrast, Gudair®, the sheep vaccine to protect against ovine Johne’s disease, requires only one dose, despite being a killed vaccine. This is because the immune response works through the cellular, rather than the antibody, system.
Live or modified attenuated live vaccines
Live or modified attenuated live vaccines are developed from a weakened virus or bacteria, allowing it to replicate in the body and generate an immune response. Due to this process of pathogen replication promoting a protective response, many live or attenuated live vaccines do not require booster shots. While attenuated live vaccines do not usually cause disease, if disease is caused it is usually significantly milder than a strain caught through animal-to-animal transmission.
Scabigard® protects sheep against scabby mouth and is a live vaccine that ensures strong immunity with a single (scratch) vaccine. The animals develop the disease without effects approximately two weeks later and then develop immunity. While immunity is not lifelong, the likely continued exposure to scabby mouth virus over time results in ‘boosting’ and therefore annual boosters are not generally considered necessary.