Types of vaccines

Killed vaccines cannot 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 neutralise the infection or toxin, hence preventing disease.

For vaccines that require two doses, the timing between doses is important. Most of the information on the vaccine pack 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.

If animals have received the first dose some time in their life, and almost certainly 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 become ill.