Booster dose

In medicine, a booster dose is an extra administration of a vaccine after an earlier (prime) dose. After initial immunization, a booster injection or booster dose is a re-exposure to the immunizing antigen. It is intended to increase immunity against that antigen back to protective levels, after memory against that antigen has declined through time. For example, tetanus shot boosters are often recommended every 10 years, after which memory cells specific against tetanus have lost their function or undergone apoptosis.

The need for a booster dose following a primary vaccination is evaluated in several ways. One way is to measure the level of antibodies specific against a disease, a few years after the primary dose is given. Anamnestic response, the rapid production of antibodies after a stimulus of an antigen, is a typical way to measure the need for a booster dose of a certain vaccine. If anamnestic response is high after receiving a primary vaccine many years ago, there is most likely little to no need for a booster dose. People can also measure the active B and T cell activity against that antigen after a certain amount of time that the primary vaccine was administered, or determine the prevalence of the disease in vaccinated populations.

If a patient receives a booster dose but already has a high level of antibody, then a reaction called an Arthus reaction could develop, a localized form of Type III hypersensitivity induced by high levels of IgG antibodies causing inflammation. The inflammation is often self-resolved over the course of a few days, but could be avoided altogether by increasing the length of time between the primary vaccine and the booster dose.

It is not yet fully clear why some vaccines such as hepatitis A and B are effective for life, and some such as tetanus need boosters. The prevailing theory is that if the immune system responds to a primary vaccine rapidly, the body does not have time to sufficiently develop immunological memory against the disease, and memory cells will not persist in high numbers for the lifetime of the human. After a primary response of the immune system against a vaccination, memory T helper cells and B cells persist at a fairly constant level in germinal centers, undergoing cell division at a slow to nonexistent rate. While these cells are long-lived, they do not typically undergo mitosis, and eventually the rate of loss of these cells will be greater than the rate of gain. In these cases, a booster dose is required to "boost" the memory B and T cell count back up again.

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