Vaccination strategies aim to generate long-lasting specific immune responses by efficient activation of the immune system’s memory. Booster vaccines are used to ensure that every individual vaccinated produces, or is still producing, an effective immune response.
Vaccination and Immune Memory
Immune responses arise from the activation of cells that recognise that specific challenge, the pathogen, whether that’s the influenza virus causing flu or bacteria infecting a cut. Each effector cell (or lymphocyte) of the immune system is specific to that challenge; it’s estimated that the human body contains as many as 10 trillion lymphocytes. A number like that, 12 million million, is way more than is likely to be needed in a lifetime, but is essential to ensure that there will be a lymphocyte capable of responding to any given infection.
On first encountering a foreign organism, either the pathogen itself or in the form of a vaccine, the immune system gears itself up by expanding that pool of specific lymphocytes. Once there are sufficient responder cells activated, the primary antibody responses and killer T lymphocytes etc eliminate the pathogen.
When the individual encounters that pathogen again, the expanded population of specific cells are ready and waiting. The secondary immune response is quicker and more effective than the primary response.
Vaccination activates the immune response in the same way as the natural infection would but avoids the often nasty symptoms and potential long term consequences of the illness itself.
Why are Booster Vaccinations Necessary?
The effects of most vaccines are long-lasting. Circulating antibodies only exist at high levels for weeks, perhaps months, but a pool of turned-on effector T lymphocytes and antibody-producing B cells are present. These remain at high enough levels, ready and waiting to mount an efficient response on re-encounter with a pathogen.
The levels of protective immunity can last for years or even decades after the first immunisation, depending on the type of vaccine administered. Antibodies are not the whole story with the body’s elimination of a pathogen but they can be used as a marker of the state of readiness of the immune response. Low levels of circulating antibodies are used as a clinical indicator that a booster is required, although protective immune memory may still persist (Van Damme et al, 2003).
A small proportion of individuals do not seroconvert when immunised with a vaccine; they do not produce antibodies and a second vaccination is required to kick start the immune system. It is usually too costly and time consuming to test everyone after the first attempt. Therefore a second vaccination is recommended with many vaccines, for example, the triple MMR (measles, mumps and rubella) vaccine used in the UK.
As with all vaccination programs, achieving high levels of herd immunity is necessary to control the spread of infections. Booster injections ensure that individuals are themselves protected and immunity in the population, as a whole, is at the required levels.
Sources
Van Damme P et al (2003). Hepatitis A booster vaccination: is there a need? The Lancet, Vol. 352, p 1065-1071.
Catherine Whitlock - After some fun years in medical research poring over a microscope, I now write about the scientific wonders that others have magnified.I ...
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