Why does the protection afforded by some vaccines last longer than for others?

Question

After reading an answer to the question of How Do White Blood Cells Learn? Or Do They?, I came to wonder something. Specifically,

The effect of this is that every new B and T cell that your bone marrow makes will recognize a unique, random protein sequence, and you will have billions of these cells floating around in your blood circulation at any given time. These cells however will only float around in circulation for a short period of time before dying and being replaced by other cells with new random sequence receptors.

Assumption: What I gather from this (and the remainder of the answer) is that basically, a vaccine "teaches" the existing white blood cells in your body; eventually, these cells will be purged, so you will no longer be immune.

However, many vaccines seem to only be once-in-a-lifetime, or at least require a vaccine only once per decade (or less).

Some examples would be the Varicella vaccine, which can last up to 20 years, the Tetanus vaccine, which recommends a booster every 10 years, and the HPV vaccine, which lasts 5-6 years. Durations also vary with Hepatitis, Influenza, Polio, and numerous other common vaccines.

I would assume that, regardless of what a cell has "learned," it is purged with the same likelihood as any other cell; yet, it seems that some "learned" defenses can be retained longer than others. In essence, my question is: Why do some vaccines take longer to "wear off" than others?

_{(Note: This question is not a duplicate of Why do I need a flu shot every year, while many other vaccinations last years or even a lifetime?; I understand that some pathogens can mutate, so immune cells are of no use against mutated strains since they were not vaccinated against the mutation.)}

Answer 1

The first thing I want to make clear is that, the B and T that "will only float around in circulation for a short period of time before dying and being replaced", are not the ones that have already learned to recognise a certain diseases (or vaccine) but rather the naive cells that haven't yet done so. This is also made clear later in the same answer you cited:

As these injected protein sequences circulate through your blood they will eventually bump into a B or T cell that has a receptor that recognizes them. When this happens, the B or T cell will now start to divide and give rise to long lasting memory cells that express the exact same receptor and will therefore recognize the exact same protein sequence again in the future. These cells will float around in your blood for most of your life and make you 'immune' to the recognized pathogen.

In order to understand why some vaccines hold for your whole life, while some need a booster every few years we need to look at the life time or recycling of those memory cells, which stay in your body for much longer than the naive immune cells.

Now we come to the actual answer(s) to your question and there multiple factors that (can) have an influence on this:

• Initial strength of the immune response: Every vaccine (and/or disease) evokes an immune response with a different strength. Generally the stronger the first response is, the more memory cells will be created initially (those actually don't have correspond to the same molecular memory either, they might recognise different aspects of the vaccine/disease) and with more cells available it will take longer until none are left

• Natural exposure: memory cells start proliferating when they encounter the molecule (protein/virus/bacteria) which they have 'memorised', if there is an acute infection they will do so a lot in order to produce a lot of antibodies against it. However, even a very small amount of virus/bacteria, that wouldn't actually make you sick, will allow induce them to proliferate and preserve their numbers. Therefore memory cells against a more common or prevalent disease should be more stable in your body, than ones against a very rare one.

• There very likely more factors that either I or the whole scientific community don't really know about. Cellular systems and the immune system in generally are rather complicated ...

Answer 2

This issue is deep but one answer is bacteria/virus genetic variability. Some organisms have the ability to change their DNA in just a few generations (for example the influenza virus which requires a new vaccine every year). When you take a vaccine for influenza virus you expect that will protect you in a short period of time (winter season) because this virus is a nasty one, i.e it has the ability to present antigen variability due to mutation/constitution of its DNA and antigen is the source of power of vaccine (a vaccine assumes the organism has a antigen that could be a protein or other biomolecule that is relatively fixed between generations and is specific to that organism) so the vaccine loses its effect because the organisms are no longer susceptible which causes immune response to be weak (no longer recognizes the organism as it recognized) and you get sick. If the organism has slow mutation/variability the vaccine will last longer because the antigens are present for a longer period of time but eventually you will need to get another vaccine years later (for example tetanus).