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When being vaccinated we are given viruses that have been killed or made 'not working' in some other way. Then we get B and T cells that can 'kill' it.

For example, for polio, memory B and T cells produced in response to the vaccine persist only six months, while tetanus disease requires a booster dose every 10 years, and chickenpox last a lifetime.

So I am assuming the B and T memory cells are being lost for polio faster than for tetanus. I am also assuming that the memory cell losing rate is related to the nature of the virus.

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What makes some viruses being 'easily forgotten' and others to be 'remembered' for a lifetime?

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For fast and easy accessible comparison of different vaccines and how long they last see

  1. Centers for Disease, Control and Prevention Vaccines and Preventable Diseases - List of vaccines used in the United States.

  2. The Immunisation Advisory Center Duration of protection by vaccine - There is a list of "Estimated duration of protection from vaccine after receipt of all recommended doses"

According to the second link, the duration of polio vaccination is at least 18 years, for tetanus it is at least 13 years. This is contrary to your stating: "... for polio, memory B and T cells produced in response to the vaccine persist only six months", please verify or add reference.

For your question about reasons why ("Why do we need booster shots, for some viruses but not the others?"), this seems to be answered on a scientific level yet in an exhilarating way by Bali Pulendran, an immunologist at Stanford University, see J. Cohen, Science, 2019, How long do vaccines last?,:

"I keep saying, ‘It's not well understood, it's not well understood.' This is one of the major issues in vaccines." (...), two essential vaccines, against diphtheria and tetanus, appear to have better durability than widely presumed".

Another quote from the Science article: "... "We simply don't know what the rules are to inducing long-lasting immunity," says Plotkin, who began to research vaccines in 1957. " (...) Everything of course depends on immunologic memory, and we have not systematically measured it."

This may answer the question as far as it inquires about different durations of immunity. However, taken literally the question may refer to the necessity of "receipt of all recommended doses", which casually may be called boosting. To this, the same applies: the immunological reasons for several "shots" to reach full vaccination status are not known, keeping in mind that according cited Science article one single natural infection suffices.

The question does not comment on the diagram showing "reinfection". According to basic knowledge any reinfection corroborates and refreshes existing immunity which, at the same time, protects against the reinfection. It may depend on the type of virus how often reinfection occurs; common cold is known to occur quite often. It may also depend on the specific viral strategy of co-evolution with host if infecting or reinfecting viruses avoid and circumvent immune cells (thus inhibiting their natural boosting). Some virus may become, by mutations, "less harmfull" as their mutations foster not getting in contact with the adaptive immune system, eventually resulting in less pathogenicity which may counteract long term immunity.

Last but not least: Some viruses mutate more often than others, and changing antigens that virus are recognized by as a mutational strategy may be conceptually distinguished from host-cell-cooperation described above. In that case, booster shots are necessary to compensate the affinity loss of antibodies or memory cells. Search term is immune evasion.

See also, e.g.,

Slifkaa/J. Amanna, Vaccine,2014, How advances in immunology provide insight into improving vaccine efficacy

J. Amanna et al., N Engl J Med 2007, Duration of Humoral Immunity to Common Viral and Vaccine Antigens (discusses several diseases)

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