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Before the seasonal flu vaccine was recommended for all healthy adults, people in developed countries might get vaccinated against on the order of 10 different pathogens and develop immunity for some more via natural exposure.

Now the same person is expected to take a seasonal flu vaccine for a new strain every year - won't this put them in an uncharted territory as far as effects on the immune systems are concerned within a couple of decades?

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    Welcome to MedicalSciences.SE! Please take the tour and read the help center. For reasons mentioned in this post and in How to Ask, we require prior research information when asking questions. Please help us to help you and edit your question to provide more information on what you have read on this subject, what made you ask this question, and any problems you are having understanding your research. If you found nothing, what did you Google? – Chris Rogers Oct 26 '18 at 8:10
  • This is one common argument against vaccinations, though as it stands the human adaptive immune system seems to possess the capacity to develop responses against far, far more pathogens than a given human will ever have the chance to be exposed to. To use the below-referenced term "immune memory", you can think of the human immune memory space as being so incredibly vast that you could spend the rest of your life injecting yourself with every pathogen on the planet and you'd likely get nowhere near close to the limit, if it exists, though such a lifestyle wouldn't do much for your longevity. – TheEnvironmentalist Feb 1 '19 at 0:03
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Won't this (having a flu vaccine every year) put them in an uncharted territory as far as effects on the immune systems are concerned within a couple of decades?

No

You are exposed to and develop a memory response to many, many more pathogens than vaccines. Your collective immune memory from natural exposure to a pathogen is much, much larger than from vaccination.

Effective vaccines reproduce the immune response of exposure to a pathogen (see the CDC pink book, Chapter 1). Some use specific subgroups of antigens important for protective immunity, some use killed viruses that don't replicate in the host, and others use live attenuated viruses that replicate, but don't cause disease. The number of vaccines you get is much smaller than the number of different antigens you are exposed to. Considering the common cold alone, the incidence in the US is 6 per person year for children, 2 per person year for adults without exposure to children (see Cecil Medicine Ch. 369). Each of these exposures involves many different antigens. Vaccine related immune memory is a drop in the bucket.

Beyond this, the basic premise (that you have a small amount of immune memory storage relative to the number of vaccines you receive) is wrong. There are over 3 billion B-cells in your body at any given time, with about 100 million different B cell receptors. This repertoire of antigen receptors is enough to match every single (or nearly every single, depending on who you ask) possible macromolecular antigen. For a protein, or protein conjugated antigen, the product of that match can produce long lived high affinity memory B cells, and long lived high affinity plasma cells. Your immune system has the memory capacity to deal with a great deal more antigens than the number of vaccines. You can read about these basic immunology details in Sompayrac's How the Immune System Works (Chapter 1 gives the numbers I've discussed here, and discusses how the repertoire of antigen receptors matches up with the number of possible antigens). Other basic immunology texts (e.g., Abbas, Janeway) cover the same, but can be a little more dense.

  • But (and correct me if I'm wrong here) - not every incidental exposure results in long-term immunity whereas a vaccine is optimized to do exactly that - which to me would suggest it makes a larger use of the immune memory - or is this not how it works ? – Macrophagus Oct 26 '18 at 7:03
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    @Macrophagus that is not how it works. In general, pathogens are more immunogenic than vaccines and live attenuated vaccines are more immunogenic than killed vaccines. – De Novo Oct 26 '18 at 15:30
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    To add on to De Novo's response in the comment...part of why vaccines need to be so 'optimized' is exactly because they otherwise wouldn't be great at resulting in long-term immunity since they don't involve an actual infection and therefore don't cause you to mount a full immune response (which would be quite unpleasant as well). – Bryan Krause Oct 27 '18 at 0:44

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