UPDATE: So it looks like I've asked a complex and technical question. I'm going to take @tyersome's advice and study Khan Academy on immunology, and from there, formulate a better question if I still need to.

The question is based on the idea that often a vaccine is based on an attenuated virus.

My understanding is that a virus isn't really alive in the same way a bacterium is, which means they can't be killed. (Correct?)

My (layman) understanding is that a virus is simply DNA or RNA wrapped in a thin layer of protein. (So is it a lifeform, or merely an extremely complex set of molecules?)

I can imagine it's possible, chemically, to obliterate the molecular structure of a virus, and thus "kill" it, but would such a "debris soup" be useful in a vaccine?

I'm not asking how a virus is attenuated, ie, by growing it in certain cultures. I'm asking what it actually means for a virus to be "attenuated"? What's different about it, yet still makes it usable as a method to teach the immune system?

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    $\begingroup$ What is the distinction you are trying to make between "attenuated" and "different strain or variant"? Have you read the Wikipedia page? ——— You seem to have multiple questions some of which (especially the last) are much too broad to be answered here. (Both of these are also criteria for closing questions.) ——— To really understand this take/read an introduction to immunology and then post any questions you still have. I have found Khan Academy a good introduction. $\endgroup$
    – tyersome
    Sep 9, 2021 at 18:56
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    $\begingroup$ Presumably, attenuated means it can't replicate. $\endgroup$
    – DKNguyen
    Sep 9, 2021 at 19:38
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    $\begingroup$ @DKNguyen - not necessarily, it may be able to enter and replicate part of the virion, but not the whole genome or perhaps has a packaging defect. Polio and MMR vaccines are both attenuated. The polio one in particular is known to occasionally cause infection and be passed on to others. $\endgroup$
    – bob1
    Sep 9, 2021 at 21:45
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    $\begingroup$ @Stewart - you can edit your post with the things that you have found out about how attenuation is done and how this helps make it less pathogenic. Note that the exact mechanisms are specific to the type of virus. Killed (inactivated) vaccines have the structures preserved by fixation or by chemically damaging the genome so that it no longer functions, but proteins remain intact. $\endgroup$
    – bob1
    Sep 9, 2021 at 21:48
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    $\begingroup$ Lower entry/binding to receptor, lower replication, lower pathogenicity, sometimes missing proteins or parts of it, and with genetic engineering more curious changes such as lesser replication fidelity, sub-optimal TRS, some proteins replaced by other ones (influenza reassortant). Historically it was just about finding a closely related virus circulating in a different species, giving fewer symptoms and cross-protection (vaccinia virus). Then it was about selecting passaged viruses giving fewer plaques in lab, adapting to eggs so that after enough passages it replicates poorly in human cells. $\endgroup$
    – reuns
    Sep 10, 2021 at 17:12


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