As I said in my OP, I understand people fear adverse side effects from vaccination. What I still I don’t understand is why anyone worries about that risk but seems unconcerned about diseases returning and exposing everyone to far greater harm.
This is a result of the way that the fear is induced by the antivax accounts.
They use overly emotional accounts generally of families who have children who are severely disabled with autism or with rare genetic conditions or with conditions which medicine can't explain, or children who are medically delicate with auto-immune conditions, or children who die of SIDS or sepsis or complications from an illness. They exploit the fact that families are often grieving, either their child who has died or the life they feel an able-bodied child would have and of course the fact that it is incredibly hard to care for a severely disabled child. Many of those families will look for more concrete answers because "We don't know why this happens, it just happens sometimes" isn't very satisfying or reassuring. The reality is science can't explain everything, and there is often not enough resources, time or support for families of disabled children.
Alternative health providers do have time and they can offer boatloads of emotional support since they can't provide much in the way of actual treatments. This can be very reassuring to someone who feels they have been sidelined or dismissed or ignored by mainstream medicine. Finally someone understands, cares, listens. They can also give you a confident, concrete, neat, tied up answer for what "happened to" your child and who to blame for it. Some people find this much easier to swallow than "random genetic mutations happen and we can't prevent it". Unfortunately these families become cash cows for the antivaxxer or alternative health practitioner - they are used to "spread the word" and of course because they genuinely believe that vaccines were the cause of their child's disability they are often happy to do this, to warn other parents away, to "save" other children.
And yes, I do think the fact that autism and physical/intellectual disability, and SIDS are concepts we are all familiar with, and to many people would be one of their worst fears, whereas the severe effects of vaccine preventable diseases are usually things we are unfamiliar with, is not a coincidence at all. It is easy to exploit this because of the way that we (humans generally) calculate/process/understand risk. It is difficult to feel very worried about something that you have no experience of at all, even second hand. It's always just a theoretical risk and it's quite hard for us to process that. OTOH something we can picture very vividly, that may affect somebody close to us, or someone we feel close to because we have heard such a personal and emotionally moving tale from them, that kind of risk feels much more real, present and frightening.
I don't think it's quite fair to say that people who are vaccine hesitant don't care about those diseases, they just have an incredibly skewed picture of the relative risk, and this is done deliberately via manipulation by the big players in the game. And I don't think it's quite fair to say they are relying on herd immunity, even though they are - a lot of the antivax propaganda, while they're bombarding you with how difficult and terrible and awful it is to live with a "vaccine injury" (most of their exampled of which are extremely unlikely to be), they are also telling you that "Big Pharma" makes up a lot of these concepts in order to control people through fear - so they often come to believe that herd immunity isn't really a thing, and that levels of these diseases went down naturally due to hygiene or other factors or that there is a big cover up about how vaccines cause other illnesses. So vaccine-hesitant families don't believe they are causing any harm by causing herd immunity levels to drop, because they have often swallowed the lie (usually using manipulated data) that vaccines aren't doing very much anyway.
The actual messages from antivax activists change their focus every so often but the key themes - those things the "mainstream media" want you to worry about are lies, vaccines and science and mainstream medicine are the real enemies, they're hiding the truth and we alone will say it - these stay the same.
There was an amazing but absolutely sickening Channel 4 Dispatches in about 2021 I think - called "The Anti-Vax Conspiracy". It isn't on their online catch up any more but it comes onto youtube every so often and then gets deleted - I think because it was sold to the BBC. If you can catch it, it's excellent but harrowing - there is a whole side aspect to this which gets extremely dark which is about controversial "treatments" for autism. But I think the biggest thing for me about this documentary was learning just how much of the antivax content online is traceable back to the same small group of people.
This goes into that a bit: https://www.mcgill.ca/oss/article/covid-19-health/dozen-misguided-influencers-spread-most-anti-vaccination-content-social-media
Also good is the episode of the podcast "You're Wrong About" called "The anti-vaccination movement". They interview Dan "Debunk the Funk" Wilson and he is incredibly patient, compassionate and thorough about what goes on.