I have long suspected that much of the objection to vaccinations stems from a dislike of needles (and "chemicals," see below). I am not trying to say that people who object to vaccinations are merely afraid of shots because they hated getting them as kids.
But it's moderately close to that. About halfway there. As that is a bit insulting, I should provide more evidence than mere impression.
I have two other streams of data that are not available to most people. I have not been methodical about gathering it, but they're there and you should know about them.
First, many psychiatric medications come in long-acting injectable forms. It is very hard to talk people into taking those. It is not the case that there are more side effects when delivered in this form. In fact for most of them, smaller doses can be prescribed, reducing the side-effects. There is a confounding factor with the mentally ill that they often don't want to take these medicines at all, and hope to cheek them or otherwise avoid them. Many don't believe they need them, so why lock oneself in to a medicine coursing through the blood that cannot be removed? Yet even subtracting those out, and including the pressure that providers and families put on people to accept the biweekly or monthly shot, there is simply a lot of refusal to switch from oral to injectable medication. I have heard from nurses that the same applies for Depo Provera, but I don't have any numbers on that. The impressions of nurses are not infallible, but neither are they chopped liver. Weigh that as you will.
Second, because of the questions at the Red Cross blood center about recent vaccines, I am always reminded to ask that day's nurse about my pet theory: Do people who refuse vaccines give blood? No one has any numbers, but the question always intrigues them. They note that none of the refusers they know are also blood donors. That may seem a small sample size, but people who don't like vaccines seem to be eager to tell nurses about this a lot. Nor, when discussing the question about recent vaccines, do they ever encounter people who say "Oh I never get vaccines." This data source has pretty much dried up now, as the questions are answered privately on computer.
I'm betting that people who don't vaccinate their kids don't give blood, either. It's unnatural.
People sometimes don't get the flu shot because they forget, or are too busy, or otherwise miss it for reasons of "soft avoidance." As they seldom get influenza, they are generally rewarded in this. Less bother, no change in health outcome. We also all encounter people who announce every year "I never get the flu shot, and I never get the flu. The only time I got sick was the year I got the shot." Which is crap, but how can you prove their evidence wrong? But these folks likely have some effect on the soft avoiders. The best one can do is to point out that you don't get into an accident every time you don't wear your seat belt either. In fact, some people go their whole lives unbuckled and uninjured. But that doesn't mean that fastening your seatbelt is a bad idea. Because when you need it, you really need it.
I note that people who do not get their children vaccinated are invariably people who prefer what they call "natural" treatments and medicines. That is only partly so for those who don't get flu shots. Those certainly tend that way, but I have encountered some who are not especial granolas who nonetheless have talked themselves into not getting flu shots. I will spare you the lengthy scientific discourse about the history of medicine except to note that 99% of the plants and animals around you are inedible, and often poisonous. People who live in safe areas that have not had epidemics tend to believe that it's all largely controllable if we just take care of ourselves. They read only selected history and science.
"Natural solutions are always better," read one of the facebook comments in my feed. Such comments are delivered with the authority of philosophical, even religious advice. Christians who say such things include a belief in naturalness in their theology, in fact. They prefer to believe that God has provided everything naturally, if we would just cleverly use it and trust in Him and not vainly pursue solutions of man's devising. There is nothing in scripture to even remotely suggest this, they just like that sort of god better. That's the way the world should be, therefore it is that way. It's a variation on some German heresies of the 18th & 19th C's. People combine it with any religion now, or have it as a religion to itself.
It would be a charming eccentricity - and in fact the individuals are often charming eccentrics - if it weren't for the dead children.
Putting a needle under your skin is unnatural. I get it. It's not in the Bible, it's not in Gram's list of home remedies (and she lived to be 99, so those millions of folks who died of the Spanish Flu must have...oh, forget it; they've never really thought about epidemics), it's got chemicals in it that you can't find in Nature (that is, God's World), or those much wiser Native or Eastern folks. Not to mention those corporations that are getting rich lying to you, so stay away from that. And it hurts, and kids cry, which parents of young children spend a lot of energy avoiding, and they don't like the feeling that I let them hurt my child, who will be traumatised and hate me at some level. I want her to think mommy's presence is a safe, gentle place.
Even the idea of spacing the vaccinations out derives from this idea that if it's a shot, then it must be very risky, and hard on your child's tiny body, which must be given some sort of rest before it can be put through the ordeal or another shot.
Here's my contention: Vaccination avoidance stems from needles and chemicals, and the associations we have of such things as unnatural, painful, and unholy. All the other arguments are post hoc.