Sources of funding and categorisations of actions here are interesting, and ill served by the common "they is all morons" rhetoric, even if I would think they are all morons even if for different reasons. Whether you know it from Sun Tzu or Rage Against the Machine then "know your enemy"
https://www.scienceofstrategy.org/main/content/know-enemy
Follow the money can also be a good one here, all sorts of things from that one if you follow it.
You usually get the
Hippy mothers
Religious types with odd readings of religious texts, some of which might even have made sense if first thought of before disinfectant, germ theory and such like when the local witch was your best bet.
General distrust of authority.
Different flavours are more prevalent in different locales, and have different regional variations as well.
I am told hippy mothers tended to be the Australian problem a few years back, potentially solved by in turn saying "no shot, no school" which means the hippy mothers could not do wine at lunch as they would be home schooling instead.
Religious types actually in Europe is one of the big ones; the Netherlands tending to provide a nice case study here as they are lumped with what sort of became what those in the US will probably associate with the Mennonites (Menno Simons of Friesland, Friesland being a Dutch speaking part of the world for those not familiar with Dutch geography) and Amish. Not limited to that though; all sorts of beliefs pertaining to medicine exist (both other wackadoodle Christians, anti West which includes medicine Islam is a rabbit hole if you want that*, native belief structures and oriental efforts, plus history but more on that in a bit) and make life harder for those that at best note a placebo effect from those practices and would rather go with the chemicals, scans and such approach that... works.
Distrust of authority varies, and authorities give reasons to distrust them all the time.
My favourites probably being the causation-correlation in Africa wherein it has been noted various places are smart enough to note the arrival of those in fancy biohazard suits and all your mates dying of some horrible disease, best point your AK at them just like you do when other arseholes come to town to mess things up) but it is also not limited to that in any way, shape or form and mistrust of authority has different causes, historical (smallpox blankets is a meme at this point but still can inform, and also make things that might be more relevant when it comes to forced treatment a harder affair, if not outright impossible under basic human rights law**) and current, all over the place.
*interesting one there is people fleeing Pakistan for India (one of the leading pharmaceutical producers in the world) tending to bring diseases with them, people went out and were all "we don't care about you trying to cross, however you have this or you don't come in".
**even without the trouble of "see many historical events" then not an easy ethical debate either ("my body, my choice" and all that), also says nothing of the pragmatism concern of whatever powers you give your presently totally benevolent and competent government now (see also "utter fantasy") will be available when some/the other guys get back in (and they will, or at least will have to be assumed to have it happen one day even by a fluke). Something to consider when the presently pondered/threatened/contemplated "vaccine passports" are being discussed.
There are potential scientific reasonings behind things here; anything you put in you will have a side effect, however you don't need to be a medic to say passing sore arm is far preferable to insides pouring out of you*** (and then probably 15-20 days later the same for all/no small amount of your nearest and dearest), however part of the risk-reward equation does see people leap to extremes (there is a fractional risk of reaction and medical negligence, neither of which would happen if you sat at home instead and pondered why if everybody else has car insurance that you need it for you). People love them some extremes and gruesome stuff as well; see most medical advertising in the US, biologically driven as well -- checking behind the bush for a lion is tedious when I could be charging off after dinner but even if it is only one in 300 then if I have to go hunting every day then you are not going to make it more than a year or two... we are all the descendants of the ones that checked behind the bush with the handful of sociopaths not being concerned by that. More sociological things vis a vis group vs individualism (there are political concerns here, and not necessarily the ones people think -- left vs right has some interesting things to ponder but by and large does nothing for the whole picture as individualism and collectivism, to say nothing of its massive variation between cultures, has different notions within each of those).
***lack of first hand knowledge/witnessing of things has also reduced their scariness to some if only hearing about it in a book, or indeed taking the words that only heard about it in a book. Especially as most people that have seen things be a widespread affair are now very old (
https://www.chop.edu/centers-programs/vaccine-education-center/vaccine-history/developments-by-year - average age is about 70 and most things there cured by the early 1960s, if not earlier, so if you wanted to be say 13 to properly comprehend things you can do the rest of the maths, and that is even before grandma talking about how she lost 3 siblings being much like grandpa talking about losing 3 buddies in the war for the in one ear-out other stakes).
Lack of scientific literacy (and scientists are generally awful moral touchstones, and even worse PR peeps as most are not even pretty) also goes both ways. Someone doing what the bespectacled person in the white coat says is probably going have a positive outcome even if they have no clue what actually just happened to them and would need months to properly understand it (if at all), if the same ignorance also means they are going to struggle when some bad science (poor method, funding conflicts, poor sampling... it really does not matter) comes along, even more so if "a lie spreads halfway around the world while the truth is strapping on its shoes (and then probably needs a lecture to explain why the lie is a lie)" has its underlying reasoning carry across.
Can also carry across to non vaccine stuff as well. Scary one from a few years back was it was noted many would be mothers were not getting pre natal vitamin k injections and often cited similar concerns.
https://pediatrics.aappublications.org/content/142/2/e20173743 is for newborns but still interesting reading.
Papers then, not that papers mean much to many of these.
Adverse reactions and prevalence thereof.
The autism-MMR thing. Be it the original, or ponderings of mercury (thimerosal). You also get the Africa men in biohazard suits type related thing where autism gets diagnosed after infant vaccinations are concluded (mainly because those end about the time the first signs of autism tend to show up in early play and development).
There is a fair bit of guesswork involved in some things as well; your annual flu vaccine is usually whatever strain pops in Australia earlier in the year (winter being in the northern hemisphere's summer) being the most likely to go worldwide so spin up a batch of that (and made troubled this time around by there not really being a flu season what with "stay in your home, citizen", possibly further by what little flu there was meaning what minor immunity might have been had being absent/diminished now).
Efficacy of the vaccines at producing antibodies is one I have heard a few times. Both as a general effect of the vaccine in question (bad batch, improper testing... all fun), and interactions (see cancer suffers and efficacy of covid vaccines if you want a more recent one).
Lack of testing of some of the vaccines. Claims the FDA did not do this or that might well be true (or true to some extent -- proper testing does take years, and if a virus only appeared and was sequenced in the last year then you might have to go for a lesser "will probably still be OK" type nod than your most convicted choice, especially when the lesser nod will probably still be just fine/acceptable casualties and potentially save more than a few lives and a lot of funds treating things that mere numbers say will be the case), and they have certainly not shown themselves to be ever competent or incorruptible either (addyi probably being my favourite for them being bullied and cajoled), never mind the wider government (see most drug laws, censorship and more besides). There are also still quite a few questions about things given to various military types despite dubious testing before various wars.
Despite my bluster above I am also aware of the lack of embracing by medics and wider scientist followers. This has swept up a few that might have one of the few genuine reasons to avoid vaccinations; immunodeficiency, allergy (wheat and eggs are often key ingredients in bulk vaccine production, also two common allergies these days it seems) as it will then be the anti set that listens to them or does not treat them like pariahs (simplistic "no vaccinations = bad" mantra of many being then rather tricky when your crotch fruit can't handle a chicken ovulation without dying, and "would love to if we could" falling on deaf ears).
Even without my snark I am also aware that the many paragraphs I just wrote are far less meaningful than "my cousin says" on a silly picture on facebook for most people, or indeed the Australian thing of make it inconvenient to live otherwise.