The only vaccines I'm really excited about are the ones based on S1 Receptor Binding Domain (RBD), none of which are yet in stage 1.
S1 Receptor Binding Domain (RBD), What make one more worrying than the other? I have not the expertise to make such inferences
What they think these side effects might be, I have no idea.
* T Cell mediated inflammatory cascade (Cytokine storm) caused by off-target cross-reactivity with IL-11: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7092895/
* Autoimmune responses: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1809466/ and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7246018/
* Antibody-Dependent Enhancement of viral entry: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4018502/
There has actually been something like this during the swine flu pandemic. There was a vaccine that is believed to have caused some cases of narcolepsy. This was a real, bad sideeffect with a vaccine that was rushed to market.
Surprisingly this isn't a big talking point in antivaccine circles. Yeah you'll hear about it sometimes, but compare it to the number of times you hear "somethingsomething MMR aluminium autism" which is all debunked bullshit - you'll hear that far more often.
What I want to say is: Antivaccine people largely don't care about real issues with vaccines. They're happy with their pseudoscientific nonsense, that's good enough for them to reject every vaccine ever made.