Search the scientific literature for examples of long-term vaccine randomized clinical trials that use inert placebos — you will not find them. Short-term studies are the preferred mode of vaccine manufacturers — even when there is no emergency — leaving long-term vaccine safety to be assessed after the vaccine goes to market.
The public is never afforded the opportunity to opt out of human subject trials in spite of federal regulations that provide protections against experimentation — with special protections for children and pregnant women.
First, the design allows rapid updates of the vaccine to match new circulating types of virus. This is a stark contrast to, say, Merck's MMR vaccine, which is showing signs of waning usefulness, not at all unexpected — in fact predicted to be obsolete in 2022 — unless updated to match the wild measles virus every two years or so.
Merck has not updated the MMR vaccine since it was created in 1960 — and the vaccine type and wild type measles lineages have evolved away from each other. Vaccine failure is now evident in that a majority of measles cases in the U.S. are in the vaccinated population (see Poland and Jacobson, 2012 and Hammond, 2020).
Given that Moderna's mRNA vaccine utilizes only a single protein, it may be expected to induce less autoimmunity than vaccines that utilize more than one SARS-CoV-2 protein. Still, Moderna did not screen out unsafe epitopes to reduce autoimmunity due to homology between parts of the viral protein and the human proteome. Thus, concern over vaccine-induced pathogenic priming is not zero, but it may be less than COVID-19 vaccines that use more than one SARS-CoV-2 protein.