I have had at least 8 mRNA Covid shots without a bad reaction.

That being said, vaccines (and other pharmaceuticals) contain many "inactive" ingredients in addition to the "active" ingredients. These may include solvents, stabilizers and preservatives. It's possible that your bad reaction was to one (or more) of the "inactive" ingredients. That's not uncommon in bad reactions to generic drugs which have different "inactive" ingredients from brand-name drugs even if the active ingredient is identical.

mRNA is sensitive to storage conditions. It's chemically unstable and can break down if it's not very cold. This was a factor in my choice of Moderna over the competing vaccine in 2020 since Moderna claimed better stability. I had already experienced the problems that could be caused in a product that had a temperature excursion during shipping.

I'm not discounting your bad reactions to mRNA vaccines. Obviously, each mRNA molecule is different. The vaccine is designed for the mRNA from the virus to be different from your own body's many mRNA molecules and to trigger an immune reaction. Your immune system may be violently rejecting the mRNA from the virus while most people have a more moderate immune reaction. Cytokine cascades (extreme immune system reactions) caused by Covid killed healthy, young people in 2020.

We are all individuals. Of course you should listen to your own body.

But it's possible that other mRNA vaccines might not cause as extreme a reaction. They may have different mRNA-containing active antigens. They may have different "inactive" ingredients.

I continue to get a Covid and flu shot every year. DH and I never got Covid due to scrupulous distancing and many vaccinations. We are both over 70 and have lung disease. If we got sick we would get very, very sick and possibly die.

Wendy