My concern about the mRNA vaccine is wondering if it could cause some strange allergies in the future?

Its impossible to be allergic to mRNA. mRNA is in every cell in your body and all RNA is made of the same 4 amino acids that are in the vaccine.   The vaccine contains several salts, acids, and fats that stabilize the mRNA and enable it to get inside cells.  Its possible to be allergic one or more of the additives.  Allergic reaction to the additives usually occurs within 20 minutes of getting a vaccine.  Rare delayed allergic reactions can occur 2-3 days later.  After a week, all of the vaccine constituents are broken down and cleared from the body and there is zero possibility of having an allergic reaction beyond that time.  

No, and what you said makes no sense. 

"No virus is in there. But it gets your body to recognize the virus, even though it's not there."

The way proteins are made in the human body is to have DNA as the master blueprint for protein making with mRNA as the messenger to the ribosomes where the protein is actually made. The protein that one is instructed to be made is the spike protein that this virus has to attach to cells. 

The vaccine is a mRNA that is injected into the muscle which in turn is taken up by the cells and taken to the cells manufacturing center that makes a viral protein. The viral protein is detected as foreign by your body and the immune response thus makes antibodies to it. 

This is basic biology go ahead an review protein synthesis. 

The mRNA only lasts for hours before it is broken down and doesn't go on forever. As far as allergies go those are immediate reactions to allergens that cause running nose after being exposed to the allergen again. We all know what allergies are. The vaccine doesn't cause random reactions that wouldn't occur normally. People with allergies develop more allergies and that has nothing to do with vaccines. Some vaccines are actually used in treating allergies like with asthma. 

These vaccines are for all intensive purposes still in large-scale human trials.  Unlike other vaccines that have decades of tests.  

This vaccine using mRNA which has never been used before has had at most a few months of testing.    No one knows what will happen in 10 years, 5 years, 2 years, or even a year from now...Because it hasn't even been around that long.  

Is it possible that nothing will happen and it will be a great vaccine...of course.    But it is also possible that in a couple of years people start to have auto-immune disorders.  Even if it is only a small number(say 5%), that doesn't seem a good trade-off for a virus where over 99% of the people survive.

Highly improbable. However, I'd rather have allergies than long-term effects from COVID. 

The thing is, no one is 100% as to what long term impacts may, or may not, be.