From Dec 2020

https://www.medpagetoday.com/infectiousdisease/covid19/89998

Take aways:

"Essentially, instead of pharma producing the proteins via an expensive and difficult process, mRNA enlists the body to do the work. The capability to produce mRNA so rapidly is one reason these vaccines are out front in the global race for a COVID-19 vaccine."

Never Been Done Before?

"That's not completely true. While an mRNA vaccine has never been on the market anywhere in the world, mRNA vaccines have been tested in humans before, for at least four infectious diseases: rabies, influenza, cytomegalovirus, and Zika."

Safety

"Safety will therefore need continued evaluation as different mRNA modalities and delivery systems are utilized for the first time in humans and are tested in larger patient populations," they wrote in the paper, which was published in 2018.

"We know broadly that the overall approach is pretty safe," Burton said, but noted that it was important that adverse events are monitored and followed up.

Going forward:

"Introducing synthetic mRNA into cells also holds promise as a type of replacement therapy for diseases in which production of vital proteins is inadequate or defective. It could thus hold advantages over gene therapies and protein replacement: less risky than the former, less frequent dosing than the latter, and cheaper than either."

"Will RNA work for all vaccines? I don't think we can say that yet," Burton said. "It's a huge leap forward. It's very quick to make and has a lot of advantages. But I think SARS-CoV-2 is an easy test relative to some of the other viruses we have to deal with."

There's also less time to production of a vaccine, now that the delivery model is established. It's then just a matter of determining what proteins to be encoded for in the mRNA that will evoke the correct antibody production.

I've read that the next big target is the flu. The benefit would be allowing production to begin only two months before flu seasons instead of a year before, allowing for better targeting of the circulating and anticipated problem strains.

A more tailored version continues to be explored for immunotherapy against cancer.

The reason that COVID was used first was the fact that it had been in development since 2003 for SARS CoV 1, the original SARS. It had also been used to create a MERS vaccine. However, since neither SARS nor MERS had emerged again as a big threat, neither was brought to market. For COVID19, it was simply a matter of identifying the appropriate protein to have the antibodies attack.

As for the coagulation / clotting concerns, I'm no hem/onc. But, we haven't seen huge numbers of people with issues, far less than some other common medications, such as birth control pills.