I don't see how they could not have been properly tested when they had started working on them back with SARS.
First, MRNA technology is new. Until the Covid vaccines, it had never been tried on human patients before. So let's consider the FDA's requirements for obtaining approval.
There are quite a number of steps involved and I'm not going to look them up again now but will give a synopsis from my memory. You can look it up yourself to confirm or deny my memory. I have provided the information I found on the FDA website in previous discussion so maybe a search of CFnet might help.
First, the short of it is that a new drug or vaccine must undergo a series of discovery/concept research to determine its potential. This process can usually take years and as you've already mentioned, MRNA technology has been in the works for over a decade so it can easily be argued that this portion of the process has been done. But that's where things kind of get fuzzy in my understanding.
The second phase is the pre-clinical testing. This phase is conducted in labs using lab animal research. It's purpose is to determine efficacy and risks such as toxicity. There are specific rules about how these tests are conducted including rules regarding personnel, facilities, the equipment used, data collected, reports, and more. This phase is probably the shortest phase of the process but even so can take time. Keep that word in mind....time.
The third phase is the clinical testing phase. This phase is divided into three or more steps starting with a small test group and with each step using a larger test group and allowing some time between each step to monitor for side effects. Here's that word time again. The purpose is to verify efficacy and safety. According to the FDA information I found the clinical testing process typically takes anywhere from 2-6 years or more because part of the process involves the aspect of time. Remember, I said keep that word in mind. It can take time to determine what side effects may be the result of the drug or vaccine.
The fourth phase is the review process. In this process if a drug developer has evidence from its early tests and preclinical and clinical research that a drug is safe and effective for its intended use, the company can file an application to market the drug. The FDA review team
thoroughly examines all submitted data on the drug and makes a decision to approve or not to approve it.
Let's think about this. The Covid pandemic in the US started in January, 2020. That's only three years ago. At that time, MRNA technology for the Covid vaccines had not yet entered the preclinical testing phase for potential treatments for Covid since Covid was not known about yet. At the very least it should have taken over 2 years to approve it after the start of clinical trials but bear in mind that MRNA technology was totally new and had no track record to work off of. So just that alone in my mind says it should take longer than a vaccine produced under known and previously tested processes like other typical vaccines. Also keep in mind that the fastest any other vaccine ever achieved full FDA approval was, going from memory here, I believe prior to the Phizer Covid vaccine, the fastest ever done was the polio vaccine and it took 6 years to approve.
The Phizer vaccine was fully approved by the FDA in August of 2021. Just 1-1/2 years after the pandemic began. How did they possibly get approval for a never before tested vaccine for human use in less time than any other vaccine ever done?
I personally can't help but wonder how it was done and what side effects might we encounter going forward? Hopefully, everything will work out okay.