Generally the long pole in vaccine approval is showing that it works - not that it's safe. If there's only a handful of cases worldwide, say for Ebola, then you have to vaccinate a few thousand people and wait for a period where a statistically significant portion would have caught the disease and then didn't. This can be years. There's only a few cases per year, you can't just give a bunch of people Ebola to see if the vaccine works. They're not waiting for 'long-term side effects' to show up, they know they don't exist.

However, with HIV as with COVID, there's a pretty significant installed base around the country so showing that it works isn't really a challenge.

Usually only a few thousand or tens of thousands of folks participate in basically any drug trial, btw. We very well understand the risk profile of mRNA vaccines at this point, having deployed billions of them and studied the technology for 40+ years. And having studied vaccines for upwards of 300 years.