How likely is it that a vaccine would have long-term side effects? Have there been past cases of it? Is there a plausible biological mechanism that could cause this?
Some Examples: - Philippines dengue vaccine debacle https://www.npr.org/2019/05/02/719366831/dengue-vaccine-cont...
- The rushed flu vaccine in the US in response to the 1976 outbreak https://en.m.wikipedia.org/wiki/1976_swine_flu_outbreak
Different underlying mechanisms (Dengue is a bitch where you get immunity against one strain but significantly increase the chances of a hemorrhagic event if you get any of the other strains for example)
Given how little we know about covid and its family (Sars burned out, MERS never got the investment it needed to understand), rushing is a very bad idea(tm). But so was pretty much most of the inept response so far, so yea...
Longer list from the CDC here, you can see a variety of causes from simple manufacturing mistakes (given how we did on producing rushed tests....) to complex side effects - https://www.cdc.gov/vaccinesafety/concerns/concerns-history....
Its worthwhile noting that the scale here is the most scary part. Most vaccines are limited to specific situations or age / risk / region groups and have years of testing and real world data behind it, following gradual rollout over the world - but the covid one will be applied universal to potentially billions in a short time. Rushing that is, well, an unfathomable risk.
Edit: Judging from the reaction this isn't what certain people wanted to hear. Well, we are in this mess because of exactly that attitude. Nature does not care for our wishes and we better grow up sooner rather than later.
There are nastier less tested vaccines out there that score worse, but still better than the actual viruses. That being said a virus doesn't infect the whole population in general, while for a vaccine to be effective you have to push it out widely thus reversing the win a little bit. I am sure there is an equation out there that balances the risk.
On phone atm so no sources.