The only reason we think that is because vaccines in the past have had most of their recorded side effects present in the first few weeks.
Past performance...
Past performance...
Vaccines are not the stock market. No one doubts gravity because "past performance is not indicative of future results".
How many genetically engineered adenovirus vaccines have there been before?
The vaccines that are your prior are typically attenuated or inactivated virus.
Majorly different category, we are in untreaded space (I'm still planning on getting one)
I don't think we know long-term how this may impact the immune system: does bypassing certain systems/mechanisms cause other problems with future immune response?
It took how long for us to realize as common sense that use of antibiotics allows superbugs to more likely evolve?