Vaccine testing takes as long as it takes to expose enough of the trial participants to see a statistically significant difference between the test and control

Vaccine testing takes as long as it takes to expose enough of the trial participants to see a statistically significant difference between the test and control groups. There isn't a bioaccumulation issue because the vaccine isn't administered daily, so the only long term harms would show up in the time it takes systems to react to it, which would be on the order of days. We have clearly observed long term harm from infection, starting a few days after infection, but no such harm from the vaccine, except for very low rates of myocarditis in young men.