Well, mRNA vaccines have been in human clinical trials for over 10 years now, so if 10 years is your metric you may want to look in to that. My greater point is that to you and I - people who do not research the human immune system - 5 or 20 or 40 years may all seem like reasonable metrics. The people closest to the science disagree with us, at least in part because they cannot identify any mechanism for these vaccines to possess stealth side effects that only emerge decades later.