Build a dam, which if it broke would kill hundreds-of-thousands, or let millions die for lack of water? Oh, we'll just over-engineer it, now we can't afford to buy food to keep the people alive long enough to need the dam; or the lead engineering firm embezzles the money and installs dodgy iron.
We can't wait around for long term studies, whatever point we decide to start vaccines - where they can still be effective for the current population - it's always possible we should have waited a bit longer.