There are a few factors -- one is that the major goal is to measure the side effects and safety of a particular new drug, in relative isolation... someone who i

There are a few factors -- one is that the major goal is to measure the side effects and safety of a particular new drug, in relative isolation... someone who is already sick is more likely to be on other drugs, thereby greatly increasing the risk of drug interactions. Another is that when someone is already sick, they are more likely to not do well with any particular new drug (pretty much nothing is 100% effective for 100% of people), so you'd have to have some way to trying to discern whether the fact they got worse or even died is related to the new drug vs. just a natural progression. This can get complicated to measure. And specifically for this particular thread, this is a vaccine, not a cure -- so in any case, when we say "least healthy" in this context, we're not talking about giving it to people who are already diagnosed with the virus, but rather to those who are _at most risk of complications_ from the virus, _if they were to contact it_. So it would be giving a drug to people who _don't_ immediately "need" it to survive, but who are still most at risk from any side effects.