A placebo is not necessary in this phase (phase 0). The most important thing is that the vaccine is safe and that there is a dose dependent relationship. The placebo will only be necessary in later trial stages. Now it's mostly to assess safety and efficacy through immunological parameters.

Not much is known about the humans immunological response to covid or this vaccine. Derek Lowe is rightly pointing out that it's way too early to say if this is even effective against COVID. The safety and dose-dependency is just the first step in the process.