Remember there is no risk of infection from these vaccines as they don’t actually contain the virus itself. Also, receiving vaccines while you are mildly ill is not a problem. The CDC recommends vaccinations during active cold or flu, so this won’t be an issue.
As to you question regarding phase 3 trials. Most likely, the vaccine will need to be tested pre-infection against a controlled placebo. The idea of the vaccine is to prevent the onset of symptoms. So, more than likely the tests will involve otherwise healthy people and may track the incidence of coronavirus infection against controls. The purpose of phase 3 is to determine efficacy.
Thanks for your question!