I presume the blinding is imperfect because some of the vaccines cause mild reactions that the placebo wouldn’t. I doubt it’s a big problem. The people doing the trial are selected for being more conscientious than the average person. (For one of the two trials, the rate of Covid seropositivity was only ~1% for people starting the trial, which is lower than the general US population). They will not want to risk their household members getting Covid, and they will have been warned that that the vaccines are unlikely to work perfectly.
I presume the blinding is imperfect because some of the vaccines cause mild reactions that the placebo wouldn’t. I doubt it’s a big problem. The people doing the trial are selected for being more conscientious than the average person. (For one of the two trials, the rate of Covid seropositivity was only ~1% for people starting the trial, which is lower than the general US population). They will not want to risk their household members getting Covid, and they will have been warned that that the vaccines are unlikely to work perfectly.