I went in today for my second jab of either the Novavax COVID-19 vaccine or a placebo, with no way of knowing which I was getting.
At which point I was informed that they've introduced a new element to the Phase 3 trial of the vaccine in which I'm participating.
I've had two shots, and will be going in two weeks from now for blood work (presumably to check for antibodies, etc.).
At some point after that, I'll be receiving two more shots. If I got the vaccine this time, I'll get the placebo next time. If I got the placebo this time, I'll get the vaccine next time. I won't know which one was which.
The "why" wasn't explained, but my educated guess is that they're not getting as many volunteers for the trial as they need. Doing it this way doubles the number of results.