This is true of the large scale vaccine trials, yes. However, there have been several small long term monitoring studies that test immunity persistence post infection recovery, and they seem to show that replication is effectively prevented when exposed to blood serum with acquired immunity with a fairly short time delay (this is what motivated the attempt to do serum transfusions from recovered patients as treatment, but I'm not sure what came of that). That said, you're right that my comparison is overstating the confidence we currently have in this.