I recall listening to some expert or other being interviewed about this a few weeks ago on TV. As the rate of infections continues to drop, it becomes more unlikely that triallists will bump into the infection to test the vaccine. They are expected to go about their normal routine (including being locked down if applicable) and not go actively seeking to meet the virus. So efforts were being concentrated on getting front line health professionals to sign up as they're more likely to come across it in the normal course of their daily lives. But even there, with new infections dropping and less and less people hospitalised, the timescale for testing the effectiveness of the vaccine disappears into the future. So perhaps a different approach is being considered.
So are there no areas of the world (Florida, Texas, Arizona, South America, Brazil) where there's plenty of emerging virus activity? We're all in this together, remember! In America, there's a whole redneck idiot population who'd LOVE to challenge C19 to a duel with Jesus! Invite em to be triallists.
CG