When the asymptomatic rate is high, it’s hard to determine if the vaccine is truly works, and to what degree. If 40% of people are asymptomatic then that would theoretically mean that 40% of challenge trial participants won’t show symptoms. Does that mean the vaccine is working? Who knows. It could be working, it may also not work.
The only way to do it and get real results is to use a challenge trial only on the elderly population where the asymptomatic rate is low and where the symptoms are exaggerated. This would mean though that if the vaccine didn’t work you would overwhelm the hospital system with a lot of sick elderly people who require the most care, when the healthcare system is already overwhelmed.
