The main problem is that you need a lot more data to make confident statements about severe cases than just about symptomatic cases. That 100% number of the Pfizer trial had a huge uncertainty interval. So even though severe cases are what you care most about, you usually don’t want to wait for a study that gives you good data on those and measure (and compare) symptomatic cases instead. Zeynep Tufekci discussed this issue of study endpoints in one of her recent articles: https://www.theinsight.org/p/vaccine-efficacy-statistical-po...