a) current hypothesis is people with prior exposure to common-cold non-COVID-19 coronaviruses are by and large either asymptomatic or have milder symptoms than those without such exposure
b) presumably it should be possible to test people for these antibodies much as we can test people for COVID antibodies
If people would volunteer for the b) anti-body test in reasonable numbers, and a subset of them who have them would volunteer for a modified trial, we could possibly do a double-blind intentional-exposure test on people to confirm that the vaccine confers BETTER immunity than the existing similar antibodies do, right? Would it be possible to ethically and realistically use that strategy to accelerate effectiveness testing (if not safety testing) of any vaccine candidates? or would the smaller scale of that have only made sense for something more like Phase 2 trials?
Arguably, as long as safety is near perfect, we should be willing to tolerate only moderate (say, 90%) effectiveness (and I'd read one of the vaccines took two doses to get 100% effectiveness but was 90% after a single dose), but I don't know how safety is evaluated and obviously long-term safety and short-term safety are pretty different/I imagine a lot of people will be more concerned about long term safety and I don't know how you can reassure them on that on any kind of accelerated schedule.