It's a double blind study. Nobody (including the researchers) knew who was getting the vaccine vs a placebo until the end of the study. That's the point. So both groups were taking the same precautions. So roughly none of that 95% is attributable to difference in precautions.
If they were wearing masks and social distancing, what % of that 95% came from that vs the vaccine?
[adding the below]
From this CNBC article in August, it seems participants are free to continue wearing masks https://www.cnbc.com/2020/08/12/coronavirus-vaccine-trial-he...
"But he is still taking precautions as best he can. "I'm not going to walk around without a mask on," he said. "I don't think I'm superman."