CDC study from this week showed 90% effectiveness when doing proactive weekly testing regardless of if presenting symptoms. That is the best data I’ve seen as evidence it prevents even a symptomatic cases as a very high efficacy. Israel data claimed it was highly effective on a symptomatic but their methodology was off (for how they defined a symptomatic)

https://www.cdc.gov/mmwr/volumes/70/wr/mm7013e3.htm?s_cid=mm...