Ann: Phase 3 Trial in COVID-19 ARDS Surpasses 50% Enrollment, page-228

The following from the NIH doesn't directly answer the question -- 'does a trial have to be stopped if overwhelming efficacy is clear at an interim analysis?'. However it clearly states that the "stopping rules' for efficacy have to be specified as part of the study design:
"For studies that are conducted in a blinded manner, the presentation of data relating specifically to outcome measures should be presented to the DSMB in the manner and timing described in the DSMP for that study. Studies that include interim analysis of efficacy data should provide the proposed number of interim analyses to be made, the specific comparisons to be made at each such analysis, the stopping rules on the basis of efficacy findings, including both standards for the determination of 'overwhelming efficacy' and an 'inevitable failed trial', and the methods of statistical correction to be used to control the final overall Type 1 error. When defining this interim review in the protocol, consideration should be given to whether the data can be prepared for the DSMB review in a sufficiently timely manner so that any resulting decision can in fact impact on the study Ð if the study is largely completed before the conclusions are made, the interim review maybe unnecessary."

From:

https://www.drugabuse.gov/research/clinical-research/guidelines-establishing-data-safety-monitoring