a retrospective trial is not considered as clean or conclusive as a double blind placebo controlled prospective trial. there may be some benefit to the drug in some patients and some settings. this study does not conclusively prove that. its just more information to consider. the latter kind of trial is considered the gold standard. and even then often we would like a second gold standard trial to confirm the first one
The main study seems to show a little benefit but I don't know if that is a statistically significant benefit. Usually in the analysis of findings they'll include that in peer reviewed papers. The WHIP study IMO is maybe more important to determine the prophylactic value.
