Olive leaf extract:
The Effect of Olive Leaf Extract on Upper Respiratory Illness in High School Athletes: A Randomised Control Trial
This is the first study investigating the effect of Olive Leaf Extract (OLE) on Upper Respiratory Infection (URI) incidence and duration in school athletes. OLE had no significant effect on incidence but significantly reduced the number of sick days by 28%. A reduction in the number of days would likely aid in ‘return to play’ and potentially improve performance. High school athletes should be educated on adequate intake of nutrients to support immune function, mainly concerning carbohydrates, prior to any consideration of supplementation. This research shows that OLE supplementation could be used to alleviate the effect of URI on high school athletes; however, adequate food-based nutrition and potentially other polyphenol products should still be investigated to decrease incidence.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6412187/
People (I know personally) who have taken olive leaf extract do not get common cold and flu.. if they get it, they get better in 2-3 days.
Problem with OLE is that if an individual is also taking BP medicine, OLE further reduces the BP and a person may experience double effect.. which might not be good. For healthy individuals, it is fine. I have been taking it for about a month.. and I can tell you I feel much better.. I had a skin allergy that's gone now.