That's true for almost all herbs. There are few studies because you cannot stick a copyright on herbs so there's no almost incentives for big companies to push for such researches.

There seem to be some conclusive studies on elderberry helping for influenza.

Ex: "In conclusion, the proprietary elderberry extract used here is effective in controlling influenza symptoms and is complementary to current anti‐viral agents. The safety and ease of administration warrant further investigation of its clinical efficacy in children, elderly and other high‐risk patients of the proprietary elderberry extracts." - http://www.omicron-pharma.com/pdfs/ElderberryClinicalOJPK_Pu...