Hi Paul.....Yes, this has been claimed about fresh onions and as with most natural substances, these are speculations/observations, based upon various studies and laboratory testing. So, while evidence is strong, in these claims, they're, as yet, not absolute.
However, I say, "any and all," which nature provides us, that consistently show significant benefit to the body, in terms of prevention or healing properties, should be taken seriously and at least considered as a viable option to health treatment.
Red onions, in particular, have high concentrations of "Quercetin." ...a plant-derived flavanol, that has anti-oxidant properties, which remove free radicals from our system.
Quercetin has been linked to numerous health benefits, in particular, it is known to support our immune system, clear excess congestion and is a very powerful anti-inflammatory agent. It seems to me, given these links....".help in preventing the flu ," is an easily understood probability. "Quercetin supplements (capsules) can be purchased OTC....but if you like onions, as I surely do, I prefer a burger or salad with ONION...to a capsule! Hope this helps, Paul
(I managed a General Nutrition Center (GNC) store for 4 years...lol...I didn't just happen to pull this info, off the top of my head!.....)