How many lives have life-jackets on planes saved???

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i have just got off a flight from udaipur to port blair.considering this flight is mostly overland ,it seems strange that at least half of the safety briefing is about how to put the lifejackets on. so while there are lots of statistics about how many plane crashes there are and how safe air travel is,i would like to know how many lives have ever been saved by those yellow life jackets?? i will bet it is very few.