I don't know - I hear stories but I work in health so I always have the flu needle and I've had it for 10 years and never had an issue (other than brief pain and self pity). For me, my motivation is that I work with sick people so I don't want myself to get them sicker with the flu. I've seen too many people die or be permanently disabled requiring 24/7 care from the Flu, including people in their early 20's and children. I've never had a real flu since I've got the flu needle whereas I have seen colleagues suffer through H1N1 (Swine Flu) and similar when they didn't have the Flu Needle. I'm very biased but I am always going to advocate for getting the Flu needle