Level 2: Mid-level evidence
IM and intranasal seasonal influenza vaccines were compared with each other and with placebo in a randomized trial involving 1,952 healthy adults during the 2007-2008 influenza season (N Engl J Med. 2009;361:1260-1267; available at content.nejm.org/cgi/content/full/361/13/1260, accessed November 9, 2009). The rate of laboratory-confirmed symptomatic influenza was 6.1% overall, with lowest incidence (3.4%) occurring in the group receiving IM inactivated trivalent vaccine (Fluzone). The rate of laboratory-confirmed symptomatic influenza was significantly higher for both live attenuated influenza vaccine (FluMist) (6.9%) and placebo (10.8%). The NNT to prevent one case of symptomatic seasonal influenza using the inactivated vaccine was 29 compared with live attenuated vaccine and 14 compared with placebo. Arm soreness was the only significant adverse effect associated with the inactivated vaccine. The live attenuated vaccine appeared superior to placebo, with an NNT of 26.