www.ncbi.nlm.nih.gov/pubmedhealth/PMH0010949/

Cochrane Reviews

www.vox.com/2015/10/14/9521807/flu-shot-vaccine-effectiveness

Summary of Cochrane

In adults, however, the vaccine's effects are more modest. "Depending on the season," explained Tom Jefferson, an author on these Cochrane reviews, "you need to vaccinate anywhere between 33 and 100 people to avoid one set of symptoms." In a good year, when the WHO guesses correctly and the flu shot matches the strains in circulation, you need to give 33 adults flu shots, on average, to prevent one case of illness. In a tsar when the WHO guesses badly, you need to vaccinate 100 people to prevent one flu case.

Sincerely,
Gargoyle