When I was growing up, no one got a seasonal flu shot. It was unheard of. Now about 42% of adults and more than half of children get one:
All those flu shots. Is it doing anything?
The independent Cochrane Group, which does exhaustive reviews on a topic, had this to say about the effectiveness of the seasonal influenza vaccine (The evidence in this review is current to 31 December 2016.):
Injected influenza vaccines probably have a small protective effect against influenza and ILI (moderate-certainty evidence), as 71 people would need to be vaccinated to avoid one influenza case. … Vaccination may have little or no appreciable effect on hospitalisations (low-certainty evidence) or number of working days lost.
Healthy adults who receive inactivated parenteral influenza vaccine rather than no vaccine probably experience less influenza, from just over 2% to just under 1% (moderate-certainty evidence).
Fifteen included RCTs [randomized controlled trials] were industry funded (29%).
They said that a person’s risk of influenza, without a flu shot, is about 2.3% and that vaccination could reduce that to about 1%. The reason it feels like more than just 2% of people get the flu is that:
Over 200 viruses cause ILI [influenza-like illness], which produces the same symptoms (fever, headache, aches, pains, cough, and runny nose) as influenza.
And the flu shot doesn’t cover any of those 200 viruses.
Also, Cochrane says there’s a real problem with bias in these studies. Many are either industry-funded or lack information needed to determine industry’s influence. (“We were unable to determine the impact of bias on about 70% of the included studies due to insufficient reporting of details.”)