Consider two Cochran Reviews.
The first, regarding “Vaccines for preventing influenza in healthy adults”, concludes
Influenza vaccines have a modest effect in reducing influenza symptoms and working days lost. There is no evidence that they affect complications, such as pneumonia, or transmission.
The second, regarding “Vaccines for preventing influenza in the elderly”, concludes
The available evidence is of poor quality and provides no guidance regarding the safety, efficacy or effectiveness of influenza vaccines for people aged 65 years or older. To resolve the uncertainty, an adequately powered publicly-funded randomised, placebo-controlled trial run over several seasons should be undertaken.