Does the flu shot actually work?

What the evidence says

Yes, the flu shot generally works, though it is not a perfect shield. Canada's National Advisory Committee on Immunization reviewed the evidence and concluded that getting the flu vaccine every year is the best way to lower your chances of catching influenza and dealing with its complications, like being hospitalized or dying from it.

A large Cochrane review, which combines many high-quality randomized trials, looked at healthy adults given inactivated flu shots (the regular injected vaccine). It found the vaccine lowers the chance of getting confirmed flu from just over 2 in 100 people down to just under 1 in 100. That is a real benefit, but it is smaller than many people expect, and the review found it was uncertain whether the vaccine clearly reduces time off work or serious complications.

For older adults, the picture looks better in relative terms. A Cochrane review focused on this age group found vaccinated people had roughly 40 to 60 percent lower risk of getting flu compared to unvaccinated people, though the researchers rated this evidence as low certainty because there were not many randomized trials in this age group specifically.

How well the vaccine works also depends heavily on how closely that year's vaccine matches the flu strains actually circulating. Canadian surveillance networks track this every season. For example, in the 2025-2026 season so far, the vaccine has been estimated to cut the risk of medically-attended flu illness from the main circulating strain by about 40 percent, even though that strain had mutated somewhat from what the vaccine targeted.

Overall, no flu vaccine gives complete protection, but the evidence consistently shows it reduces your risk of getting flu and its complications, which is why NACI recommends it annually for everyone six months of age and older who has no contraindication.