About a week to ten days after the vaccine is injected, your body makes antibodies to the vaccine viruses. These antibodies help to protect you against any similar viruses you then come into contact with. This protection lasts for about a year. Flu vaccinations are pretty effective. If you do catch flu, it is likely to be milder than if you had not been vaccinated.
Flu vaccinations protect against flu.
They will not protect you against the many other viruses that are around every winter. Some of these cause similar symptoms to flu, and are often called 'flu'. But flu is generally more serious than these other infections, so it still makes sense to have a flu vaccination. The vaccine does not contain any live virus, so it cannot cause flu. Flu vaccinations are very safe.