The flu vaccine helps your immune system to produce antibodies to the flu virus. If you then come into contact with the virus, these antibodies will attack it and stop you from getting sick.The flu vaccine starts to work within 2 weeks. You need to have the flu vaccine every year. This is because the antibodies that protect you decline over time. Flu strains can also change from year to year.The flu vaccine doesn't contain any live viruses. This means it can't give you the flu.You are in an at-risk group if you:are a child aged 2 to 12are 65 years of age and overare pregnantlive in a nursing home or other long-term care facilityhave a long-term medical condition – for example, a heart, lung, kidney or neurological disease or cancerhave a weak immune system – for example, if you have diabetes or you're having chemotherapyare a child with a moderate to severe neurodevelopmental disorder such as cerebral palsyare obese who have a body mass index (BMI) of over 40were born with Down syndrome