A flu shot usually takes about 2 weeks to become fully effective.

Time to protection

After the injection, your immune system needs roughly 14 days to build up enough antibodies to provide strong protection against the flu. During those first 1–2 weeks you can still catch the flu, so it is wise to be more cautious about exposure.

How long it lasts

Protection is generally strongest for several months and then gradually wanes, which is why the shot is recommended every flu season. Many health organizations suggest getting vaccinated in early fall so that your immunity is high during the typical peak months of flu activity.