We all know that person...the one who says every year that they don't get the flu shot because it "gave them the flu" the last time they got it. Well, I'm here to tell ya...that is not happening.
The flu vaccine cannot cause a person to get sick with influenza because the stains of influenza contained in the shot are either inactivated or attenuated, so they cannot replicate and cause illness in the body.
If your friend says they tested positive for the influenza right after the flu shot, they were most likely already infected with the flu before the shot, but not yet symptomatic. The incubation period (the period between when someone is infected and when they begin to show symptoms) for the flu is 2-4 days.
Also, there are many other respiratory viruses that have similar symptoms to the flu, so if your friend was not "officially" tested, it may also have been another virus altogether.
Sometimes side effects from the flu vaccine can also make people feel sick and can be mistaken for the flu itself, but it’s just the body’s immune system mounting a response. The most common symptoms after the vaccine itself are muscle aches, headache, and fever.
Also, fun fact: It takes about 2 weeks for the body to mount an optimal response to the flu vaccine, so infections after vaccination and during that 2-week window can occur because the body has not developed full immune protection.
As with every vaccine or medication, the flu shot is not 100% effective. During some flu seasons the match of the vaccine to the circulating virus is better than others, so the protection from the vaccine against infection does vary from year to year. So, it is possible to get infected even if you have been vaccinated. It does, however, give you the best chance of avoiding infection, and most definitely protects against severe illness from flu.
So don't listen to that person, and go get your flu shot!