A close friend of mine recently told me that he came down with the flu six hours after getting a flu shot (influenza virus vaccine). He was convinced that the flu shot had given him the flu and was adamant that he would never get vaccinated against influenza again. It’s not the first time I have heard this argument, and it begs the question:
Can the flu shot give you the flu?
The short answer is no. A flu shot might give you some mild symptoms related to the injection, but it will not give you the flu. This is because the flu vaccine is inactivated (killed) virus. Flu virus manufacturers kill the vaccine and batch test it to make sure that there is no active virus in the vaccine. Killed virus cannot replicate in your body, so you cannot get an infection. Well controlled studies show that the only side effect differences seen when giving the flu vaccine or a salt-water shot are some soreness at the site of the injection. People who were given the flu shot did not develop the flu.
Now, the flu nasal spray vaccine is different from the flu shot, as the mist does contain live virus. These are not full-strength viruses; in the nasal spray, the influenza virus has been modified to be attenuated (weakened). The flu nasal spray may cause some mild symptoms, but it should not cause a full-blown flu infection. The modifications prevent the viruses from replicating at normal body temperature.
So, if the flu shot didn’t cause my friend’s flu infection, then how did he get the flu? We can never know for sure, but it seems likely that he had been exposed to the influenza virus before he got the flu shot. The incubation time for the influenza virus is about two days, meaning that there is often a two day lag between getting infected and starting to experience flu symptoms. The incubation time can be shorter, but it can also be as long as four days.
A less likely option is that my friend was exposed to a flu strain that was not included in the flu shot. The flu shot protects against strains that scientists believe post the greatest threat for that year, but it does not protect against all influenza virus strains. So, even those people who have been vaccinated can still catch the flu.
One final possibility is that my friend never had an influenza infection at all. Very bad colds, which are caused by viruses other than influenza, can often seem like the flu, causing fever and achiness.
All of these scenarios are more likely than getting the flu from a flu shot, especially since the flu seems to be everywhere these days. The influenza virus can spread before a person has symptoms, so we may be frequently exposed to people who look and feel fine, but are really unwittingly spreading the flu. And if we are, we will feel fine for a few days too. That is why it is important to follow good hygiene all of the time—cover your mouth when you cough or yawn, wash your hands frequently, and keep those fingers out of your nose and mouth. These strategies can help you prevent from getting—or giving—the flu. And getting a flu shot helps too!
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