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Question: I once got a flu shot and then came down with the flu afterward. Now I won’t get the shot anymore. Why did the shot give me the flu?
Answer: First and foremost, it’s not possible to get the flu from a flu shot. There are two common forms of influenza vaccination. The first is an injection of inactivated virus, which is essentially virus that has been killed, and sometimes dismantled into pieces. Even though the virus has been inactivated, the immune system can still respond to it and “learns” the shape of the flu virus from the pieces present in the inoculation. This allows the immune system to respond rapidly upon subsequent exposure to the flu such that the virus will be eliminated before it can cause illness. Inactivated virus is incapable of causing illness, much the way the remains of a chicken dinner are incapable of laying an egg.
The second type of influenza vaccination is live, attenuated (meaning weakened) virus in the form of a nasal spray called FluMist. Despite the fact that the nasal spray contains live virus, the virus isn’t able to proliferate in the body and cause illness, because it’s been genetically modified to survive only at temperatures lower than core body temperature. As such, the virus can survive in the nose (such that the immune system can learn to recognize it and prevent future infection), but it can’t survive in the lungs or elsewhere in the body. The latter would be necessary for infection to occur.
There are a few possible reasons that an individual who’d been vaccinated for influenza could come down with “the flu.” The first, and most likely, is simply that the body reacts to the flu vaccine by generating an immune response to build immunity against a real attack. Because of this, people vaccinated against the flu feel kind of icky, tired, and may even come down with a headache or low grade fever. These symptoms are indicative of the body’s activated immune response to the flu vaccine, which is necessary to develop immunity against the live influenza virus.
Alternatively, what’s called “the flu” in common parlance is often a virus other than true influenza. The influenza vaccination can’t protect against colds or any of the many other respiratory viruses that are common during the winter months. As such, getting an upper respiratory infection after having been vaccinated for influenza isn’t an indication that the shot has failed to work. Influenza symptoms are characterized by fever (in most cases), muscle aches, headache and fatigue. It comes on much more rapidly — and is more severe in its presentation — than the common cold and many other seasonal respiratory viruses.
In the rare event that an individual is exposed to influenza shortly after vaccination, it is possible to come down with the flu. It takes the influenza vaccination approximately two weeks to take effect. During this time, the immune system is working on learning to recognize and respond to the virus, and an exposure soon after vaccination could result in illness. To preclude this, physicians generally recommend getting a flu shot well before the anticipated start of flu season.
Finally, there’s the outside chance that the flu shot could be ineffective against the flu in a particular year. Because the most common strains of influenza virus differ from year to year, health organizations must make educated predictions about which strains will likely be most prevalent. This creates a bit of a dilemma; the earlier predictions are made, the sooner the flu vaccine will be available, but the greater the chance the predictions won’t be completely accurate. Waiting until flu season starts to produce the vaccine would nearly guarantee the vaccine’s effectiveness, but would be unwise, because susceptible populations would be left unprotected for the beginning of the flu season.
In general, the flu shot is quite effective and the vast majority of vaccinated individuals do not get influenza. The flu can be quite serious — particularly in patients with respiratory disease, the elderly, and the immunocompromised. Approximately 200,000 hospitalizations and thousands to tens of thousands of deaths occur every year because of the flu . Since it’s not possible to get the flu from a flu vaccine, and the virus hospitalizes and kills more people in this country than any other vaccine-preventable disease, the benefits of the flu vaccine clearly outweigh its riskss, even if it doesn’t 100% preclude the possibility of the flu.
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- Key Facts About Seasonal Influenza (Flu). Centers for Disease Control and Prevention. Updated 2007, Sep 17.