According to the CDC, the flu vaccine is designed to protect against the influenza viruses predicted to spread in the community during that flu season. However, during years when the flu vaccine is not well matched to circulating influenza viruses, it is possible that little or no benefit from flu vaccination may be observed.
Recent average flu vaccine efficacy rates show:
2017-18 - 38%
2016-17 - 40%
2015-16 - 48%
2014-15 - 19%
2013-14 - 52%
2012-13 - 49%
2011-12 - 47%
That's an average 42% efficacy rate over 8 years with efficacy rates ranging from 19-52%.
Additionally, flu vaccines do NOT protect against infection or illness caused by other viruses that can cause flu-like symptoms.
A recent report examining studies from 2010-11 to 2014-15 concluded that the effectiveness of a flu vaccine may be influenced by vaccination the prior season or during many prior seasons (1). In some seasons, protection against influenza A(H3N2) virus illness may have been lower for people vaccinated in the current season and the prior season compared to those who had only been vaccinated in the current season. This fits with findings on immune response to vaccination that suggest repeated influenza vaccination can weaken immune response to vaccination and especially to the H3N2 vaccine component.
Like any medical product, vaccines can cause side effects.
Common side effects from the flu shot include:
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Soreness, redness, and/or swelling from the shot
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Headache
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Fever
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Nausea
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Muscle aches
The flu shot, like other injections, can also occasionally cause fainting.
Some studies have found a possible association of injectable flu vaccine with Guillain-Barré syndrome (GBS).
With any vaccine, look for any unusual conditions, such as a high fever, behavior changes, or signs of a severe allergic reaction after vaccination.
Signs of a severe allergic reaction can include:
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Difficulty breathing
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Hoarseness or wheezing
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Swelling around the eyes or lips
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Hives
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Paleness
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Weakness
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A fast heart beat or dizziness