So, what does the CDC's website have to offer as an excuse?
There are several reasons why someone might get flu-like symptoms even after they have been vaccinated against the flu.
People may be exposed to an influenza virus shortly before getting vaccinated or during the two-week period that it takes the body to gain protection after getting vaccinated. This exposure may result in a person becoming ill with flu before the vaccine begins to protect them.
People may become ill from other (non-flu) viruses that circulate during the flu season, which can also cause flu-like symptoms (such as rhinovirus).
A person may be exposed to an influenza virus that is not included in the vaccine. There are many different influenza viruses. For more information, see Influenza (Flu) Viruses.
Unfortunately, some people can remain unprotected from flu despite getting the vaccine. This is more likely to occur among people that have weakened immune systems. However, even among people with weakened immune systems, the flu vaccine can still help prevent influenza complications. For more information about the effectiveness of the flu vaccine, see How Well Does the Seasonal Flu Vaccine Work?
So, do I have to get a bonus flu-shot to protect me from the viruses not included in the initial shot?