The Center for Disease Control and Prevention conducts studies each year to estimate how well the flu vaccine protects people from getting sick against the Influenza virus.  This is called vaccine effectiveness (VE) and is largely based on the information that the CDC is able to collect during the flu season.

According to the CDC, effectiveness against flu A (H3N2) virus (the main virus spreading this season) is estimated to be 47%, while effectiveness against flu B was 67% for all ages. “This indicates that vaccination with the 2012-2013 flu season vaccine reduced the risk of flu-associated medical visits from flu A (H3N2) viruses by one half and from flu B viruses by two-thirds for most of the population.” However, the CDC notes that the exception to the results was the effectiveness of the flu vaccine against flu A (H3N2) viruses among people aged 65 years and older which was lower at 9%.

Overall, the CDC says that vaccine effectiveness estimates are within the range expected during flu seasons.

Resources:
http://www.cdc.gov