Every year is unique when it comes to influenza - both the strains that are circulating and the timing of its arrival in a geographic area vary tremendously year-to-year. The Centers of Disease Control and Prevention (CDC) continually conduct studies to determine the amount of influenza in all areas of the country, and publish this information on a weekly basis. If you are interested in seeing the map documenting the spread of influenza, you can find it at cdc.gov.
The flu shot vaccine for a particular year is typically available in late August or September. You can receive your flu shot once it is available until well into the flu season, which usually occurs between February and March in Central Pennsylvania.
The ideal time to get a flu vaccine is at least two weeks before exposure to the virus. This gives the body enough time to respond to the vaccine and have its antibody defenses at peak readiness. To prepare for the winter of 2013 in Central Pennsylvania, now is the best time to get that flu shot, because the prevalence of the flu is expected to increase over the next few months. Once the influenza illnesses begin to wane, it is no longer as useful to get a flu shot, and it is probably smarter to wait until the next season.
Recommendation: Get your flu shot before mid-February 2013 to give your body time to respond and prepare antibodies for the influenza virus. You can get a flu shot at a number of places, including your primary physician, health clinics and pharmacies.