The waning efficacy of flu vaccines has traditionally forced people to choose immunization dates carefully. Receive the vaccine too soon and you risk being unprotected at the end of the season; get it too late and you lack immunity if the virus arrives earlier than expected.
Improved vaccines have increased the margin for error, though. Whereas in the past, a vaccine given in August would offer little protection come February, new formulations are extending the period of immunity. Healthcare professionals have responded by adjusting the advice they give to their patients.