From Flu.gov: It's Not Too Late to Vaccinate!
Yes, flu season is here (it usually peaks in January or February), but there is still time to help protect your family from the flu! The very BEST protection against the flu is the seasonal flu vaccine, and January is not too late since it takes about two weeks for the vaccination to provide protection against the flu. Other recommendations to reduce the spread of the flu are to wash your hands frequently, cough/sneeze into your elbow if you don't have a tissue, and stay home if you're sick.