Dear all:My antiHbs is about 471IU/L in 2012,June, 371IU/L in 2014,Jan and 200 in 2014,Sep.They drop quickly,and I don't know when should I take a test again.Cause one of my best friends get infected of HBV and we connect really often,I am wondering when should I take vaccinum again?Last time I take it was when I was born,and now 21 years have passed.