In order to optimize HBsAg clearance in CHB patients with low level HBsAg, the investigators enrolled patients who had received, and responded to, NAs for more than 12 months(see the inclusion criteria), and patients are switched to receive peginterferon alfa-2a 180 micrograms/week or peginterferon alfa-2b 80 micrograms/week for 48 weeks and follow up for 24 weeks.