A 24-year-old new graduate completed a three-dose hepatitis B vaccine course at age 13 but was never tested afterwards. Her anti-HBs is now under 10 mIU/mL. She received a single booster dose and anti-HBs measured 4 weeks later is still under 10 mIU/mL. According to the Australian Immunisation Handbook, what is the most appropriate next step?