This patient should receive a third dose of the hepatitis B virus (HBV) vaccine to complete the vaccine series. HBV vaccination is indicated in this patient who is traveling to China, an area with a hepatitis B surface antigen (HBsAg) prevalence of 5% to 7%. Typically, the vaccine series is administered as a three-dose regimen over a 6-month time period, with doses administered at 0, 1, and 6 months. Administration of the entire series in accordance with the recommended schedule leads to seroconversion and adequate immunity in more than 95% of persons. This patient has only received two doses over a period of 18 months; consequently, the series is considered to be interrupted. When a vaccine series is interrupted, the most appropriate approach is to resume rather than restart the series, as development of immunity is progressive and giving doses at longer-than-recommended intervals does not usually decrease the final immunologic response. Therefore, the third dose of the HBV vaccine should be administered to this patient now.

Serologic testing to determine this patient's immunity against hepatitis B is unnecessary, because this patient has yet to complete the primary HBV vaccine series, and routine postvaccination testing is not recommended by the Centers for Disease Control and Prevention. Postvaccination serologic testing is indicated only in patients whose subsequent management depends on their immunity status. These individuals may include health care and public safety workers, HIV-positive individuals, persons who are receiving chronic hemodialysis, sexual partners of HBsAg-positive individuals, and injection drug users.

Administering only two of the three recommended doses of HBV vaccine decreases the likelihood of seroconversion. Therefore, discontinuing the vaccine series following only two doses would not be appropriate.