Inclusion Criteria:

          -  Patients with a diagnosis of nonmetastatic or metastatic breast cancer in complete
             clinicla response classic HER2pos (ie, IHC 3+ or FISHpos) breast cancer (BC) who have
             previously been vaccinated with DC1 HER2-pulsed vaccines on any of several prior
             clinical trials for ductal carcinoma in situ (DCIS) or inflammatory breast cancer
             (IBC) are eligible; however, we also allowed HER2 2+ patients in many of these prior
             trials and they will also be allowed to participate in this trial. Note: HER2pos BC is
             defined by tumor tissue HER2 overexpression and or tumor HER2 amplification. The lack
             of HER2 overexpression by IHC is defined as 0 or 1+ whereas overexpression is defined
             as 3+. In the event of equivocal IHC, 2+, the tumor must be gene-amplified by
             fluorescent in situ hybridization (FISH) performed upon the primary tumor or
             metastatic lesion (ratio > 2 and HER2 copy number > 4 define HER2negdisease).

          -  Patients with nonmetastatic HER posBC must have completed all standard-of-care
             treatment for nonmetastatic BC (e.g., surgery, chemotherapy, radiation therapy, and
             HER2-targeted therapy). Note: antiestrogen therapy is permitted while on trial. Note:
             antiestrogen therapy is permitted while on trial.

          -  Patients with diagnosis of metastatic HER2 pos breast cancer must have complete tumor
             response to current treatment per RECIST 1.1 and completed all standard-of-care
             chemotherapy. Note: maintenance treatment with approved HER2-targeted agents and/or
             antiestrogen therapy is permitted while on trial.

          -  Age ≥ 18 years.

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1.

          -  Participants must have normal organ and marrow function within 2 weeks of
             registration.

          -  Left ventricular ejection fraction above institutional lower limit of normal (by
             echocardiogram or multigated acquisition (MUGA) scan within 6 months of registration).

          -  For both male and female patients, effective methods of contraception must be used
             throughout the study and for 3 months following the last dose.

          -  Must have the ability to understand and the willingness to sign a written informed
             consent prior to registration on study.

        Exclusion Criteria:

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection, congenital prolonged QT syndrome, symptomatic congestive heart failure,
             unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations
             that would limit compliance with study requirements.

          -  Current use of immunosuppressive agents or systemic corticosteroids. Topical, ocular,
             intra-articular, intranasal, inhalational corticosteroids (with minimal systemic
             absorption) are allowed. Patients who have received systemic corticosteroids ≤ 30 days
             prior to starting study drug will be excluded.

          -  No other prior malignancy is allowed except for the following:

               -  Adequately treated basal cell or squamous cell skin cancer

               -  In situ cervical cancer

               -  Any other cancer from which the patient has been disease free for at least 3
                  years.

          -  Pregnant or breast feeding.

          -  Known to be HIV positive.

          -  Known current or a history of hepatitis B or C virus, including chronic and dormant
             states, unless disease has been treated and confirmed cleared.

          -  Major surgery within 4 weeks of initiation of study drug.

          -  Have not recovered to ≤ Grade 1 or tolerable Grade 2 adverse events (AEs) due to
             agents administered ≥ 28 days earlier, as documented by the treating investigator.

          -  Currently enrolled in any other clinical protocol or investigational trial that
             involves administration of experimental therapy and/or therapeutic devices, or
             investigational drug.

          -  Not able to comply with the treatment schedule and study procedures for any reason.