Inclusion Criteria:

• - <20 years old.

• - Male or Female.

• - Diagnosis of essential thrombocythemia according to the 2016 WHO criteria.

• - Platelet count ≥ 450 × 109 / L for more than 6 months（If the patient has JAK2 V617F, CALR or MPL gene mutation, the history may be less than 6 months） - Platelet count ≥ 1000 × 109 / L or other therapeutic indications at screening.

• - The guardians has provided written informed consent prior to enrollment.

Exclusion Criteria:

• - Known to meet the criteria for primary myelofibrosis or polycythemia vera by 2016 WHO criteria.

• - Presence of any life-threatening co-morbidity.

• - Secondary thrombocytosis.

• - Familial thrombocytosis.

• - Resistance, or intolerance, or any contraindications to interferon.

• - Interferon is used in the past 1 month before enrollment.

• - Patients with previous or present thrombosis or active bleeding.

• - WBC<4× 109 / L.

• - HGB<110g/L.

• - Poor control of thyroid dysfunction.

• - Patients with a prior malignancy within the last 3 years.

• - Patients with severe cardiac or pulmonary dysfunction.

• - Severe renal damage (creatinine clearance < 30 ml / min) - Severe liver dysfunction (ALT or AST > 2.5×ULN) - Patients diagnosed as diabetes with poor control.

• - Patients with hepatitis B virus, hepatitis C virus replication or HIV infection.

• - Patients with a history of drug / alcohol abuse (within 2 years before the study) - Patients that have participated in other experimental researches within one month before enrollment.

• - History of psychiatric disorder.

- Any other circumstances that the investigator considers that the patient is not suitable to participate in the trial

This is a prospective, open-label, nonrandomized, single-center clinical trial between Interferon Alfa and Pegylated Interferon Alfa-2b in childhood and adolescent essential thrombocythemia (<20 years). Patients will be divided into the following two treatment groups: 1. Recombinant Interferon Alpha, with an initial dose of 300 wu twice a week. Other interferons that have been listed can be used if Recombinant Interferon Alpha (300 wu) is not available, and the specific dose will be determined by the researchers; 2. Pegylated Interferon Alfa-2b, with an initial dose of 135 ug once a week (body surface area < 1.73 m2) or 180 ug once a week ( body surface area≥1.73 m2). The current drug therapies and possible risks of Pegylated Interferon Alfa-2b and Interferon Alfa in the treatment of childhood and adolescent essential thrombocythemia will be fully introduced to the guardians (childhood patients) or patients (adolescent patients) by the researchers. Then the patients will be divided into one of the two groups according to the guardians' (childhood patients) or patients' (adolescent patients) will. The dosage will be adjusted according to the results of laboratory examinations and patient tolerance. The patient will be transferred to the other group if intolerance or resistance occurs.

Arms

Active Comparator: Recombinant Interferon Alpha

Recombinant Interferon Alpha, with an initial dose of 300 wu twice a week. Other interferons that have been listed can be used if Recombinant Interferon Alpha (300 wu) is not available, and the specific dose will be determined by the researchers.

Experimental: Pegylated Interferon Alfa-2b

Pegylated Interferon Alfa-2b, with an initial dose of 135 ug once a week (body surface area < 1.73 m2) or 180 ug once a week ( body surface area≥1.73 m2).

Interventions

Drug: - Recombinant Interferon Alpha

Recombinant Interferon Alpha, with an initial dose of 300 wu twice a week. Other interferons that have been listed can be used if Recombinant Interferon Alpha (300 wu) is not available, and the specific dose will be determined by the researchers;

Drug: - Pegylated interferon alfa-2b

Pegylated Interferon Alfa-2b, with an initial dose of 135 ug once a week (body surface area < 1.73 m2) or 180 ug once a week ( body surface area≥1.73 m2).