Inclusion Criteria:

Patients are eligible to be included in this study only if all of the following criteria apply:

1. Must be capable of providing written, signed, and dated informed consent; and for patients who are minors, age-appropriate assent (performed according to local regulations) and parent/caregiver consent.
2. Males and females aged ≥12 and <29 years old at the time of Screening.
3. Diagnosed with T1D (according to the American Diabetes Association [ADA] classification) ≤6 months at the time of Screening.
4. Possess the HLA DR3-DQ2 haplotype (all patients will be tested; prior genetic testing results will not be accepted).
5. Fasting C-peptide ≥0.12 nmol/L (≥0.36 ng/mL) on at least one occasion (maximum two tests on different days during the Screening period).
6. Possess detectable circulating GAD65 antibodies (lowest level of detection defined by the method used by the central laboratory).
7. Possess HbA1c levels between 35 to 80 mmol/mol (5.4 to 9.5%) on at least one occasion prior to randomization (maximum one additional test within one month from Visit 1B).
8. Be on a stable insulin dose or insulin dosing regimen for one month prior to inclusion with limited fluctuation of daily insulin requirement based on investigator's assessment. For example, if the average insulin dose/kg/24h over a 7-day period compared to the previous 7-day period does not vary more than approximately 15% and/or if the daily insulin dose does not vary more than 0.1 U/kg/24h, the dose can be considered stable. Individuals that are diagnosed with T1D according to the ADA classification but are not taking insulin are eligible to participate.
9. i. Females of childbearing potential (FOCBP) must agree to avoid pregnancy and have a negative pregnancy test performed at the required study visits.

FOCBP must agree to use highly effective contraception, during treatment and, until 90 days after the last administration of study medication. Birth control methods, which may be considered as highly effective (e.g., a failure rate of less than 1% per year when used consistently and correctly) include:

• Combined (estrogen and progestogen containing) hormonal contraception associated with inhibition of ovulation:
  • Oral.
  • Intravaginal.
  • Transdermal.
• Progestogen-only hormonal contraception associated with inhibition of ovulation:
  • Oral.
  • Injectable.
  • Implantable.
• Intrauterine device.
• Intrauterine hormone-releasing system.
• Bilateral tubal occlusion.
• Vasectomized partner (vasectomized partner is a highly effective birth control method provided that partner is the sole sexual partner of the FOCBP trial patient and that the vasectomized partner has received medical assessment of the surgical success).
• Sexual abstinence (sexual abstinence is considered a highly effective method only if defined as refraining from heterosexual intercourse during the entire period of risk associated with the study drugs. The reliability of sexual abstinence needs to be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient).

  9. ii. Male patients must agree to remain abstinent from heterosexual sex during treatment and for 90 days after treatment or, if sexually active, to use two effective methods of birth control (e.g., male uses a condom and female uses contraception) during and for 90 days after treatment. Acceptable male contraception is as follows:

• Condom (male).
• Abstinence from heterosexual intercourse.
• Vasectomy. The agreement to remain abstinent or use two effective methods of birth control will be clearly defined in the informed consent; the patient or legally authorized representatives (e.g., parents, caregivers, or legal guardians) must sign this specific section.

Exclusion Criteria:

• Patients are not eligible to be included in this study if any of the following criteria apply:
  1. Participation in any other trial aimed to influence beta cell function from time of diagnosis of T1D.
  2. Treatment with any oral or non-insulin injectable anti-diabetic medication within 3 months prior to Screening.
  3. History of maturity-onset diabetes of the young (MODY).
  4. Pancreatic surgery, chronic pancreatitis, or other pancreatic disorders that could result in decreased beta cell capacity (e.g., pancreatogenous diabetes).
  5. History of DKA or severe hypoglycemia requiring hospitalization within one month before Screening, or severe episodes of hypoglycemia requiring third party assistance within one month before Screening.
  6. Signs or symptoms suggesting very poorly controlled diabetes e.g., ongoing weight loss, polyuria or polydipsia.
  7. Hematologic condition that would make HbA1c uninterpretable including:
     1. Hemoglobinopathy, with the exception of sickle cell trait or thalassemia minor; or chronic or recurrent hemolysis.
     2. Donation of blood or blood products to a blood bank, blood transfusion or participation in a clinical study requiring withdrawal of >400 mL of blood during the 8 weeks prior to the Screening visit.
     3. Significant iron deficiency anemia.
     4. Heart malformations or vaso-occlusive crisis (VOC) leading to increased turnover of erythrocytes.
  8. Treatment with marketed or over-the-counter Vitamin D at the time of Screening and unwilling to abstain from such medication during the 120 days when the patient will be supplemented with the study-provided Vitamin D. A patient currently taking Vitamin D at the time of Screening must be willing to switch to the study-provided Vitamin D treatment and to administer it per the study requirements.
  9. Any clinically significant history of an acute reaction to a vaccine or its constituents (e.g., Alhydrogel).
  10. Treatment with any (live or inactive) vaccine, including influenza vaccine and Coronavirus Disease 2019 (COVID-19) vaccine, within 4 weeks prior to planned first study dose of study drug; or planned treatment with any vaccine up to 4 weeks after the last injection with study drug.
  11. Any acute or chronic skin infection or condition that would preclude intralymphatic injection.
  12. Recent (past 12 months) or current treatment with immunosuppressant therapy, including chronic use of glucocorticoid therapy. Inhaled, topical, and intranasal steroid use is acceptable. Short courses (e.g., ≤5 days) of oral or intra-articular injections of steroids will be permitted on trial.
  13. Continuous/chronic treatment with prescribed or over-the-counter anti-inflammatory therapies. Short-term use (e.g., <7 days) is permissible, for example to treat a headache or in connection with a fever.
  14. Known or suspected acute infection, including COVID-19 or influenza, at the time of Screening or within 2 weeks prior to Screening. After confirmed recent COVID-19 infection, a negative polymerase chain reaction test will be required before randomization.
  15. A history of epilepsy, head trauma or cerebrovascular accident, or clinical features of continuous motor unit activity in proximal muscles.
  16. Known diagnosis of human immunodeficiency virus (HIV), hepatitis B or hepatitis C infection. Patients with previous hepatitis C infection that is now cured may be eligible.
  17. Any clinically significant concomitant medical condition, including but not limited to other autoimmune diseases, cardiovascular, gastrointestinal, hematological, immune, renal including a history of renal transplantation, neurological (including Batten disease), significant diabetes complication, any underlying conditions or receiving treatments that could affect red blood cell turnover or other diseases that in the opinion of the investigator would interfere with trial participation or procedures. Celiac disease with adequate diet before diagnosis or discovered by increased autoantibodies at Screening will be permitted.
  18. History of significant hepatic disease or Screening alanine aminotransferase (ALT) >2.5 x upper limit of normal (ULN) or aspartate aminotransferase (AST) 3 x ULN and/or total bilirubin >2 x ULN. Patients with documented Gilbert syndrome and total bilirubin level ≥2 x ULN due to unconjugated hyperbilirubinemia, without other hepatic impairment, are permitted.
  19. Estimated glomerular filtration rate (eGFR) calculated by Chronic Kidney Disease Epidemiology Collaboration (CKD-Epi) for those >18 years old, and by the Schwartz equation for those 12 to 18 years old, <90 mL/min per 1.73 m or rapidly progressing renal disease.
  20. Patients with hypothyroidism or hyperthyroidism must be on stable treatment for at least 3 months prior to Screening (with normal free thyroxine [T4] levels if hypothyroid).
  21. Any clinically significant abnormal findings during Screening, and any other medical condition(s) or laboratory findings that, in the opinion of the investigator, might jeopardize the patient's safety or ability to complete the trial.
  22. History of malignancy not in remission within the last 5 years other than adequately treated basal cell or squamous cell skin cancer or cervical carcinoma in situ.
  23. Patients with any mental condition rendering him/her unable to understand the nature, scope and possible consequences of the trial, and/or evidence of poor compliance with medical instructions at Screening or showing non-compliance during the Run-In Period.
  24. A history of alcohol or drug abuse or dependence within the past 12 months based on DSM IV criteria.
  25. Current or previous participation in a trial of Diamyd.
  26. Participation in a clinical trial involving administration of an investigational drug in the past 3 months or 5 half-lives (whichever is longer) prior to first dosing of study drug or during the trial.
  27. Females who are breastfeeding, pregnant or plan to become pregnant during the trial.
  28. Patients who in the opinion of the investigator will not be able to follow instructions and/or follow the study procedures or patients that are unwilling or unable to comply with the provisions of this protocol.
  29. An employee or immediate family member of an employee of Diamyd Medical AB.