The purpose of this trial is to test if treatment with the trial drug, tralokinumab, can affect the body's immune response to vaccines. The trial will also evaluate the efficacy of tralokinumab when it is given concomitantly with vaccines.

The trial includes a screening period of 2 to 6 weeks, a treatment period of 16 weeks (Weeks 0 to 16), and a 14-week off-treatment follow-up period for the assessment of safety (Weeks 16 to 30). Eligible subjects may transfer to an open-label, long-term trial at Week 16 or later.

Subjects with atopic dermatitis (AD) will be treated with either tralokinumab or dummy treatment (placebo) for 16 weeks. All subjects will receive 2 vaccines at Week 12. The vaccines are:

1. Tetanus (lockjaw), diphtheria (infection of the nose and throat), and pertussis (whooping cough) vaccine. This combination vaccine is also known as the Tdap vaccine and is used to prevent these 3 diseases.
2. Meningococcal vaccine. This vaccine is used to prevent meningococcal diseases (infection of the brain and spinal cord) and blood poisoning.

The primary objective of the trial is to demonstrate non-inferiority of tralokinumab versus placebo with respect to immune responses to concomitantly administered vaccines.

The secondary objective is to evaluate efficacy of tralokinumab concomitantly administered with vaccines.

Inclusion Criteria:

• Age 18 to 54 years
• Diagnosis of AD as defined by Hanifin and Rajka (1980) criteria for AD
• History of AD for ≥1 year
• Subjects who have a recent history of inadequate response to treatment with topical medications or for whom topical treatments are otherwise medically inadvisable
• AD involvement of ≥10% body surface area at screening and baseline
• An EASI score of ≥12 at screening and 16 at baseline
• An IGA score of ≥3 at screening and at baseline
• Subjects must have applied a stable dose of emollient twice daily (or more, as needed) for at least 14 days before randomisation

Exclusion Criteria:

• Subjects for whom administration of the meningococcal vaccine provided in this trial is contraindicated or medically inadvisable, according to local label of the vaccine
• Subjects for whom administration of the tetanus, diphtheria, and pertussis vaccine provided in this trial is contraindicated or medically inadvisable, according to local label of the vaccine
• Active dermatologic conditions that may confound the diagnosis of AD or would interfere with assessment of treatment
• Use of tanning beds or phototherapy within 6 weeks prior to randomization
• Treatment with systemic immunosuppressive/immunomodulating medications and/or systemic corticosteroids within 4 weeks prior to randomization
• Treatment with the topical medications topical corticosteroids (TCS), topical calcineurin inhibitor (TCI) or phosphodiesterase 4 (PDE-4) inhibitor within 2 weeks prior to randomization
• Receipt of any vaccine (except influenza virus vaccines) within 3 months prior to screening, any meningococcal vaccine within 1 year prior to screening, or any tetanus-, diphtheria-, or pertussis-containing vaccine within 5 years prior to screening
• Receipt of any marketed (i.e. immunoglobulin, anti-IgE) or investigational biologic agent, including dupilumab
• History of any active skin infection within 1 week prior to randomization
• History of a clinically significant infection (systemic infection or serious skin infection requiring parenteral treatment) within 4 weeks prior to randomization

Neither the subject nor any of the investigator or LEO staff who are involved in the treatment or clinical evaluation and monitoring of the subjects will be aware of the treatment received. The packaging and labelling of tralokinumab/placebo will contain no evidence of their identity.

Since tralokinumab and placebo are visually distinct and not matched for viscosity, they will be handled and administered by a qualified, unblinded healthcare professional at the trial site who will not be involved in the management of trial subjects.