Herpes Zoster (Shingles): This is the most likely diagnosis due to the description of a papular and vesicular rash, which is characteristic of shingles. The rash typically appears in a dermatomal distribution, which could include the upper back.

Contact Dermatitis: This could be a possible diagnosis if the patient has been exposed to an allergen or irritant that caused a reaction on the upper back, leading to a papular and vesicular rash.
Eczema (Atopic Dermatitis): Eczema can cause papular and vesicular lesions, especially if the skin is scratched, leading to further irritation and possible infection.
Pityriasis Rosea: Although it typically starts with a herald patch, followed by a wider rash, the description could fit if the patient is seen later in the course of the disease.

Varicella (Chickenpox): Although less common in adults, varicella can cause a severe illness and should not be missed. The rash is typically vesicular and can appear anywhere on the body, including the upper back.
Herpes Simplex Virus (HSV) Infection: While HSV typically causes oral or genital lesions, it can cause lesions anywhere on the body, including the upper back, especially in immunocompromised patients.
Impetigo: A highly contagious bacterial infection that can cause papular and vesicular lesions, which can become secondarily infected and lead to more serious complications if not treated.

Pemphigus Vulgaris: An autoimmune disease that causes blistering of the skin and mucous membranes. It's rare but should be considered in the differential diagnosis due to its potential severity.
Bullous Pemphigoid: Another autoimmune disease that causes blistering of the skin, which could present with papular and vesicular lesions.
Dermatitis Herpetiformis: A skin manifestation of celiac disease, characterized by intensely itchy, blistering, and papular lesions, typically on the elbows, knees, and buttocks, but can appear on the back.