Linear lichen planus (LLP); Elevated AST (79 U/L with normal range 1759); Elevated [AST and] creatine kinase (906 U/L with normal range 55-250); Skin biopsy showed a lichenoid tissue reaction with basal vacuolar change with as band like lymphocytic infiltrate, apoptotic keratinocytes, and pigment incontinence consistent with lichen palnus; Multiple well-demarcated hyperpigmented macules; Slghtly raised agminated papules; [Coalescing into] linear plaques, unilaterally on the left posterior trunk, flank, and chest; Dermatitis continued to spread; Brown lesions that started on his chest and spread to his trunk; Pruritic; This is a serious literature case, initially received on 10-Jun-2019, concerning a 43-year-old, adult male patient.    The article describes a rare case of linear lichen planus following intramuscular influenza vaccination, not associated with an injection site reaction.   The patient had a medical history significant for herpes labialis treated with acyclovir three times a year. The patient had received multiple, annual vaccinations for influenza and regular vaccinations for other diseases. It was reported that his vaccinations were up-to-date, including 14 consecutive, annual influenza vaccinations, 11 of which were intramuscular.  On an unspecified date, the patient was administered INN Flu Vaccine Seasonal (influenza virus vaccine polyvalent, route of administration: intramuscular, dose, anatomical location, batch number, expiry date, trade name and manufacturer: not reported) for an unknown indication. It was reported that it was the 14th influenza vaccination that the patient received.  On an unspecified date, reported as within 8 to 10 days after vaccination, the patient experienced brown lesions (reported as a 5-month history) that started on his chest and spread to his trunk which gradually became pruritic, weeks after onset. The patient associated onset with intense physical workouts; however, he admitted these workouts were no different than his routine. The patient also denied new personal care products or medications. On an unspecified date, his examination was significant for multiple well-demarcated hyperpigmented macules and slightly raised agminated papules; some were coalescing into linear plaques, unilaterally on the left posterior trunk, flank, and chest. It was reported that the patient had no remote or recent cutaneous trauma at the sites of lesions. On an unspecified date, the laboratory workup was significant for elevated AST (79 U/L with normal range 1759) and creatinine kinase (906 U/L with normal range 55250); additional infectious workup, including for sexually transmitted diseases and viral hepatitis, were negative. A skin biopsy showed a lichenoid tissue reaction with basal vacuolar change with a band-like lymphocytic infiltrate, apoptotic keratinocytes, and pigment incontinence consistent with lichen planus. Given the unilateral blashko-linear distribution, the patient was given the clinicopathologic diagnosis of linear lichen planus (LLP). The clinical differential diagnosis included lichen striatus, adult blaschkitis, inflammatory linear verrucous epidermal nevus, linear psoriasis, and linear DarierWhite disease and these could be histologically distinguished on skin biopsy. The patient was given a topical corticosteroid but failed to use the medication as prescribed, and dermatitis continued to spread. Though the patient was advised to regularly use a topical corticosteroid, at 1-year follow-up, the patient reported the inconsistent and minimal use of the topical corticosteroid and received another annual flu vaccination with further cutaneous dissemination and continued symptoms.   The outcome of all the events was not recovered.  The reporter (author) assessed the causality of the event LLP as related to the suspect vaccine as it was reported that the temporal relationship of the onset of LLP to the influenza vaccination was strongly suggestive that the inactivated influenza vaccine was the antigenic trigger. It was reported that immune-related dermatoses might be underreported, and a vaccine history should be considered in all patients with a skin condition that is otherwise attributed as idiopathic. The reporter concluded that the somatic mosaicism could be exhibited by genetically predisposed patients who had experienced a triggering environmental input, such as a vaccine.  This case is linked to 201903719 (same patient).; Reporter's Comments: A 43-year-old male patient had diagnosis of linear lichen planus confirmed with skin biopsy, within 8 to 10 days after vaccination with INN flu vaccine. Considering the plausible temporal relationship and absence of alternate aetiologies, the company assessed the causality of linear lichen planus as related to suspect vaccine. The events as aspartate aminotransferase increased, blood creatine phosphokinase increased and biopsy skin abnormal can be explained by the lichen planus, hence assessed as not related to suspect vaccine.  The company assessed the event 'lichen planus' as serious (medically significant).