Unilateral laterothoracic exanthem (ULE), also known as asymmetric periflexural exanthem of childhood (APEC), is a self-limited exanthem commonly seen in early childhood. The etiology is unknown, but it is thought to be a cutaneous reactive process to viral infections, with the most common being parvovirus B19, Epstein-Barr virus (EBV), adenovirus, and SARS-CoV-2. Other reported associations include influenza, parainfluenza 2 and 3, human herpesvirus 6 and 7, molluscum contagiosum, poxvirus, rubeola, and Campylobacter jejuni. It has been hypothesized that a postzygotic mutation involving the keratinocytes on one side of the body contributes to the accentuated unilateral response.

ULE is more common in the late winter and early spring. It usually affects children aged 2-3 years (although it has been described in children aged 8 months to 10 years). It is predominantly seen in females, with a female-to-male ratio of 2:1. Although uncommon in adults, many cases have been reported following both COVID-19 infection and COVID vaccination (mRNA and recombinant) in both children and adults. The eruption tends to appear 2 days after vaccine administration. The Centers for Disease Control and Prevention (CDC) encourages subsequent COVID-19 vaccines, as the reaction is not considered a severe cutaneous reaction.

A prodrome of constitutional, upper respiratory, or gastrointestinal symptoms may precede the exanthem, which begins in an axilla (or the inguinal fold, or a popliteal or antecubital fossa) and spreads to involve the trunk and/or the extremities. Associated symptoms and signs include low-grade fever (40%) with mean occurrence on the sixth day, pruritus (65%), and lymphadenopathy (70%) located adjacent to the primary area of involvement.

The exanthem usually resolves within 3-6 weeks without cutaneous sequalae. Recurrence is unusual.