A young patient presents with a fever and rash: Is this an adverse effect of the mRNA vaccine, vasculitis, or rickettsiosis?

The aetiology may be complex in patients presenting with fever and rash. The differential diagnosis may include coronavirus disease 2019 (COVID-19) infection, an adverse effect of the COVID-19 vaccine, infection, and vasculitis. We reported a patient who presented with fever and vasculitic rash, which we hypothesized was an adverse vaccine effect. A 35-year-old male patient presented to the emergency department reporting headache, fever, rash, weakness, and myalgia. The first dose of the mRNA vaccine, COVID-19, had been administered five days before his presentation. A nasopharyngeal severe acute respiratory syndrome coronavirus two-challenge test was negative. Antinuclear antibody, antineutrophil cytoplasmic antibody, rheumatoid factor, and cryoglobulin were negative. No hypocomplementemia was detected. Skin biopsy was predominantly lymphocytic, with a vasculitic reaction with a few neutrophils. The Rickettsia conorii immunoglobulin M test examined using enzyme-linked immunosorbent assay (ELISA) was positive. COVID-19 should be excluded in patients with fever, rash, and headache. Symptoms that occur after vaccination may indicate adverse reactions. Even though we are in the pandemic phase, rickettsiosis should not be forgotten. Key Points • COVID-19 is suspected first in the patient who applied to the emergency department with the complaints of fever and maculopapular rash during the pandemic era. • Although the COVID-19 vaccine is widely used, possible side effects of the vaccine may raise doubts among physicians and patients. • The preliminary diagnosis of a patient with a vasculitic rash who complains of fever and myalgia should be thought to be Rickettsiosis.