A woman in her 40s known to have systemic lupus erythematosus presented with a maculopapular rash on her face, neck and chest following measles exposure. She had received a single-dose measles vaccine as a child in the 1970s and was therefore presumed to be immune, and thus not infectious. As a result, she was initially managed in an open bay. Measles virus IgM antibody in serum was undetectable; however, measles virus RNA was subsequently detected in throat swab by PCR, which is consistent with current infection. Measles is one of the most transmissible diseases in the world and cases are rising both in the UK and globally. Our case and literature review highlight the risk of vaccine failure in measles, especially in people who have not received two doses of the measles, mumps and rubella vaccine. It also highlights the challenges in diagnosing measles in previously vaccinated individuals.