According to official reports, the infected individual had previously received one dose of the MMR (measles, mumps, and rubella) vaccine, which likely provided a degree of partial protection but did not prevent infection. While the single-dose MMR schedule was historically used in the past, current public health guidelines recommend two doses to achieve the most reliable and long-lasting immunity.

One dose typically offers about 93% protection—high, but not complete—meaning that some people may still contract the virus if they encounter a large enough viral load or if their immune response is weaker than average. Shortly after the exposure window at O’Hare, the individual developed symptoms, including fever and a distinctive rash that first appeared on April 25.

These symptoms prompted them to seek medical care, and laboratory testing soon confirmed that they were infected with measles. Upon diagnosis, the patient immediately began isolating at home, following both medical guidance and public health laws requiring individuals with measles to avoid public settings until they are no longer contagious.