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A recent report from the Centers for Disease Control and Prevention (CDC) found polymerase chain reaction (PCR) tests that simultaneously test for multiple causes of a rash may show false positives for measles in children who recently had a dose of the measles, mumps, rubella (MMR) vaccine. About approximately 5% of kids experience a rash 7–10 days after the MMR shot. A study of 1,548 syndromic PCR panels from the Tennessee Department of Health found 14 children with detectable measles virus, all of which had received an MMR vaccine dose previously, including 13 who were vaccinated within 21 days of testing.

The implications play out: CDC specifically says: “These positive test results most likely represented detection of measles vaccine virus in patients with rashes from a vaccine reaction or other cause, rather than measles infection.” Some clinicians who noted measles detection by syndromic PCR panels in the study anecdotally indicated they didn’t suspect measles infection in the patient and didn’t know the test panel included measles. Patients were diagnosed with other common illnesses such as roseola or impetigo.

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Positive Measles Test May Actually Indicate Recent Vaccine
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