Tests can confuse recent MMR vaccination as measles infection, report advises
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Key takeaways:
- Syndromic PCR panels may incorrectly indicate a child with rash has measles if they recently received an MMR vaccine.
- Clinicians should assess clinical features and vaccine history after a positive test.
Tests that look for multiple causes of rash in children may incorrectly identify a measles infection if a child has recently received an MMR vaccine, investigators reported in MMWR.
They advised physicians to practice caution in reviewing PCR tests for measles to avoid misdiagnoses.
The CDC recently alerted health care providers to be on the lookout for measles amid a rise in cases. Cases have also been on the rise globally.
According to the authors of the new report, around 5% of people experience a rash 7 to 10 days after receiving an MMR vaccine.
They began their investigation after the Tennessee Department of Health received two reports of measles in children who did not have documented risk factors for the virus. Both children had rashes, and both had received an MMR vaccine.
Six state health departments collaborated with the CDC to understand the frequency and characteristics of people receiving a positive measles PCR panel test result. Syndromic PCR testing panels are commonly used to test for pathogens that can cause rash, including measles.
“A commercial laboratory that included measles testing as part of a syndromic multiplex PCR panel provided the numbers of such panels ordered in the United States during May 2022-April 2023 and the number that detected measles virus by state,” the authors wrote.
They found that syndromic PCR panels may incorrectly indicate a child with rash has measles if they recently received the MMR shot. According to their report, approximately 1% of tests reported a positive measles result after recent MMR vaccination.
“As demonstrated by this analysis, inclusion of measles virus in syndromic PCR panels can result in incidental detection of measles vaccine virus,” they wrote. “Some clinicians who received reports of measles detection by syndromic PCR panels anecdotally shared with health departments that they had neither suspected measles infection in the patient nor realized that the test panel included measles. These clinicians had diagnosed common childhood illnesses, such as roseola or impetigo before receiving test results.”
If measles is suspected, or detected by a test, the authors recommended carefully assessing clinical features and measles vaccination history and contacting the local health department immediately for guidance.
According to the authors, because one dose of MMR vaccine is 93% effective at preventing measles, if someone recently received the vaccine and has no risk factors for measles, “additional public health response is likely unnecessary.”
“However, if a person has not recently received MMR vaccine, subsequent public health response should include necessary measures to prevent measles transmission,” the authors wrote. “For a person who recently received MMR vaccine and has a risk factor for acquiring measles, additional testing for measles vaccine virus is needed to determine subsequent response measures.
References:
CDC. Measles cases and outbreaks. https://www.cdc.gov/measles/cases-outbreaks.html. Accessed Mar. 29, 2024.
Thomas CM, et al. MMWR Morb Mortal Wkly Rep. 2024;doi:10.15585/mmwr.mm7312a3.
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