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Differentiating True Measles Cases From Vaccine Shedding

Differentiating True Measles Cases From Vaccine Shedding 1024 683 Mary Bates, PhD

Shedding of measles vaccine RNA can occur for up to four weeks post vaccination.

Detection of measles vaccine RNA following vaccination is not uncommon and may occur up to four weeks post-vaccination, according to a new study from Nationwide Children’s Hospital.

“The frequency at which detection after MMR occurs in this patient population really highlights, in a measles outbreak setting, the need to distinguish wild type versus the vaccine strain in terms of your PCR,” says Matthew Washam, MD, MPH, a chief of Epidemiology and Infection Prevention at Nationwide Children’s and coauthor of the study.

Measles, mumps and rubella (MMR) vaccination is a critical component of measles outbreak responses. But since this vaccine contains live, weakened viruses, viral genetic material (RNA) can be “shed” into the body, making case classification difficult. Recently immunized individuals may have side effects from the vaccine that look like symptoms of wild-type infection, as well as detectable levels of measles vaccine RNA. It may be especially challenging to differentiate true measles cases from cases of recent vaccination in individuals with overlapping respiratory symptoms from another viral illness.

In 2022, central Ohio experienced a measles outbreak resulting in 85 cases. The Clinical Microbiology Laboratory at Nationwide Children’s Hospital developed an in-house Pan-measles PCR test to aid the rapid diagnosis of patients with suspected measles. The assay detects measles RNA from nasopharyngeal samples and can detect both vaccine and wild-type measles strains. The team at Nationwide Children’s used an additional genotype A PCR assay to differentiate vaccine strains among samples testing positive by the Pan-measles PCR.

For the new study, Dr. Washam and colleagues identified children who were tested with a respiratory panel within 30 days after receiving the MMR vaccine during the recent measles outbreak. Their remaining nasopharyngeal samples were screened for measles vaccine by Pan-measles PCR.

“We found a surprisingly high percentage of these kids, approximately one-third of them, had detectable measles vaccine RNA after vaccination,” says Huanyu Wang, PhD, D(ABMM), director of Molecular Microbiology Laboratory at Nationwide Children’s, associate professor of Pathology at the Ohio State University College of medicine, and senior author of the study.

The results suggest that most measles RNA detection will occur within two weeks after vaccination, but detection out to 29 days can occur. However, the findings also point to a measure that can differentiate vaccine RNA and wild-type infection. As indicated by median cycle threshold (Ct) values, the magnitude of vaccine RNA shedding is significantly lower than that of true measles cases.

Dr. Washam, who is also an assistant professor of Pediatrics at The Ohio State University College of Medicine, emphasizes that this does not mean that the MMR vaccine gives children measles. “It’s a highly safe and effective vaccine,” he says.

Rather, the results indicate that clinicians and public health officials should be aware of the possibility of measles vaccine detection in recently vaccinated individuals. While awaiting genotyping testing, the Pan-measles PCR Ct value may help with time-sensitive public health decisions.

 

Reference:

Washam MC, Leber AL, Oyeniran SJ, Everhart K, Wang H. Shedding of measles vaccine RNA in children after receiving measles, mumps and rubella vaccination. J Clin Virol. 2024 Aug;173:105696. doi: 10.1016/j.jcv.2024.105696. Epub 2024 May 24.

Image credit: Adobe Stock

About the author

Mary a freelance science writer and blogger based in Boston. Her favorite topics include biology, psychology, neuroscience, ecology, and animal behavior. She has a BA in Biology-Psychology with a minor in English from Skidmore College in Saratoga Springs, NY, and a PhD from Brown University, where she researched bat echolocation and bullfrog chorusing.

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