A child received 2 dose of mesales vaccine 7 to 8 months ago and child developed the measles symptom. Both IgM and PCR test are positive for measles. Can this be a false positive ?
Given that both the IgM serology and the PCR (Polymerase Chain Reaction) tests are positive, it is highly unlikely to be a false positive.
While a standalone IgM test can occasionally yield a false positive due to cross-reactivity with other febrile rash illnesses (like Parvovirus B19 or Rubella), a concurrent positive PCR provides definitive, independent virological confirmation.
Here is why this occurs and what it means for a fully vaccinated child:
Why a Double Positive is Not a False Positive
The two tests measure entirely different things, making simultaneous false results extremely rare:
- PCR (Molecular Confirmation): Detects the actual genetic material (RNA) of the measles virus. It is highly specific.
- IgM (Serological Confirmation): Detects the body's acute antibody response to an active infection.
- The Vaccine Timeline Rule: If the child had been vaccinated within the last 14 to 21 days, a positive PCR/IgM could simply be detecting the live-attenuated vaccine strain. However, because the child was vaccinated 7 to 8 months ago, the vaccine virus has long since cleared. The PCR is detecting wild-type measles virus.
After ruling out the possibility of false-positive result due to cross-reactivity associated with IgM test as confirmed by PCR test, as rightly pointed out by Wendwosen Nibabe. Another possibility may be due to secondary vaccine failure; as the child may not develop a long lasting immunity after the two doses. And can still contract the wild-type measles virus, which may eventually make the IgM and PCR test positive.
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