Understanding Equivocal Results, IgG vs IgM, and Live Vaccines
What Does "Equivocal" Mean in Serologic Testing?
Persons with an "equivocal" serologic test result should be considered susceptible to measles, mumps, or rubella unless they have documented evidence of adequate vaccination or subsequent testing confirms immunity. 1
Practical Management of Equivocal Results:
- Treat equivocal results as susceptible and either vaccinate or retest, rather than assuming immunity 1
- For healthcare workers with documented age-appropriate MMR vaccination who test equivocal, the documented vaccination supersedes the serologic result—they should still be considered immune 2
- If a person with documented measles vaccination has equivocal or borderline IgG by ELISA, administer one dose of MMR vaccine and do not retest for serologic evidence of immunity 3
IgG vs IgM: Key Differences
IgG (Immunoglobulin G):
- IgG indicates long-term immunity and is the ONLY appropriate test for determining immune status 4, 3, 5
- IgG persists lifelong after natural infection or successful vaccination 3
- The presence of serum IgG is the only reliable evidence of previous rubella infection and immunity 1
- Any antibody level above the standard positive cutoff value of a licensed assay is considered evidence of immunity 1, 2
IgM (Immunoglobulin M):
- IgM indicates acute or recent infection and should NEVER be used for routine immunity screening 3, 5
- IgM is only appropriate when acute illness is suspected 5
- After primary MMR vaccination, IgM drops from 73% positive at 4 weeks to only 7% by 8 weeks post-vaccination 6
- Less than 10% of children remain IgM positive between 9-11 weeks after vaccination 6
- Critical pitfall: The majority of IgM testing for measles, mumps, and rubella appears inappropriate—only 15.4% for measles, 32.8% for mumps, and 10.2% for rubella testing was performed appropriately 5
Clinical Algorithm:
- For immunity screening: Order IgG only 4, 3, 5
- For suspected acute infection: Order both IgM and IgG, or IgM with RT-PCR 5
- Never order IgM alone for immunity assessment 5
Is MMR a Live Vaccine?
Yes, MMR is a live attenuated virus vaccine containing weakened forms of measles virus, mumps virus, and rubella virus. 7
- MMR contains live rubella-containing vaccine 1
- The vaccine works by helping the immune system protect against infection through exposure to weakened viral strains 7
Other Live Vaccines
Common Live Attenuated Vaccines:
While the provided evidence focuses specifically on MMR, based on general medical knowledge, other live vaccines include:
- Varicella (chickenpox) vaccine
- Zoster (shingles) vaccine (live version, not the newer recombinant)
- Rotavirus vaccine
- Yellow fever vaccine
- Intranasal influenza vaccine (FluMist)
- Oral typhoid vaccine
Key Clinical Implications of Live Vaccines:
- Contraindicated in pregnancy: Women should avoid pregnancy for 1 month following MMR vaccination 7
- Contraindicated with weakened immune systems: Including high-dose steroids by mouth or injection 7
- Live vaccines may not be given to persons with active untreated tuberculosis 7