Vaccinated students with negative enzyme immunoassay results show positive measles virus-specific antibody levels by immunofluorescence and plaque neutralisation tests

BACKGROUND: The prevalence of measles antibodies was investigated by an enzyme immunoassay (EIA) in students aged 14 every year since 1996 in a Swiss municipality. This region has wide measles vaccine coverage (first dose < or = 95%, second dose < or = 65%) without any reported measles outbreaks since 20 years. In 2003 and 2004, in contrast to previous years, surprisingly many negative results (33% and 54%, respectively) were observed. OBJECTIVES: To corroborate the measles antibody values by different methods. STUDY DESIGN: Serum samples from 101 students with known vaccination status were available. Sera with equivocal and negative results obtained by two different EIAs were retested by indirect immunofluorescence test (IFT) and plaque neutralisation test (PNT). RESULTS: Retesting by IFT showed a positive result in 17/21 sera (81%) and retesting by PNT indicated that 46/49 sera (94%) were positive; the three sera with negative PNT result were from unvaccinated individuals. Only 3/96 vaccinated students showed measles antibodies below the putative protective level of 0.2 IU/ml after retesting by PNT. CONCLUSIONS: Negative EIA results should be interpreted with caution in a widely vaccinated population without booster by circulation of wild viruses. Retesting by IFT or PNT is recommended.