Read this study very carefully. Of 122 patients with seroconfirmed measles, 10 of the patients (all previously vaccinated) did not show any detectable IgM antibodies specifically against measles. This group had a significantly milder illness than those who did show IgM antibodies in the blood, whether they were vaccinated or unvaccinated!

Edmonson MB, Addiss DG, McPherson JT, Berg JL, Circo SR, Davis JP. Department of Pediatrics, University of Wisconsin, Madison 53792. Mild measles and secondary vaccine failure during a sustained outbreak in a highly vaccinated population. JAMA. 1990 May 9;263(18):2467-71.

A prolonged school-based outbreak of measles provided an opportunity to study "vaccine-modified" mild measles and secondary vaccine failure. Thirty-six (97%) of 37 unvaccinated patients had rash illnesses that met the Centers for disease Control clinical case definition of measles, but 29 (15%) of 198 vaccinated patients did not, primarily because of low-grade or absent fever. Of 122 patients with seroconfirmed measles, 10 patients (all previously vaccinated) had no detectable measles-specific IgM and significantly milder illness than either vaccinated or unvaccinated patients with IgM-positive serum. Of 108 vaccinated patients with seroconfirmed measles, 17 patients (16%) had IgM-negative serology or rash illnesses that failed to meet the clinical case definition; their mean age (13 years), age at the time of vaccination, and time since vaccination did not differ from those of other vaccinated patients. The occurrence of secondary vaccine failure and vaccine-modified measles does not appear to be a major impediment to measles control in the United States but may lead to underreporting of measles cases and result in overestimation of vaccine efficacy in highly vaccinated populations.