Dutch researchers studied immunologic memory against meningococci two and a half years after 177 children received either the hexavalent meningococcal outer membrane vessicle (OMV) vaccine or hepatitis B vaccine. The children were revaccinated with monovalent P1.7(h),4 meningococcal OMV vaccine. According to the authors, the findings reflect the presence of immunologic memory in toddlers and school-aged children previously given three hexavalent meningococcal vaccinations. The researchers concluded that "booster vaccination with monovalent P1.7(h),4 meningococcal OMV vaccine induces a significant increase in [serum bactericidal antibodies] against serosubtype P1.7(h),4 and cross-reactivity against other serosubtypes in the hexavalent vaccine."