If I have the combined ACWY vaccine against meningococcal meningitis, how long should it make me immune to this group of diseases for?
About 5 years. However, vaccine efficacy can vary from person to person. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm6003a3.htm
Referring to the meningococcal vaccine, the CDC's ACIP wrote: "After licensure, additional data on bactericidal antibody persistence, trends in meningococcal disease epidemiology in the United States, and VE have indicated many adolescents might not be protected for more than 5 years."
The purpose of the vaccine is to protect children against the peak incidence of infection which is at ages 16-21. The at risk groups are
Certain groups are at increased risk for meningococcal serogroups A, C, W, and Y but not serogroup B. These risk factors include HIV infection, travel to places where meningococcal disease is common (such as certain countries in Africa and in Saudi Arabia), and college students living in a dormitory. Other risk factors for serogroups A, C, W. and Y include having a previous viral infection, living in a crowded household, having an underlying chronic illness, and being exposed to cigarette smoke (either directly or second-hand).
Therefore the vaccination schedule for otherwise healthy children is
All adolescents should receive a dose of MenACWY at 11 or 12 years of age. A second (booster) dose is recommended at 16 years of age. Adolescents who receive their first dose at age 13 through 15 years should receive a booster dose at age 16 through 18 years. The minimum interval between MenACWY doses is 8 weeks. Adolescents who receive a first dose after their 16th birthday do not need a booster dose unless they become at increased risk for meningococcal disease. Colleges may not consider a second dose given even a few days before age 16 years as valid, so keep that in mind when scheduling patients.
The aim is to ensure that bactericidal level of antibody present. Although a single vaccination creates antibodies, the disease progresses so rapidly that that body is unable to respond fast enough using memory cells to create a bactericidal response. Therefore the schedule calls for the booster shot.
The Meningococcal vaccines: WHO position paper, November 2011 says regarding effectiveness:
Recent estimates of the effectiveness of the first licensed quadrivalent vaccine suggest that within 3 to 4 years after vaccination, effectiveness is 80% to 85%.