The purpose of the study is to evaluate and compare the immune response to two vaccines against 4 related bacteria: meningococcal serogroups A, C, W−135 and Y. These bacteria can cause meningitis and /or septicaemia (blood poisoning). The two vaccines are a protein−polysaccharide conjugate vaccine (MenACWY)and a meningococcal plain polysaccharide vaccine(MenACWY PS). Both vaccines are licensed and are currently used for travellers to areas with a high incidence of invasive meningococcal disease. However, plain polysaccharide vaccines are known to be poorly immunogenic in children and they do not stimulate immunological memory, apart from the serogroup A component. In contrast, a protein-polysaccharide conjugate vaccine against meningococcal serogroups A, C, W−135 and Y has been found to be immunogenic in infants and to be able to induce immunological memory.