In this single centre, open−label, randomised, controlled study of 150 healthy adults aged 18−70 years we will be evaluating the immune response to immunisation with 2 different vaccines against 4 related bacteria known as Neisseria meningitidis serogroups A, C, W−135 and Y. These bacteria (also known as meningococci) can produce meningitis and septicaemia (blood poisoning). The first vaccine, which has been used as a travel vaccine in the UK for several years, is known as the MenACWY plain polysaccharide (MenACWY PS). The other vaccine, known as the MenACWY conjugate vaccine (MenACWY) was licensed in the UK in March 2010 and is now recommended as a travel vaccine by the Department of Health.