The present invention describes methods of immunizing a patient with a combined vaccine that offers protection against meningococcal disease caused by pathogenic bacteria Neisseria meningitidis serogroups A and C. The vaccine comprises at least two distinct polysaccharide-protein conjugates that are formulated as a single dose of vaccine. The purified capsular polysaccharides of Neisseria meningitidis serogroups A and C are chemically activated and selectively attached to a carrier protein by means of a covalent chemical bond, forming polysaccharide-protein conjugates capable of eliciting long-lasting immunity to a variety of N. meningitidis strains in infants.
