Infections by encapsulated bacteria are important causes of infant mortality worldwide. Over the last 20 years protein-polysaccharide conjugate vaccines have been developed to protect against the major invasive bacterial diseases of childhood, Streptococcus pneumoniae, Haemophilus influenzae type b (Hib) and Neisseria meningitidis. These vaccines are highly immunogenic and have resulted in a huge reduction in the diseases caused by these bacteria in the countries that have introduced them in their immunisation schedules. However, it has been reported that infant immunisation is associated with a relatively short duration of antibody levels and vaccine effectiveness, despite the demonstrable presence of booster responses to further vaccine dose. In contrast, at older ages, more sustained protection has been described with just a single dose of a conjugate vaccine. Understanding the generation of long-term immunity, by protein-polysaccharide conjugate vaccines, is essential to reduce infant mortality through the improvement of vaccine formulation and scheduling.