Lipsitch M, et al. - The magnitude of an individual’s serotype-specific immunoglobulin G (IgG) response to a pneumococcal conjugate vaccine (PCV) has been associated with the vaccine’s protective efficacy against carriage of pneumococci of that serotype, though the relationship with other serotypes needs to be understood. In this current study, it was suggested that for optimizing vaccination schedules and identifying robust correlates of protection, improved understanding of the nature and causes of variation in immune response may be helpful.