Background: The minimum number of doses of pneumococcal conjugate vaccine required for protection is not known. We studied the immunogenicity of a reduced schedule in infants and toddlers.
Methods: U.K. infants were given either 2 or 3 doses (at 2 and 4 or 2/3/4 months of age) of a 9-valent pneumococcal conjugate vaccine (9VPCV) followed by boosting at 12 months of age. In a separate study, toddlers (12 months) received 1 or 2 doses (2 months apart) of 9VPCV followed by pneumococcal polysaccharide vaccine at 18 months of age.
Results: For infants, serotype-specific IgG geometric mean concentrations were similar post-primary immunization between the groups with both showing avidity maturation and similar booster responses. For toddlers, the primary response to 4 of the 9 serotypes was lower in the 1- compared with the 2-dose group (type 6B, 0.77 versus 7.1; type 14, 4.67 versus 14.98; type 19F, 5.05 versus 7.75; type 23F, 2.48 versus 5.05), although for all serotypes booster responses were similar between groups, and the postprimary responses in the 1-dose group were at least as high as those after infant immunization.
Conclusions: The 2-dose infant priming schedule of 9VPCV is comparable with the 3-dose schedule and may thus be equally protective, whereas 1 dose in toddlers may suffice for a catch-up.
From the *Institute of Child Health, University College London, and the †Health Protection Agency, London, ‡Health Protection Agency, Porton Down; §Health Protection Agency, Gloucester and ∥Health Protection Agency, Manchester, United Kingdom; and ¶Princess Margaret Hospital for Children, Perth, Western Australia
Accepted for publication November 21, 2005.
Supported by U.K. Department of Health RDD project grant 1216520.
Presented in part at the International Symposium on Pneumococci and Pneumococcal Disease, May 2004, Helsinki.
Tables 1A, 1B and 1C are available on line at the journal website, www.pidj.com.
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