© 2013 The Cochrane Collaboration. This is the protocol for a review and there is no abstract. The objectives are as follows: To assess the effects of needle size for administering vaccines to children and adolescents on vaccine immunogenicity (the ability of the vaccine to elicit an immune response), procedural pain, and other reactogenicity events (adverse events following vaccine administration). We will assess the effects of two dimensions of needle size (gauge and length) separately and in combination, i.e. we will assess the effects of: needle gauge on vaccine immunogenicity, procedural pain and other reactogenicity events needle length on vaccine immunogenicity, procedural pain and other reactogenicity events needle gauge and length on vaccine immunogenicity, procedural pain and other reactogenicity events.