The 2013-2014 influenza vaccines include those that are trivalent (protects against 3 strains) and quadrivalent (protects against 4 strains). Both types of vaccine contain new strains from the previous season. Because of the new strains and the fact that immunity drops up to 50% 6-12 months after vaccination, influenza vaccine is recommended every year for all those over 6 months of age.
Please note the AAP Recommendations for Influenza Immunization of Children, 2013-14 will be released Sept 2, 2013 and published in the October issue of Pediatrics.
For more information: http://www2.aap.org/immunization/pediatricians/influenzaguidance.html