8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19A, 19F, 20, 22F, 23F, and 33F), and so covers 90 percent of the types found in pneumococcal bloodstream infections. Young children Children under the age of two years fail to mount an adequate response to the 23-valent adult vaccine, and instead a 13-valent pneumococcal conjugated vaccine (PCV13; for example, Prevnar 13) is used instead. PCV13 replaced PCV7, adding six new serotypes to the vaccine. While this covers only thirteen strains out of more than ninety strains, these thirteen strains caused 80–90 percent of cases of severe pneumococcal disease in the U.S. before introduction of the vaccine, and it is considered to be nearly 100 percent effective against these strains. Special risk-groupsChildren at special risk (e.g., sickle cell disease and those without a functioning spleen) require additional protection using the PCV13, with the more extensive PPSV-23 given after the second year of life or two months after the PCV13 dose: See also Streptococcus pneumoniae Pneumonia Conjugate vaccine References Further reading External links Pneumococcal Vaccines World Health Organization (WHO) PATH's Vaccine Resource Library pneumococcus resources Vaccines