However , in children and older infants , a second dose of conjugate vaccine had no incremental effect on antibody titres and , at all ages , concentrations of antibodies increased substantially 6 months after vaccination ( from 55 U/mL [ 33-94 ] to 63 U/mL [ 35-114 ] in adults , from 23 U/mL [ 15-34 ] to 51 U/mL [ 34-76 ] in children , and from 21 U/mL [ 14-31 ] to 22 U/mL [ 14-33 ] in older infants ) .