Subjects receiving HPV4 vaccine at month 30 ( mean age 15 years ) had a similar baseline rate of seropositivity to 1 of the 4 HPV types to those vaccinated at day 1 ( mean age 12 years ; 1.9 % [ 9 of 474 ] vs 1.7 % [ 20 of 1157 ] ) ; however , 4 of the 9 subjects vaccinated at the later age were seropositive to 3 vaccine types , indicating previous HPV exposure .