HMM: HPV Vaccine Expanded for People Ages 27 to 45. “If a person has already been exposed to a particular strain of HPV, the vaccine will not work against that strain. For that reason, vaccination has been strongly recommended for young people before they become sexually active. But even someone who has already been exposed to a few strains — but not to all nine in the vaccine — can still gain protection against the strains they have not encountered.”