Studies are under way to determine if a booster, in addition to the three initial intramuscular injections, will be necessary for long-term protection. Clinicians know that the new cancer vaccine remains effective for up to at least 4 years, but more research is needed to find out what happens after that time. An NCI study in progress will follow vaccinated women for several more years to obtain information on the vaccine's long-term safety and the extent and duration of its protection.