<qa>  What is the long-term follow-up study of the V503-001 base study?Protocol V503-021 is a long-term follow-up study of the V503-001 base study NCT00543543 to evaluate the safety, immunogenicity, and long-term effectiveness of V503 vaccine in preventing cervical cancer and related precancers caused by human papillomavirus HPV types 16, 18, 31, 33, 45, 52, and 58. Because of the high retention of V503-001 participants from the Nordic countries, and the highly efficient screening and surveillance system there, study V503-021 will evaluate only participants from V503-001 sites in Denmark, Norway, and Sweden. The hypothesis being tested is that V503 vaccine will remain effective for at least 14 years after the start of vaccination. 