If a single dose of HPV vaccine were effective, that would be an important advance. A large observational study using national data from women across Australia found that one dose of HPV vaccine was as effective as two or three doses in preventing high-grade cervical lesions (34). An analysis of data from a community-based clinical trial of Cervarix in Costa Rica, found that even one dose of the vaccine caused the body to produce approximately nine times more antibodies against HPV than the body produces in response to a natural HPV infection, and those antibody levels persisted for 11 years (35). In addition, the rates of HPV infection remained low for at least 10 years (35).
Two NCI-led clinical trials have been launched in Costa Rica to confirm and extend these findings. The ESCUDDO study, a randomized double-blind controlled trial involving 20,000 girls ages 12–16 years, is testing whether one dose of either Cervarix or Gardasil 9 is as effective as two doses at preventing persistent cervical infection with HPV. PRIMAVERA-ESCUDDO, a non-randomized open-label trial, will provide earlier and complementary results to ESCUDDO about the immunogenicity of one dose of Cervarix in girls ages 9–14 years compared with three doses of Gardasil in women ages 18–25 years.
Another prevention strategy that is being explored is topical microbicides. Carrageenan, a compound that is extracted from a type of seaweed and used widely in foods and other products, has been found to inhibit HPV infection in laboratory studies. An interim analysis of data from a randomized clinical trial showed that consistent use of a lubricant gel that contains carrageenan reduced the risk of genital HPV infection in healthy women (36).
Researchers are working to develop therapeutic HPV vaccines, which instead of preventing HPV infection would prevent cancer from developing among women previously infected with HPV (37–40). These vaccines work by stimulating the immune system to specifically target and kill infected cells. Ongoing clinical trials are testing the safety and efficacy of a therapeutic DNA vaccine to treat HPV-related cervical and vulvar lesions.