In several industrialized nations, such as the USA, a significant number of girls do not complete the entire three-vaccination course. In resource-poor countries, where logistics and cost can sometimes be overwhelming problems, these findings will come as good news.
In 2010, 49% of female American 13-17 year olds received one HPV vaccine dose, and only 32% completed all three. Gardasil, the vaccine most used in America, has a different formulation to Cervarix. Gardasil protects against HPV strains 16 and 18, but also 6 and 11, which cause 90% of genital warts.
About half-a-million females are diagnosed with cervical cancer each year globally, and approximately 250,000 die. Most new cervical cancer cases are currently occurring in developing nations. Persistent HPV infection is responsible for nearly all cases of cervical cancer.
A Cervarix course consists of three vaccines over a six-month period. Investigators say so far they have observed that Cervarix provides up to 8 years protection from persistent HPV infection. How long protection will last has not been determined yet.
The aim of the Costa Rica Vaccine Trial, which was sponsored by the National Cancer Institute, was to assess Cervarix in a community-based setting. Females were randomly selected to receive either Cervarix or a Hepatitis A vaccine (controls). They had intended to give all 7,466 females the full course of three vaccines. However, approximately 20% of them only completed two doses of HPV or control vaccine. A third did not finish the course because they had cervical abnormalities, became pregnant, and some other reasons.
Those who only had two doses were found to have the same strong protection as the ones who received three, after four years of follow up. They even observed a high level of protection from the females who only had one dose. However, they warn that the long-term efficacy of one dose is doubtful.
They wrote that further studies are needed to determine how effective a single dose, or two doses are - long-term.
Lead author, Aimée R. Kreimer, Ph.D., and investigator in NCI's Division of Cancer Epidemiology and Genetics, said:
"The main question will be whether the duration of protection from fewer doses is adequate. Our study provides evidence that an HPV vaccine program using two doses will work. It may be that vaccinating more women, with fewer doses for each, will reduce cervical cancer incidence more than a standard three-dose program that vaccinates fewer women."
The authors stressed that the Cervarix study findings apply to women in Costa Rica and may not be relevant to populations everywhere, such as those with malnutrition, endemic diseases, or HIV infection - diseases that may impact on the immune system.
Also, one cannot extrapolate the findings of this trial and apply them to Gardasil, which has a different formulation.
"Further studies are needed to confirm our findings in other populations as well as to quantify the duration of protection for fewer than three doses. If other studies confirm that fewer than three doses provide adequate protection against persistent cervical HPV 16 and 18 infection, we may be one step closer to prevention of cervical cancer, especially for women in resource-poor settings, where the need is greatest."
Kerimer added that the FDA and other regulatory authorities approved the HPV vaccines for their cervical precancer protection qualities and not persistent infections. Persistent infections are the initial steps towards precancer.