Call it immunological kismet.
Just days after federal health officials released surveys that showed many young women aren't getting the full three-dose vaccine series recommended to protect them against cervical cancer-causing human papillomavirus, a new study has come out that suggests one less dose may provide just as much immunity.
The research, carried out by scientists from the National Cancer Institute along with experts from Costa Rica and the Netherlands, was a four-year trial of the vaccine on women aged 18 to 25.
About 7,500 Costa Rican women were randomly assigned to receive either three doses of an HPV vaccine called Ceverix, or the same number of doses of vaccine against Hepatitis A. But about 20 percent of the women only got one or two shots, either because they became pregnant or were found to already have cervical abnormalities.
Researchers were able to follow this lightly vaccinated group as well, and at the end of a four-year follow-up period, women who had received just two doses of the HPV vaccine were found to have just as strong immunity against persistent infection from key strains of the virus as those who got the full three-dose schedule.
"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," said Aimee Kreimer, an investigator with the cancer institute who lead the project.
She cautions that more research is needed to confirm the findings and determine how long lasting the immunity is with two doses. She also noted it is not clear that the results would be the same with the other, more common, HPV vaccine formula licensed in the U.S., Gardasil, or if the same level of benefit would be seen in other populations, such as those with chronic infections or malnutrition, that can impair immune function.
The study is the latest in a long line of research aimed at finding the optimal dosing and timing of vaccines, particularly among teens and young adults.
A 2010 survey of teen vaccination coverage conducted by the federal Centers for Disease Control and Prevention found that 49 percent got at least one dose of HPV vaccine, but less than a third got the recommended three doses.
Teen girls were less likely to get all three doses if they were black or Hispanic or living in poverty. The shots, which cost nearly $400 for the full series, are covered under a federal subsidy program for childhood vaccinations for girls up to 18.
About 6 million people become infected with HPV in the United States each year and the CDC reports that 12,000 women are diagnosed with cervical cancer. Some 4,000 women die from cervical cancer each year.
Acceptance of HPV vaccination has been slowed in part because it is intended to prevent a sexually transmitted virus among teenage girls, a prevention questioned by some parents who fear it endorses sexual activity.
Unlike most vaccines, the HPV shots are intended to prevent infections with long-term consequences, rather than block diseases that can strike at any time. Experts say almost all HPV infections occur before women are in their mid-20s, but cervical cancer is most diagnosed among women in their 30s and 40s.
Dr. Melinda Wharton, deputy director of CDC's National Center for Immunization and Respiratory Diseases, during a briefing on the survey results last month, said many pediatricians and family physicians have not urged the vaccine for teen girls in part because the recommended starting age for the shots at 11 or 12 is perceived as "just too young.
"We know that their daughters are young. That's why we recommend a vaccine at that age. The vaccine needs to be given before kids become sexually active. Even if we know our children very well, we don't really know when that's going to be, so we give them the vaccine very young so we know we've got it in to provide them protection when the time comes."
While some parents have raised concerns about adverse effects from the vaccines, Wharton said with 35 million doses given over more than four years, "we have a lot of experience with the vaccine and that experience provides a reassuring picture of the safety of the vaccine."