Just 35 percent of girls 13 to 17 have received a full course of the vaccine, which inoculates against the strains of human papillomavirus that can cause cervical cancer, according to 2011 data from the Centers for Disease Control and Prevention. And a study in Pediatrics this month, also based on C.D.C. data, says the intent to vaccinate is declining: 44 percent of parents in 2010 said they did not intend to vaccinate, up from 40 percent in 2008.

Alarmed by the stubbornly low rates, doctors and federal health officials are brainstorming about how to get more children vaccinated.

“Behind these numbers are people who will develop cervical cancer that could have been prevented,” said Dr. Bruce Gellin, director of the National Vaccine Program Office at the Department of Health and Human Services. At a meeting in Washington last month, federal and local officials, doctors and other health workers explored ways to make the shots more accessible. Some suggested giving the first of the three doses required to complete the vaccine at a doctor’s office and the other two at schools or pharmacies.

Others argued for a greater emphasis on cancer prevention, playing down the fact that the vaccine prevents a sexually transmitted disease. The STD link has put off many parents who are loath to talk about sex with their children.

“These vaccines are safe and effective and people should really have their teens get them,” says Dr. Paul Darden, lead author of the study and professor of pediatrics at the University of Oklahoma College of Medicine. “Parents say pediatricians are telling them about the vaccines, yet they just don’t seem to understand why they are necessary or are skeptical about their safety.”

When parents of teens were asked why their children didn’t receive certain forms of the tetanus, diphtheria, pertussis (Tdap) and meningitis vaccine, some parents noted these shots were not recommended or not necessary, according to the study. Others did not have a reason.

Regarding the controversial and fairly new vaccine that protects against the sexually transmitted human papillomavirus – which has been linked to cancer – some parents also said it was not necessary. In other cases parents noted their children were not sexually active or were not the appropriate age to receive the vaccine.

Concerns of mothers and fathers about the safety of the HPV vaccine grew each year, from 4.5% in 2008 to 16.4% in 2010, according to the study. The number of parents who said they would not vaccinate their children for HPV increased from 39.8% in 2008 to 43.9% in 2010. The main concern was safety.

Investigators were surprised, because the vaccine has been found to be very effective in preventing the virus that causes cervical cancer in young women.

“We thought perhaps many parents would think the HPV vaccine would give kids permission to have sex, and therefore not allow their children to get it. But that wasn’t it,” explained Darden. “They seemed to be skeptical of its safety, which is odd, because it’s shown to be effective with few side effects. We have a vaccine that protects against cancer. Why not vaccinate your child? I don’t get it.”

Looking at a sample of nearly 1,400 girls, the researchers found no evidence that those who were vaccinated beginning around age 11 went on to engage in more sexual activity than girls who were not vaccinated.

“We’re hopeful that once physicians see this, it will give them evidence that they can give to parents,” said Robert A. Bednarczyk, the lead author of the report and a clinical investigator with the Kaiser Permanente Center for Health Research Southeast, in Atlanta. “Hopefully when parents see this, it’ll be reassuring to them and we can start to overcome this barrier.”

HPV, the most common sexually transmitted virus in the United States, can cause cancers of the cervix, anus and parts of the throat. Federal health officials began recommending in 2006 that girls be vaccinated as early as age 11 and last year made a similar recommendation for preadolescent boys. The idea is to immunize boys and girls before they become sexually active to maximize the vaccine’s protective effects.