The prevalence of dangerous strains of the human papillomavirus — the most common sexually transmitted infection in the United States and a principal cause of cervical cancer — has dropped by half among teenage girls in recent years, a striking measure of success for a vaccine against the virus that was introduced only in 2006, federal health officials said on Wednesday.

The sharp decline in the infection rate comes at a time of deepening worry among doctors and public health officials about the limited use of the HPV vaccine in the United States. Health departments across the country are scrambling for ways to increase vaccination rates, while nonprofit groups are using postcard reminders and social media campaigns and pediatricians are being encouraged to convince families of the vaccine’s benefits.

There are some signs that resistance to the vaccine may be growing. A study published in the journal Pediatrics in March found that 44 percent of parents in 2010 said they did not intend to vaccinate their daughters, up from 40 percent in 2008. Because it prevents a sexually transmitted infection, the vaccine comes with a stigma. Some parents worry it promotes promiscuity. And it has been controversial. During the Republican primary in 2011, Representative Michele Bachmann, Republican of Minnesota, said the vaccine could have “dangerous side effects,” a concern that health officials say is unfounded.

The magnitude of the decline in HPV infections surprised public health experts because only about a third of teenage girls in the United States have been vaccinated with the full course of three doses. By comparison, vaccination rates in countries like Denmark and Britain are above 80 percent. Even Rwanda, in East Africa, has reached 80 percent.