WEDNESDAY, June 19 (HealthDay News) -- Vaccination against the cancer-causing human papillomavirus (HPV) is proving highly effective in reducing the spread of the sexually transmitted virus among young women, new research shows.
The rate of new infections with strains of HPV targeted by the vaccine have dropped by 56 percent among females aged 14 to 19 since the first vaccine was approved in 2006, report researchers at the U.S. Centers for Disease Control and Prevention.
According to the CDC, HPV infection is known to be the major cause of cervical cancer, and has also been strongly linked to vulvar, vaginal, penile, anal, and certain throat/oral cancers.
The new findings "are striking results, and I think they should be a wake-up call that we should increase vaccination rates, because we can protect the next generation of adolescents and young girls against cancer," CDC director Dr. Tom Frieden said in a press conference held Wednesday.
The findings are published in the June issue of The Journal of Infectious Diseases.
Each year in the United States about 19,000 cancers caused by HPV occur in women, with cervical cancer being the most common. Virtually all cases of cervical cancer are caused by HPV, and just two HPV subtypes, 16 and 18, are responsible for seven out of every 10 cervical cancers.
About 8,000 cancers caused by HPV occur each year in men in the United States, most commonly being throat cancer.
The decline in HPV infections among girls is actually higher than had been expected, indicating that "herd immunity" may have taken effect as more girls receive the vaccine, study lead author Dr. Lauri Markowitz said at the press conference.
Herd immunity occurs when widespread vaccination shrinks the pool of infected people to the point that it becomes less likely that a person can catch the disease from someone else. In this type of scenario, "even people who aren't vaccinated will have some protection due to the vaccination program," Markowitz said.
Another possible reason for the better-than-expected results could be that the vaccine is so effective that a girl receives protection from HPV even if she receives only one or two shots out of the full recommended three-dose series of vaccination.
Whatever the reason, the new statistics are "great news," according to Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "I think every young woman should have the opportunity to have this vaccine. That's what I've been telling people for however long the vaccine has been available."
HPV is highly infectious. In fact, about 79 million Americans, most in their late teens and early 20s, are thought to be currently infected with HPV, and each year about 14 million people become newly infected.
The study was based on data from the CDC's National Health and Nutrition Examination Survey. It compared HPV infections in teenage girls from 2003 to 2006 (before the start of the United States' HPV vaccination program) against infections that occurred from 2007 to 2010, after girls began receiving the vaccine.
The CDC team showed that infections by the HPV types covered by the vaccine fell dramatically -- from 11.5 percent in the pre-vaccine era to 5.1 percent post-vaccine.
There has been some public resistance to the HPV vaccine, of which there are now two versions, Gardasil (approved in 2006) and Cervarix (approved in 2009). The CDC currently recommends routine immunization at ages 11 to 12 for both boys and girls -- before most young people become sexually active.
A series of three shots is recommended over six months for both girls and boys. HPV vaccination is also recommended for older teens and young adults who were not vaccinated when younger.
"Unfortunately, only one-third of [U.S.] girls aged 13 to 17 have received the full three-dose series of the HPV vaccine," Frieden pointed out. In contrast, "countries such as Rwanda have vaccinated more than 80 percent of their teen girls. [The U.S. rate] is simply unacceptable. Our low vaccination rates represent 50,000 preventable tragedies; 50,000 girls alive today will develop cervical cancer over their lifetime that would have been prevented if we had reached our goal of 80 percent vaccination rates. For every year we delay in doing so, another 4,400 girls will develop cervical cancer in their lifetimes."
Brooks believes resistance to vaccination stems from a couple of factors, one being the difficulty in explaining to people how the vaccine works. "It's a conceptually difficult thing for people to recognize -- that a virus causes a cancer," he said. "That's a major, major issue."
Parents also have been reluctant to have their adolescent girls and boys receive a vaccine against a sexually transmitted disease, said Dr. Jill Rabin, head of urogynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y.
"This is the tack that I take with patients and with families: the reason we give it to younger women is that they will build up the best antibody response," Rabin said. "It's not that we're giving it now because we expect them to have sex in their teenage years. We're giving it because they will make the best antibody response so when they do become sexually active, they will have protection at that point.
"If you give it to a 26 year old, they aren't going to mount as good an antibody response as someone who is 11 or 12 years old," she continued. "You're going to be giving better protection if you give it younger. It's not permission for them to go out and have intercourse."
Doctors also play a role in the failure to achieve widespread HPV vaccination, Frieden added.
"Providers are not consistently giving strong recommendations for the vaccine, and they are not encouraging vaccination at every encounter," he said.
Frieden said the vaccine has a safe track record, with no serious long-term complications associated with it even though more than 56 million doses have been distributed in the United States.
"The time has come to ramp up our efforts to vaccinate the next generation against cancer," he concluded. "This is an anti-cancer vaccine."
For a helpful Q & A on the HPV vaccine, head to the American Cancer Society.