An experimental vaccine appears to be 100 percent effective against the virus that is the major cause of cervical cancer, according to a study being published today.
If the results stand up, they could lead to the first human vaccine whose specific purpose is to prevent cancer.
The vaccine protects against a form of the human papilloma virus, which causes a painless infection of the cervix, the necklike entrance to the womb.
The sexually transmitted virus is extremely common, occurring in about half of American adults. While relatively few of the women infected go on to develop cervical cancer, nearly all cervical cancers are the result of long-standing HPV infection.
Cervical cancer affects 470,000 women a year worldwide and kills 225,000. In the United States, there are 13,000 cases a year and 4,100 deaths.
In a study of 2,392 young women, half of them were vaccinated and half were given placebo shots. Followed for 17 to 27 months, no vaccinated woman developed infections or precancerous growths from the virus, whereas 41 non-vaccinated women did become infected, including nine with precancerous cervical growths.
"These are tremendous results," said Dr. Laura A. Koutsky of the University of Washington in Seattle, director of the study, published in the New England Journal of Medicine. The study was carried out by researchers from 16 universities and Merck Research Laboratories, a unit of Merck & Co. Merck made the vaccine and paid for the research.
Although there are more than 30 types of HPV that can infect human beings, one of them _ type 16 _ is responsible for about half of all cervical cancers. The experimental vaccine protected only against that one, although future formulations are likely to also protect against the less common HPV types that can cause cervical cancer.
The vaccine, which is not yet available, can be used to prevent cervical cancer but will not be able to treat it. As a preventive, the vaccine will have to be given to girls and young women before they become sexually active.
More research also is needed to find out how long the vaccine protection lasts, Koutsky said.
Dr. Allan Hildesheim, a senior investigator at the National Cancer Institute, called Merck's results extremely promising, but, he said, larger and longer-term trials were needed to make sure the vaccine is safe and effective, especially since it is intended for large numbers of young, healthy people. He said researchers at the cancer institute were also developing a cervical cancer vaccine. So is GlaxoSmithKline, a spokeswoman for the company said.
Not every type of HPV causes cancer. Even with those that do, the infection is harmless in the vast majority of people, and their immune systems either eliminate the virus or suppress it. They then become immune to further infection.
But in some women the virus persists and causes lesions on the cervix. Most lesions go away, but some turn cancerous. On average the cancers take about 20 years to develop, but some can emerge much faster. Teenage girls and young women in their early 20s are more prone to develop lesions than older women, possibly because normal cellular changes in the cervix in those years make it more vulnerable to the virus, said Dr. Carol L. Brown, a gynecologic cancer surgeon at Memorial Sloan Kettering Cancer Center in New York.
But researchers cautioned that the vaccine would not do away with the need for Pap tests to screen for cervical cancer, because it cannot prevent every virus that can cause the disease.
In the United States, where 50-million to 60-million Pap tests are done every year, a great benefit of a vaccine would be a drop in the number of women who, because of the virus, have abnormal Pap tests and then need repeated examinations and tests to look for cancer. Most of the abnormalities will not turn cancerous, but it is hard to predict which ones will, so all must be monitored. Some women have surgery they may not need. By preventing the infections and abnormal growths, a vaccine could spare close to a million women a year the worry, expense and unpleasant procedures. Cost savings could be in the billions.
Another vaccine that Merck is now testing, and hopes to market, will immunize patients against both HPV-16 and HPV-18, which together cause 70 percent of all cases of cervical cancer. The vaccine will also protect against two other HPV types, 6 and 11, which cause about 90 percent of genital warts.
The wart protection was included to "give incentive to young men to also take the vaccine," said Dr. Kathrin U. Jansen, senior director of microbial vaccine research at Merck.
She explained: "Men don't get cervical cancer. I always strongly felt that if we only go for cervical cancer types, there would be no reason for men to accept the vaccine. Even though they are vectors for transmitting the virus, they don't usually have effects."
But men do get genital warts, and many young men are eager to avoid them since they are ugly and may repel potential sex partners, Jansen said.
In an separate study, a different group of researchers reported that an experimental vaccine against genital herpes protected most women from getting that virus. (It did not protect men.)
Genital herpes infection is less common than HPV, affecting about one-in-four women and one-in-five men. It can have catastrophic effects in the rare instance when it is transmitted to a fetus during pregnancy or delivery.
Together, the studies raise the possibility that the vaccines might eventually become a routine part of the medical care of teenage girls, administered before most are sexually active.
Koutsky said it had been suggested that many parents, loath to acknowledge that their teenage daughters could be sexually active, would refuse the HPV vaccine.
But, she said, "I think a generational shift has occurred, where most parents recognize that their children will make decisions about sexual activity that they have no control over, and they want them to be protected."
_ Information from the New York Times and Washington Post was used in this report.