A study involving almost 2,400 women has shown that a vaccine that prevents infection with one of the viruses linked to cervical cancer is generally safe and effective. In most of the world, cervical cancer is the second leading cause of cancer-related death among women.
“More studies involving a larger group of women need to be done, but we are very excited thus far about the lifesaving potential of vaccines to prevent cervical cancer. This could be a golden opportunity to eliminate a major cause of cancer-related death in large parts of the world,” says Dr. Laura Koutsky, a professor of epidemiology at the UW’s School of Public Health and Community Medicine, and principal investigator of the study whose results are published in the Nov. 21 issue of The New England Journal of Medicine.
The vaccine, produced by Merck Research Laboratories, which funded the research, seeks to prevent the infection of human papillomavirus type 16 (HPV-16). This extremely common virus infects about 20 percent of the adult population. In most cases, there are no symptoms; however, HPV-16 has been linked to about half of the cases of cervical cancer in women. Cervical cancer is a cancer that occurs at the opening of the womb.
The study followed women age 16 to 23 at 16 centers throughout the nation, and found that the incidence of HPV-16 infection was 3.8 per 100 woman-years at risk among women who received a placebo, versus none per 100 woman-years at risk among women who received the vaccine. The paper includes information from the first two years of the study; the researchers expect to follow the women for an additional two years.
There were a total of 41 women in the placebo group who tested positive for HPV-16 infection during the study; no cases were recorded in the vaccine group. Nine cases of HPV-16-related cervical intraepithelial neoplasia occurred among women who received the placebo; no cases were recorded among women who received the vaccine. Cervical intraepithelial neoplasia (CIN) is a precancerous lesion of the cervix. These lesions may linger for long periods of time before turning into invasive cancer, or may disappear and never turn into cancer at all.
The vaccine appears to prevent HPV from lingering in the genital tract where, in addition to causing a lesion, it can be transmitted to a sex partner.
“The implications of having a vaccine are enormous. Even though Pap screening has worked in our country, cervical cancer continues to be an important cause of cancer-related death in countries where we have not been able to establish routine Pap screening. A vaccine could make a real impact in those countries,” Koutsky says.
An editorial that accompanies the paper says that more than 450,000 cases of cervical cancer are diagnosed annually, with nearly a quarter of a million deaths each year.
In the United States, most cases of cervical cancer linked to HPV are already prevented via programs that include Pap smear screening, with diagnosis and treatment of precancerous cervical lesions. However, a vaccine would still have many advantages; for example, it could reduce the numbers of women who have abnormal Pap smears and who then have to undergo colposcopy, biopsy and treatment.
“Treatments are low risk, but they still provoke anxiety and cost to women and their partners. Often, a woman who has an abnormal Pap smear will worry that she has cancer. In almost all cases, she will not – but the worry is enormous,” Koutsky says.
Since a vaccine that eliminates all HPV types that are linked with cervical cancer is many years from clinical use, Koutsky strongly encourages women to continue to undergo routine Pap smear screening.
“These are very promising results, but the bottom line is that until a vaccine is fully tested and available, the best way to prevent cervical cancer is to participate in routine Pap screening. Pap screening is remarkable, especially when you compare it to the effectiveness of other cancer screening strategies,” Koutsky says.
Investigators at the UW and other centers are currently enrolling women in a study of a vaccine that targets four types of HPV, including HPV-16. By including three other types of HPV, the combined vaccine targets two HPV types (HPV-16 and HPV-18) that are a cause of about 70 percent of cervical cancers, and two HPV types (HPV-6 and HPV-11) that cause about 90 percent of genital warts.