Vaccine May Help Reduce Risk of Cervical Cancer
An experimental vaccine prevented young women from becoming persistently infected with two types of a virus that, together, cause more than two-thirds of all cases of cervical cancer.
The Lancet Oncology, May 2005 (see the journal abstract).
Human papilloma viruses (HPV) are extremely common sexually transmitted infections. In more than 90 percent of cases, the infections are harmless and go away without treatment. However, certain types of HPV increase women’s risk for cancer of the cervix (the neck of the womb). Two types (HPV-16 and HPV-18) cause about 70 percent of all cervical cancers. Of the remaining 30 percent, most cases are associated with a dozen or so other “high-risk” HPV types.
In addition, two types of HPV (HPV-6 and HPV-11) cause 90 percent of genital warts, which are benign (noncancerous) tumors.
Although most HPV infections do not progress to cervical cancer, infections that persist for many years are more likely to do so. Most cervical cancers develop slowly through a series of abnormal changes in the cells of the cervix. Regular Pap tests can detect these changes and the abnormal tissue can be removed, preventing it from ever developing into cancer. Pap tests are not 100 percent accurate, however, and many women do not have the tests regularly.
A previous study showed that another experimental vaccine effectively prevented persistent infections with HPV-16, which causes about half of all cervical cancers. However, that vaccine was not designed to prevent infections with other forms of HPV.
The current study involved 552 women ages 16 to 23 in the United States, Europe, and Brazil. The young women were healthy and had had no abnormal Pap smears. They were assigned at random to receive three injections of either the experimental HPV vaccine or a placebo vaccine (a shot with no active ingredients). Those in the vaccine group received either a low dose, an intermediate dose, or a high dose.
The vaccine was designed to protect recipients against four types of HPV infection: HPV-16 and HPV-18, the types associated with most cases of cervical cancer, and HPV-6 and HPV-11, the types associated with most cases of genital warts.
The study was double-blinded - that is, neither the investigators nor the study participants knew who got the vaccine and who got the placebo. Participants were followed for 30 months after getting the third shot.
The study was supported by Merck Research Laboratories, maker of the experimental vaccine. The research team was led by Luisa L. Villa, Ph.D., of the Ludwig Institute for Cancer Research in Sao Paulo, Brazil.
Some women were found to have HPV infections at the time they received the first shot; others developed an infection before they received all three shots. These women were excluded when the researchers calculated the vaccine’s effectiveness.
Compared with women who got the placebo, vaccine recipients had 90 percent fewer persistent infections or disease associated with one of the four types of HPV under study. The vaccine’s effectiveness was similar regardless of whether women got a low dose, an intermediate dose, or a high dose.
Slightly more women in the vaccine group than in the placebo group reported pain at the injection site and headaches. However, most of these adverse events were not severe.
According to John T. Schiller, Ph.D., of the National Cancer Institute’s Center for Cancer Research, this is the first study to show that this particular vaccine - designed to protect against four types of HPV infection - is effective. Some researchers had been concerned that a vaccine intended to protect against multiple HPV types would actually be less effective than one aimed at a single HPV type.
“So it is certainly good news that the vaccine appears to work so well at preventing persistent infection by these four HPV types,” he said.
The study’s major shortcoming is that it involved a relatively small number of patients, said Schiller. This means it’s impossible to be certain that the vaccine’s effectiveness against cervical abnormalities and genital warts was not due to chance, though protection against persistent infection was highly statistically significant. Merck, the vaccine’s maker, is now testing the product in a larger number of patients.
In addition, two large studies of a vaccine that protects against HPV-16 and 18, which cause most cases of cervical cancer, are being conducted by the National Cancer Institute and GlaxoSmithKline.
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