According to the American Cancer Society, 13,000 women in the United States were diagnosed with invasive cervical cancer in 2002, and about 4,100 women died from it that year. Thanks in large part to advanced screening methods, deaths from cervical cancer in the US have declined over the past 30 years. Worldwide, however, cervical cancer is responsible for more deaths in women each year than any other type of cancer.

Studies have established that cervical cancer is caused by human papillomavirus (HPV) , which is transmitted sexually. Two types in particular—HPV-16 and HPV-18—account for about 70% of cervical cancers. Previous research has shown that a vaccine aimed at HPV-16 was effective in protecting against this infection and the precancerous changes associated with it.

Now, in a study published in the November 13, 2004 issue of The Lancet , researchers demonstrate that a vaccine aimed at both HPV-16 and HPV-18 effectively protects against both types of HPV infection as well as precancerous cervical changes.

About the Study

The researchers randomly assigned 1,113 women aged 15–25 years to receive either three doses of the HPV-16/18 vaccine or a placebo. The vaccines were given at the start of the study, and then one and six months after that.

The women were tested for infection with HPV-16 or HPV-18 as well as for precancerous changes to cervical cells at regular intervals for up to 27 months. Women with abnormal test results were advised to follow-up for appropriate treatment. In addition, the researchers repeatedly collected blood samples from the women to measure levels of protective antibodies against HPV-16 and HPV-18, which indicate how well the body reacted to the vaccine. Finally, the study participants reported any side effects they experienced within 30 days of receiving the vaccine.

Of the 1,113 women enrolled in the study, only 721 completed the entire study protocol, involving all three doses of HPV vaccine and follow-up tests. The researchers analyzed the effectiveness of the vaccine in all of the women, noting how many doses of the vaccine they received.

Women were excluded from the study if they had had more than six sexual partners; a history of an abnormal Pap test; surgical removal of part of the cervix; or already tested positive for antibodies to HPV-16 or HPV-18 within 90 days of beginning the study.

The Findings

The researchers found that women who received at least one dose of the HPV vaccine had significantly fewer cases of HPV-16 or HPV-18 infection than women who had received the placebo.

In women who received all three vaccine doses and who participated in all of the specified testing and follow-up, the vaccine was 100% effective in preventing long-term HPV-16 and HPV-18 infection through 27 months.

Strikingly, the vaccine was also very effective in women who had only one or two of the three vaccine doses and who did not complete all of the recommended follow-up and testing. In these women, the vaccine was 95% effective in preventing long-term infection with HPV-16 or HPV-18 and 93% effective in preventing abnormal cellular changes of the cervix associated with HPV-16 or HPV-18. The vaccine also completely protected these women against precancerous changes to cervical tissue associated with HPV-16 and HPV-18.

By month seven, of the women who had received all three vaccine doses, 100% tested positive for antibodies to HPV-16 and 99.7% tested positive for antibodies to HPV-18.

None of the women in the vaccine or placebo groups reported any serious symptoms related to the injection.

There were two main limitations to this study. First, precancerous changes of the cervix occur very slowly, so 27 months is a relatively short follow-up for testing the ability of a vaccine to prevent cervical cancer. Second, while the study examined the relationship between the HPV vaccine and precancerous cervical changes, it did not test whether the vaccine protects against the development of cervical cancer itself.

How Does This Affect You?

This study found that the HPV vaccine effectively protects against infection with HPV-16 and HPV-18 as well as the precancerous changes to the cervix that often accompany them.

An editorial, also published in this issue of The Lancet , states that licensing of the HPV vaccine is not far away. This is encouraging news, as it could potentially prevent a significant number of HPV infections—and, it is hoped, cervical cancers—worldwide. However, important questions remain unanswered. For example, for how long is a woman protected against HPV once she is vaccinated?

Once available, health providers must emphasize that the HPV vaccine only protects against infection with HPV, not any other sexually transmitted diseases. In other words, the HPV vaccine should never be viewed as a license to practice unsafe sex. The best way to prevent HPV infection or any sexually transmitted disease is to use a condom. But remember that even condoms don’t work 100% of the time.