The cervical cancer vaccine

The Family Health Guide

Cervical cancer once killed many American women. But over the past 30 years, the number of cervical cancer deaths in the United States has dropped by half. Today, fewer than 4,000 American women die each year from the disease.

The main reason for the decrease is the Papanicolou test, or "Pap smear," which is perhaps the most effective cancer-screening test we have. Under current guidelines, women should start getting regular Pap tests within three years after first having sexual intercourse, or at age 21, whichever comes first. Before their 30th birthdays, they're supposed to have the test every year — thereafter, just every two to three years, as long as they've had three normal tests in a row.

But now cervical cancer prevention is entering a new era. The FDA has approved a vaccine against the human papillomavirus (HPV), the virus that's believed to cause almost all cases of cervical cancer.

It's a remarkable achievement: The viral origins of a serious cancer have been identified, and researchers have successfully harnessed the immune system to prevent it. But it's also controversial because 11- and 12-year-old girls are prime candidates for receiving the shots, and some conservative groups fear the vaccination will send a message that it's okay for preteens to be sexually active.

HPV mingles with our DNA

Researchers discovered in 1986 that several strains of HPV cause cervical cancer. HPV is the most common sexually transmitted viral disease in the world, and experts estimate that at least half of all sexually active people will be infected by it at some time in their lives.

There are about 100 different strains of the virus, but only 30 are associated with cervical cancer, and just two, strains 16 and 18, are responsible for about 70% of all cases. Most of the time, people are able to clear the infection, but when HPV successfully evades the immune system and persists in cells, cancerous changes can occur. Fortunately, it can take decades for the cancer to develop. That's one reason regular Pap tests work: There's ample opportunity to detect abnormal cell growth before it turns into full-fledged cancer.

Vaccine made of selected proteins

Scientists at the National Institutes of Health (NIH) developed the HPV vaccine. The vaccine does not contain the killed or weakened virus, as do many vaccines. Instead, it uses just some of the virus's important proteins. The immune system's response to the proteins is enough to stop the virus from infecting the cells of the cervix.

The vaccine targets HPV strains 6 and 11, which cause 90% of genital warts — warts that grow in the genital areas of men and women — as well as the major cervical cancer–causing strains, 16 and 18. Given as a set of three shots over six months, the vaccine will protect against only those four strains. And it isn't therapeutic, which means that if there is an existing infection, the vaccine doesn't bring much to the fight.

CDC recommendations

The Centers for Disease Control and Prevention (CDC) added the HPV vaccine to its official vaccination recommendations in July. It proposed that all 11- and 12-year-old American girls get the shots, although girls as young as 9 could receive it if they're sexually active. For "catch-up," the CDC also recommended that girls and women ages 13–26 be vaccinated against HPV, regardless of their Pap test results.

The reason the CDC recommends vaccinating preteens is that the vaccine works best before an individual has been exposed to HPV. From a public health standpoint, early vaccination stands the greatest chance of making a dent in cervical cancer incidence.

Older girls and young women were included in the recommendations because even if they've had some exposure to HPV, it may not be to the strains contained in the vaccine, so they'll get some protection.

Will Pap tests still be needed?

By making the Pap test less routine, some worry that the HPV vaccine might have the unintended effect of eroding women's ties to their doctors. For now, though, Pap test screening is still essential. The new vaccines don't include HPV strains believed to be responsible for about 30% of cervical cancers. Regular Pap tests will still be the best way to prevent cancers caused by those strains.

September 2006 Update

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