Recent news headlines reported an alarming rate of sexually transmitted infections (STI) in US teenagers. The Centers for Disease Control and Prevention released a study in March conducted between 2003 and 2004 of 843 girls, estimating that one out of every four teenage girls is infected with an STI. Looking at four frequently occurring STIs, the study found that the one that occurred most often was human papilloma virus (HPV), with 18 percent of the group tested affected.

HPV is a common virus that is transmitted by sexual contact. Some experts estimate that a woman's lifetime risk of being infected with HPV is as high as 80 percent. It is not surprising then that the number of teenage girls infected is so high. There are a least 40 different strains of the HPV virus that infect the genital tract and they can be divided into low-risk and high-risk types. The low-risk types can cause genital warts, or condyloma. The high-risk types can cause pre-cancerous changes of the cervix, also known as cervical dysplasia that over time can become cervical cancer.

Unless a wart is present there are no symptoms of HPV. Thus, it is difficult to know who is infected. Infection rates are the same in men and women, but at this time there is no HPV test for men. Most people with a healthy immune system clear an HPV infection within a couple of years. But since there are so many different strains, one can become re-infected. A considerable barrier to clearing HPV is cigarette smoking, which increases the likelihood of sustained infection and ultimately the development of cancer.

HPV is not completely prevented by condom use because infection may be transmitted beyond the protection of a condom. The only absolute way to prevent HPV infection is abstinence.

However, as of June 2006, a prophylactic HPV-vaccine has been approved for use. The Gardasil vaccine is FDA-approved for girls and women ages nine to 26. This age group was chosen in hopes of protecting women before, and as they became exposed to HPV, but also to include a group that may have higher risk sexual behavior. Studies have not yet been released to understand the vaccine's effect in men or women over the age of 26.

Gardasil is protective against the two most common low-risk HPV types that cause 90 percent of genital warts and the two most common high-risk types that cause 70 percent of cervical cancer. It is given in a three injection series on day 1, month 2, and month 6. It is thought to be most effective if given before the patient becomes sexually active, although if given after someone is infected by one type of HPV it is still protective against the others in the vaccine.

The side effects of the vaccine are thought to be minimal. As the vaccine may have been given after infection already has occurred and is not protective against all HPV types, it does not replace the need for cervical cancer screening with Pap smears Currently, there is 4 years of published long-term data on the vaccine. It promotes immunity to these HPV types for at least this long, but more studies are underway to see if booster doses are needed. The HPV vaccine is the first of its kind -- a vaccine that can prevent cancer.

A Pap smear, which is the screening test for cervical cancer, is recommended annually for women starting 3 years after first intercourse or age 21, whichever comes first. All sexually active teenagers should be screened at least annually for other STIs, regardless of whether or not they undergo a Pap smear. If you would like more information regarding HPV or other STDs, check out www.cdc.gov/STD or consult your physician.