Most parents don't like to think about the fact that, one day, their kids will be sexually active. Before that occurs, parents can do something that will protect their children and teens from a common sexually transmitted infection — and in the process, potentially save their lives.

The American Academy of Pediatrics now recommends that all girls and boys get vaccinated for the human papilloma­virus, or HPV, between the ages of 9 and 14. Dr. Lewis First, chief of pediatrics at University of Vermont Children's Hospital, explains why the vaccine is a real game-changer in public health.

KIDS VT: What is HPV?

LEWIS FIRST: HPV is one of the most common sexually transmitted infections. It affects more than half of all unvaccinated sexually active people at some point in their lives, usually in their teens and 20s. It's estimated that about two-thirds of sexually active preteen and teenage girls who haven't been vaccinated are infected. About 75 percent of new cases reported each year occur in people 15 to 25 years of age, according to the U.S. Centers for Disease Control and Prevention.

KVT: What does HPV do?

LF: Most commonly, HPV in females and males causes genital warts, which, by themselves, are not very harmful. But the virus is also associated with an increased risk of several worrisome cancers, most notably, cervical cancers in women. In both women and men, it can also cause cancer of the mouth, throat and anus, as well as cancer of the penis in men. Since 2006, when the vaccine was first licensed, we have seen a 65 percent decline nationally in cervical cancers attributed to the strains included in this vaccine.

KVT: How does someone know if they have been infected?

LF: Oftentimes you don't know. In fact, many people have it and don't know it, and it may be a couple of years before they develop a genital wart. All that time, they may remain sexually active. Once you're infected, you're infected forever and can pass it to others without knowing it. So, we want to get this vaccine out there before teenagers become sexually active, because we also know that it only works before you've been infected.

KVT: Is there a benefit to someone getting the vaccine who already has been infected with HPV?

LF: There are different strains of HPV, so getting some protection is better than none. The strain someone is carrying may not be one that causes cancer. The current vaccine protects against nine different strains of the virus, which are also the ones most commonly associated with cancer-causing HPV.

KVT: Why should kids who aren't yet sexually active get vaccinated?

LF: Three reasons. First, full protection from the vaccine between ages 9 and 14 requires two shots administered over a six- to 12-month period. Between ages 15 and 26, people need three shots. In order to insure that people get the full series, it's better to start giving them early, before teens gain their independence and forget or refuse to return to the doctor. Secondly, and more importantly, we now know that preteens make more antibodies and develop better protection against HPV when they get the vaccine earlier in life. This also explains why kids 9 to 14 need only two shots. Finally, we want kids fully vaccinated before they become sexually active.

KVT: How effective is the vaccine?

LF: We only have 10 years' worth of data to study, but we know already that people who were immunized at age 11 continue to be protected. And, we've seen the decrease in cervical cancers and know from research studies that protection against the strains of HPV in the vaccine persist for at least 10 years after being vaccinated.

KVT: Will the vaccine completely prevent women from getting cervical cancer later in life?

LF: No, so when a woman turns 21 — or if her doctor thinks it should be done sooner — she should still get a pap smear to screen for evidence of cervical cancer. Also, a male getting the vaccine doesn't mean that he doesn't need to use a condom. There are many other sexually transmitted infections for which a condom can be protective. And condoms alone won't protect against HPV, which can still be transmitted through close skin-to-skin contact in genital areas that the condom may not cover.

KVT: Is there evidence that teens who get the HPV vaccine become sexually active earlier than those who don't?

LF: No. The good news is, studies have shown that preteens and teens who got the HPV vaccine did not start having sex at an earlier age than those who did not receive it. And, giving kids the vaccine is not implicitly telling kids or teens that it's OK for them to have sex. Parents should see this vaccine as an opportunity to start talking to their kids about their values around sex and making smart choices with their bodies.

KVT: Does the vaccine have side effects?

LF: Nothing more than minor effects, such as soreness, redness and swelling at the injection site. There are reports of people, mostly females, fainting after receiving the injection. We don't know what causes it, but if patients sit or lie down for 15 minutes after the injection, this shouldn't occur. Fevers occur about 10 percent of the time, as do headaches. But the rate of allergic reactions to the vaccine are rare — only one in 1 million, according to the Vaccine Adverse Event Reporting System. Worldwide, 170 million doses have been distributed in 80 countries, and there have been no serious safety concerns reported.