About half of American teenagers have been vaccinated against the human papillomavirus (HPV), the most common sexually transmitted infection in the United States.

But women ages 18 to 26 should also get vaccinated, says Lauri Markowitz, a Centers for Disease Control and Prevention medical epidemiologist.

When should you get vaccinated?

The CDC recommends getting vaccinated at 11 or 12.

The vaccines are most effective if administered before a woman becomes sexually active. The longer a woman has been sexually active and the more partners she has had, the more opportunities she has had to become infected with an HPV strain that overlaps with the vaccine.

Those who didn’t receive the vaccine at the recommended age are still urged to get vaccinated to increase the odds of protection against HPV.

Can HPV lead to cancer?

Certain types of HPV can lead to cancer.

According to the American Cancer Society, about 12,820 new cases of cervical cancer will be diagnosed in U.S. women this year, and more than 4,000 will die of the disease.

HPV is thought to be responsible for more than 90 percent of all cervical and anal cancers in men and women. The virus also causes vaginal, vulvar and throat cancers and genital warts.

There is an effective way for women, vaccinated or not, to reduce their risk of cervical cancer: routine cervical cancer screening. Pap smears are recommended every three years for women between ages 21 and 65.

However, a majority of HPV infections do not cause cancer. Most people with an infection never show any symptoms, and infections usually go away on their own.

How will the vaccine protect me?

Gardasil 9, the newest HPV vaccine approved by the Food and Drug Administration, protects against nine such strains. (Older vaccines protect against fewer strains of HPV.) It’s approved for women up to age 26.

Researchers say the HPV vaccine may be able to prevent up to 90 percent of cervical cancers.

Like other vaccines, it spurs the body’s immune system to defend itself against a virus. The FDA and CDC say the HPV vaccines are safe and extremely effective: HPV rates in women ages 14 to 19 years fell 64 percent within six years of the vaccine’s introduction in the United States in the mid-2000s and 34 percent in women ages 20 to 24.

Should men get vaccinated?

The CDC recommends that men up to age 21 have the vaccine to protect against genital warts, anal and throat cancers and other conditions, and that men who have sex with men who did not get vaccinated before they were 21 get the vaccination up to age 26.

Will my insurance cover it?

Some insurance plans do not cover the vaccine for those older than 18 — the shots can be costly, though the manufacturer may provide assistance — but it really varies across the board.

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