HPV vaccine: Who needs it, how it works

Human papillomavirus, also called HPV, is spread by sexual activity. Some strains of HPV cause genital warts. Other HPV strains can cause cancers.

Most of the time, the body can find and clear out HPV. But if the virus stays in the body for a long time, it can cause cancer. Getting vaccinated against HPV helps prevent cancer in men and women.

Here's what you need to know about the HPV vaccine.

What does the HPV vaccine do?

The HPV vaccine protects against genital warts and most cases of cervical cancer. It protects against cancer of the vagina, vulva, penis or anus caused by HPV. The HPV vaccine also protects against mouth, throat, head and neck cancers caused by HPV.

The vaccine gives the body a safe way to build immune system awareness of some HPV strains. This means the body has an easier time clearing out those strains of the virus if a person catches them later.

Who is the HPV vaccine for and when should it be given?

The Gardasil 9 vaccine is approved by the U.S. Food and Drug Administration (FDA). It can be given to people age 9 and older. This vaccine can be given at the same time as other vaccines.

The Centers for Disease Control and Prevention (CDC) suggests routine HPV vaccination at age 11 or 12. The ideal age for vaccination is before a person is sexually active.

Once a person gets HPV, the vaccine may not be as effective. That's because the vaccine's goal is to prevent a new infection. If a person has the virus, the vaccine may not help the body clear out the vaccine.

Research has shown that receiving the vaccine at a young age isn't linked to an earlier start of sexual activity.

People younger than age 15 can be vaccinated with two doses, 6 to 12 months apart.

People who start the vaccine series later, at ages 15 through 26, should get three doses of the vaccine. These shots are given over six months.

The CDC suggests catch-up HPV vaccinations for all people through age 26 who aren't fully vaccinated.

The FDA approved the use of Gardasil 9 for males and females ages 9 to 45. If you're ages 27 to 45, discuss your risks with your healthcare team. Together you can decide if you should get the HPV vaccine.

Who should not get the HPV vaccine?

The HPV vaccine isn't given during pregnancy.

The HPV vaccine is not recommended if a person had an allergic reaction after the first HPV shot, or if a person has severe, life-threatening allergies.

Also, people who are moderately or severely ill should wait until they feel better to get vaccinated for HPV.

Does the HPV vaccine offer benefits if you're already sexually active?

People who are sexually active should talk with their healthcare team about the benefits of getting an HPV vaccine.

Most people catch HPV soon after they become sexually active. But even if you have one strain of HPV, you might still benefit from the vaccine. It can protect you from other strains that you don't yet have.

But none of the vaccines can treat an existing HPV infection. The vaccines protect you only from specific strains of HPV you don't have.

Does the HPV vaccine carry any health risks or side effects?

The HPV vaccine has been found to be safe in many studies.

Overall, the effects tend to be mild. The most common side effects of HPV vaccines include soreness, swelling or redness at the injection site.

Sometimes dizziness or fainting occurs after the shot. Staying seated for 15 minutes after the shot can lower the risk of fainting. Headaches, nausea, vomiting, fatigue or weakness also may occur.

The CDC and the FDA continue to monitor the vaccines for unusual or severe problems.

Is the HPV vaccine required for a child to go to school?

The HPV vaccine is part of the routine childhood vaccines schedule. Each state in the U.S. decides which vaccines from that schedule are needed for school attendance.

Do women who've received the HPV vaccine still need to have Pap tests?

Yes. The HPV vaccine doesn't replace Pap tests. Screening for cervical cancer with regular Pap tests starting at age 21 is an essential part of preventive healthcare.

Get medical attention right away if you notice any symptoms of cervical cancer. Some symptoms are vaginal bleeding after sex, between periods or after menopause; pelvic pain; or pain during sex.