DEAR DOCTOR K >> A friend says that my young son should get the vaccine that protects girls against cervical cancer. That doesn't seem to make sense. Can you explain?

DEAR READER >> Your friend is right, and here's why: The vaccine is against a virus called human papilloma virus (HPV). There are more than 100 strains of HPV; about 40 of these strains can be transmitted by sexual contact. So-called low-risk strains cause genital warts. High-risk strains can cause cancers of the cervix, anus, penis and throat. I'll call these the HPV-related cancers. Not all of these cancers are caused only by HPV, but the virus is an important cause of each. Most cases of cervical cancer in women in the United States are caused by HPV.

In other words, HPV can cause cancers in men, and can be passed by men to women through sex. So giving the vaccine to boys protects them when they become men, and protects women with whom they may have sex.

HPV is very common. Most sexually active adults become infected with HPV before the age of 50 — and most of them don't know they have it. It doesn't cause symptoms in most people. However, infected adults can still transmit HPV to their sexual partners regardless of symptoms. Safe sexual practices such as using condoms can reduce the risk of infection.

Fortunately, many people with HPV infection never develop cancer from it. However, people with HPV infections — including people with "silent" infections that cause no symptoms — are at much higher risk for getting HPV-related cancers.


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The Advisory Committee on Immunization Practices (ACIP) recommends that boys receive one of two forms of the vaccine — Gardasil or Gardasil 9. The vaccine is given in three doses. Ideally, it is given to boys when they are 11 or 12 years old, because that's the age at which it works best. However, it can be given to boys as young as 9 or to men as old as 26. It is particularly important in teenage or young adult males who have sex with other men.

The Gardasil vaccine (also known as HPV4) was approved for boys in the United States in 2009. The vaccine protects against two low-risk strains of HPV and two high-risk strains. These four strains of HPV are responsible for most cases of genital warts and HPV-related cancers. The Gardasil 9 vaccine covers five additional strains of the virus.

Even though the vaccine has been shown to benefit boys later in life, and to benefit those with whom they have sex, not all health insurance companies pay for the vaccine. Don't ask me why; ask them.

Dr. Komaroff is a physician and professor at Harvard Medical School.