The U.S. Centers for Disease Control and Prevention recommends that all women and girls ages 9 to 26 receive a new vaccine against human papillomavirus. That is generating questions to physicians.

"Many of the patients I see have already had an abnormal Pap" test, which is used to detect cervical cancer, said Kenneth Noller, chairman of the Department of Obstetrics and Gynecology at Tufts University School of Medicine. "Their question is, 'Does it make sense for me to be immunized?' "

We posed that question and others to experts, including Noller and vaccine manufacturers. Here's what we learned:

Q. How serious a problem is HPV?

A. Fifty to 75 percent of people who have ever had sex will get HPV, according to the American Cancer Society; nearly half of those cases will occur in people ages 15 to 25.

There are more than 100 types of HPV; about 30 types infect the genital area. While most genital HPV infections are cleared by the immune system, causing no signs or symptoms, some infections persist and cause genital warts, precancers and cancers of the cervix, and other types of cancer. About 70 percent of HPV infections resolve — that is, go away — within a year; 90 percent disappear within two years, according to the ACS. There are no treatments available to cure HPV infections, though there are methods used to treat cervical cell abnormalities caused by the virus.

Q. Doesn't the Pap test detect cervical cancer? Why do I need a vaccine?

A. You need both, say experts. The Pap test detects cervical cancers — including the small percentage that aren't caused by HPV — and abnormal cells in the cervix, which could turn into cancer over time. If abnormal changes are seen, women can be treated early to prevent cancer from developing.

Q. Who should be immunized?

A. Ideally, the vaccine should be given before females have sex for the first time, because almost any sexual encounter carries a risk of infection, according to the CDC's Advisory Committee on Immunization Practices. The committee recommended that the vaccine routinely be given to girls age 11 or 12, because girls receive other vaccinations at that age. The recommendation also allows for females as young as 9 or as old as 26 to get the vaccine.

Q. Is just one type of HPV vaccine available?

A. For now, yes. The Food and Drug Administration has approved Gardasil, made by Merck. GlaxoSmithKline has developed an alternative vaccine, Cervarix; the company says it hopes to apply for FDA approval by April 2007.

Gardasil protects against four HPV types — 16, 18, 6 and 11. Types 16 and 18 together account for about 70 percent of all cervical cancers; types 6 and 11 cause about 90 percent of genital warts cases. The vaccine has been shown to be nearly 100 percent effective in protecting against those four HPV strains and is considered largely free of side effects.

Cervarix is designed to protect against types 16 and 18 and may protect against other HPV strains in addition to the strains it's designed to target.

Q. Are there protective options for me if I'm older than 26?

A. Not yet — but stay tuned. Merck is testing Gardasil in women up to age 45; Cervarix has shown promise in women up to age 55.

Q. Does it do me any good to be immunized if I've already had HPV or if I have it now?

A. Women up to age 26 should be vaccinated regardless, advises the CDC committee. That's partly because experts aren't certain whether having had one strain of the virus gives you total immunity against reinfection by that strain. Also, getting the shot will protect you against any of the included strains that you haven't had already, according to Richard M. Haupt, executive medical director at Merck.

The CDC committee recommended immunization for women previously infected with HPV, despite conflicting findings on the likelihood of their developing cervical lesions. In one of Merck's studies, previously infected women who received the vaccine had higher rates of precancerous lesions than those in the placebo group; in two other studies, previously infected women in the vaccine group had slightly lower rates of precancerous lesions than did those in the placebo group.

Because most people will get HPV at some time — though they may never be aware of it — routine testing for the virus is not recommended. A DNA test for the virus is FDA-approved for women older than 30; the test is also approved for women of any age whose Pap tests show abnormalities.

Q. How many shots are involved, and over what time period are the shots administered?

A. Gardasil is given as three shots over a six-month period.

Q. How long am I protected once I've had the shot?

A. No one is sure, but Gardasil appears to remain effective for at least five years, according to Merck. Cervarix has been shown to keep working for four years, according to research published in The Lancet. Longer-term studies need to be carried out to see if booster shots are needed later in life.

Q. I hear the vaccine is expensive. Are there any options if I don't have insurance to help me pay for it?

A. You're right, the vaccine isn't cheap. The three shots included in Gardasil cost $360. (It can be as high as $495 in Hawai'i.) Insurance companies have started to cover the vaccine for those in recommended age groups. In Hawai'i, HMSA covers a portion of the cost, depending on the plans. HMSA's PPO plan covers 90 percent of the cost. Its HMO requires a $10 co-pay.

The CDC's Vaccines for Children program will include the HPV vaccine, making it available to uninsured kids and to children whose insurance doesn't cover vaccines. To aid those 18 and older, Merck has a program under development that will make the vaccine available to lower-income adults.

Q. Don't men and boys need to be vaccinated, too?

A. Both manufacturers plan to study their vaccines for use in males, but at this point, they are only approved for women and girls. There is at present no approved method to test for HPV in men, and no tests are approved for early detection of HPV-linked cancers in men, reports the ACS.

Catherine E. Toth contributed Hawai'i information to this report.