Girls as young as 11 and young women up to age 26 should receive Merck and Co.‘s Gardasil vaccine aimed at preventing a human papillomavirus, a sexually transmitted virus responsible for most cases of cervical cancer, a panel of U.S. experts said on Thursday.

In a complicated vote, the Advisory Committee on Immunization Practices (ACIP) agreed to recommend the vaccine for three age groups - all young girls between 11 and 12; girls and women 13 to 26 who have not received the vaccine yet; and women who have had abnormal Pap smears, genital warts or some other conditions.

At their discretion, physicians could vaccinate girls as young as 9, the panel decided.

The ACIP advises the U.S. Centers for Disease Control and Prevention, which in turn advises schools districts and other authorities.

The U.S. Food and Drug Administration licensed Gardasil for use in women and girls 9 to 26 years of age earlier this month. It protects against four types of human papillomavirus (HPV) that cause genital warts. The idea is to vaccinate girls before they become infected.

“I think this is going to be a great cancer prevention tool, but it is not going to be effective for about 10 years,” said Dr. Carol Baker of the national Foundation for Infectious Disease, who was at the meeting. This is because genital wart infection takes a while to cause cancer.

Clinical trials have shown that a three-dose course of the vaccine can prevent close to 100 percent of lesions that cause genital warts and cervical cancer. Women in most developed countries get regular Pap smears to detect these lesions before they turn into tumors - however, most women in developing countries do not.

The HPV 16 and 18 strains of virus are responsible for about 70 percent of cervical cancer cases.

HPV can also cause more rare cases of penile cancer in men

The CDC has no enforcement powers. It will be up to insurers to decide whether to pay for the $125-a-dose vaccine, school districts and universities to decide if they will require or recommend it, and individuals to decide if they will get the vaccine.

Dr. Cynthia Rand of the University of Rochester in New York said she believed most people would get the vaccine. She has started a series of studies on how many people would take the vaccine if offered.

“Most were accepting of the vaccine,” Rand said in a telephone interview. “Teens were surprising in that they seemed to know more about HPV than their parents did.”

Rand has no hard numbers yet but found no controversy about whether the vaccine might somehow encourage young people to have sex.

“The minority of parents we interviewed didn’t think their children would be needing it because their children wouldn’t be having sex. But they thought it would be needed in the general community,” she said.

At least one state legislative group, Women In Government, will be supporting the vaccine.

“Many state legislators around the country are ready to support a public health effort that will make sure that all age-appropriate girls and women can receive the HPV vaccine, regardless of their socioeconomic status,” said Sarah Wells of the group, a nonprofit, bipartisan organization representing women state legislators.

GlaxoSmithKline has a slightly different HPV vaccine in development.

The CDC says genital HPV infection is the most common sexually transmitted infection in the United States. More than 50 percent of sexually active women and men will be infected with one or more genital HPV types during their lifetimes.

Merck has already been planning ahead for sales of the vaccine.

“Merck has already been shipping Gardasil since its approval. We model Gardasil sales of $3.2 billion in 2010,” said Dr. Tim Anderson, Senior Pharmaceutical Analyst at Prudential Equity Group.