The Centers for Disease Control and Prevention (CDC) issued its updated immunization guidelines today, with two minor changes that are nevertheless sure to stir up controversy.

The CDC is tweaking its recommendations on the vaccine against the human papilloma virus, or HPV. Nearly all adults are exposed to HPV, which is sexually transmitted.

vaccination schedule

The virus dramatically increases the risk of cervical, anal, penile, and head and neck cancers. HPV is the biggest risk factor for cervical cancer.

Current guidelines call for children to be vaccinated beginning at age 11 to provide immunity before they become sexually active. HPV vaccines are given as a series of three shots over six months.

Read More: Teens Are Missing HPV Vaccinations Because Doctors Are Reluctant to Talk About It »

HPV Vaccine Has Been Effective

After HPV vaccination was first recommended in 2006, the number of infections among teen girls in the United States dropped by more than half. Cervical precancers, detected with PAP smear tests, also dropped among young women.

But conservative groups have questioned whether immunizing pre-teens encourages them to become sexually active sooner. While there is no evidence that the vaccine promotes sexual activity, the questions have resulted in fewer vaccinated children.

“People say they’ve always wanted a vaccine against cancer, now we’ve got it and it’s not being used as widely as it should be,” said Dr. Cody Meissner, a fellow at the American Academy of Pediatrics (AAP) and the chief of pediatric infectious disease at Tufts University.

The AAP is one of several medical groups that provide recommendations that the CDC uses in creating the comprehensive vaccination guidelines for doctors and parents.

The CDC is likely to start up another round of controversy with its latest guidelines. The public health body now advises that children who have been the victims of sexual abuse begin HPV vaccination at age 9.

The vaccines have already been proven safe for children as young as age 9, but this is the first time the public health body suggested they be given that early to higher-risk youth.

New evidence has shown that children with a history of abuse have a higher risk of HPV infection. That risk may come from the assault itself, or it may come as a result of stress or behavior changes associated with it.

But Meissner thinks that the focus on how HPV is spread is misguided.

“We know that this vaccine is very effective at preventing infection by the virus that does cause cancer,” he said. “This vaccine shouldn’t be viewed as preventing a sexually transmitted infection.”

Read More: HPV Doesn’t Lead to Unsafe Sex »

Newer Vaccine Offers Better Coverage

The new guidelines also suggest that pre-teens receive the latest generation of HPV vaccine whenever available.

The latest vaccine — called a 9-valent vaccine or 9vHPV because it targets nine of the most common cancer-causing strains of HPV — was approved for general use slightly less than a year ago. Today’s guidelines are the first annual update, which include graphic timelines for parents to incorporate the 9vHPV vaccine.

The 9vHPV vaccine provides immunity against five additional strains of HPV that account for about 15 percent of cervical cancers and 5 percent of cancers in men.

Some healthcare providers may still have the previous generation of vaccine in stock, however. Parents are encouraged to go ahead with these vaccines.

“What they’re trying to avoid is people saying ‘No, I don’t want HPV-2 or 4, I want to wait for 9.’ But then they may not ever come back and get the 9-valent,” Meissner said.

A young person who receives the first of the three-shot vaccine series using an HPV-4 vaccine and receives two shots of the 9-valent vaccine will likely have immunity for all nine strains, according to Meissner.

Opponents of the vaccine have argued that PAP smears and other forms of screening are adequate to control the cancers that HPV can spur. The CDC aggressively refutes this approach.

“HPV vaccination is critical to protecting the next generation against cancers caused by HPV infections,” the CDC said in recently issued materials.

Four thousand U.S. women die from cervical cancer every year, even with routine screening and treatment. And there is no regular screening test for the other cancers HPV causes, according to the CDC.

The newest vaccine cuts cervical, vulvar, and anal cancers linked to HPV by 80 percent.

Read More: ‘Leaky’ Vaccines Can Produce Stronger Versions of Viruses»