When it comes to the vaccine against human papillomavirus (HPV), there’s good news but also some news that’s concerning.

On the positive side, a new analysis of 26 studies has confirmed that the vaccine is up to 99 percent effective at preventing the development of several types of abnormal cervical cells that can progress to cervical cancer. Other recent research has found that the number of people going for the vaccine is on the rise.

But the overall numbers of people vaccinated against HPV are still too low. And according to a study out today in the American Journal of Public Health, fewer people may be finishing the series of shots.

Not getting all the recommended doses of the vaccine (two or three, depending on age) or waiting more than a year between doses could be a problem, says study author Jennifer Spencer, M.S.P.H., a Ph.D. candidate at the University of North Carolina at Chapel Hill.

“We don't know how effective it is if you have this very delayed completion” or don’t finish the series, she says. 

At age 11 or 12, children should get two shots of the HPV vaccine 6 to 12 months apart, according to the Centers for Disease Control and Prevention. Those who are 15 or older when they begin the series need three shots within 6 months for full protection.

In 2006, according to the new study, 67 percent of girls and young women who started the HPV vaccine series had completed the shots within a year. But by 2014, just 38 percent had done so.

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That may be because some doctors don't consider completing the series as important for patients as other issues, Spencer says. Also, doctors might not see kids this age in a timely manner. Adolescents may be less likely than younger children to go to a doctor for a yearly checkup.

But it's important, experts say. HPV, which can often show no signs or symptoms early on, is responsible for 90 percent of all cervical and anal cancers, CDC statistics show, as well as a large share of cancers of the vagina, penis, vulva, and throat. In total, HPV causes about 31,500 cancers in the U.S. each year.

"Every parent should make it a priority to get their child—girl or boy—vaccinated against HPV," says Consumer Reports' medical director, Orly Avitzur, M.D. "The vaccine provides a window of opportunity to prevent HPV-related cancers in adulthood."

Here’s what else you need to know about the HPV vaccine, and how to make sure kids and teens finish the series.

Vaccination Rates Are Better but Not Ideal

The federal government’s official goal is to have 80 percent of girls and boys ages 13 to 15 receive all necessary doses of the vaccine by 2020. According to a March study in the Journal of Infectious Diseases, 27.4 percent of boys and men ages 9 to 26 reported receiving at least one dose of the vaccine in 2015 or 2016, up from just 7.8 percent a few years prior. For girls and women, 45.7 percent had at least one dose of vaccine in 2015 or 2016.

“It’s encouraging that we are seeing an overall increase among males,” says Eshan Patel, M.P.H., a public-health researcher at the Johns Hopkins University School of Medicine and the study’s lead author. “However, overall, we’re still doing not that great.”

Rates of HPV infection in the U.S. remain high. An April 2017 report from the National Center for Health Statistics found that 25 percent of men and 20 percent of women ages 18 to 59 are infected with what's called high-risk HPV, the type that can cause cancer. Previous studies of just women put the prevalence closer to 15 percent.

"That’s pretty startling," says Geraldine McQuillan, Ph.D., an infectious-disease epidemiologist at the CDC and the lead author of the NCHS report. "People tend to ho-hum this whole thing, not think it’s really an issue. This is an infection that leads to cancer—it's important."

Completing the Series

Research has shown that primary care physicians don't recommend the HPV vaccine as strongly as they do other vaccines for adolescents, in large part because it takes longer to discuss and they perceive that parents don't support it for their kids. So if your doctor doesn't bring up the HPV vaccine, ask about it.

To make it easier for your youngster to finish the series, schedule an appointment for the second (or third, when needed) dose right after he or she gets the first, Spencer says. That way, it’s on your schedule in advance.  

Or ask your doctor’s office staff whether they can send you a notice when it’s time for your child’s next dose. A 2016 study found that not receiving a reminder was a key reason parents forgot to take kids in to finish the HPV vaccine series.

If your child hasn’t received the first dose at age 11 or 12, you have some time. It's best for people to get the HPV vaccine before they become sexually active because the vaccine is somewhat less effective once people have been exposed to the virus.

But it’s still recommended until age 21 for men and age 26 for women, men who have sex with men, and a few other specific groups, the CDC says