Heads up to the millions of girls and women who've gotten an HPV vaccine in the last decade -- you might want another round of shots to get even more protection from the human papillomavirus.

The first two HPV vaccines on the market were Gardasil in 2006 and Cervarix in 2009. But last December, the FDA approved Gardasil 9, a new vaccine that offers additional protection against the virus.

Should you get Gardasil 9 if you've already been vaccinated? Research presented at an American Association for Cancer Research meeting on Sunday (April 19) suggests that yes, you should at least think about getting it. But here's what you need to know before heading to your doctor's office:

First off, HPV is wayyy more common than you think.

Your sex ed class might have glossed over this very important fact: HPV is the most common sexually transmitted infection in the U.S. The CDC says about 9 in 10 people will be infected with HPV at some point in their lives, though most of the virus strains -- there are over 100 of them -- don't cause any physical symptoms.

The virus spreads through skin-to-skin contact and contact with bodily fluids, which means it can be transmitted through any type of sexual activity. And since it can affect the entire genital area, condoms -- while they provide some valuable protection -- aren't 100% effective at preventing HPV transmission.

People with HPV often don't even know they're infected. This means carriers can unknowingly transmit the virus to their partners. The virus can also lay dormant in your body for years before symptoms emerge, so all these factors combined make it difficult to (1) know when you're infected and (2) identify who passed it to you in the first place. People with HPV often don’t find out about it unless it leads to genital warts or an abnormal pap test.

Gardasil 9 protects against more HPV strains than previous vaccines do.

According to the CDC, Cervarix protects you from HPV strains 16 and 18, the strains that account for around 70% of all cervical cancer cases. The original Gardasil protects against those same strands plus two more, 6 and 11, which cause genital warts.

The new Gardasil 9 protects against those same four strains plus five more: 31, 33, 45, 52 and 58. These additional five strains together cause 20% of cervical cancers.

A new study suggests the original Gardasil might increase(!!) your risk for HPV strains not covered by the vaccine.

For this study, which has been presented but not yet published in a peer-reviewed journal, researchers from the University of Texas Medical Branch examined 600 women -- some who received the original Gardasil and some who were unvaccinated against HPV -- between the ages of 20 to 26.

The Gardasil-vaccinated women were less likely to be infected with HPV strains 6, 11, 16 and 18 as compared to unvaccinated women, which is what you'd expect. However, they also discovered that the Gardasil-vaccinated women -- though they were protected from those four specific strains -- were actually at a higher risk for contracting other HPV strains not covered by the vaccine.

Overall, after controlling for factors like number of sexual partners, women who received the original Gardasil vaccine had a 40% greater risk of contracting a high-risk HPV strain not included in the vaccine protection.

That's why Fangjian Guo, one of the study's researchers and a postdoctoral fellow at the University of Texas Medical Branch, suggests getting the new Gardasil 9 vaccine even if you've already been vaccinated. You'll be protected from nine strains of HPV instead of just four or two.

"Vaccinated women who got the quadrivalent [four-strain] vaccine may get the nine-valent [strain] vaccine as further protection for them," Guo told LiveScience. But this is by no means an official recommendation, because Guo also stated that more research needs to be done to replicate and confirm these findings. For example, no one has examined how effective Gardasil 9 is on women who have already completed the original Gardasil vaccination in their past.

Getting vaccinated doesn't 100% protect you from all HPV types.

HPV vaccines, no matter which one you choose, are a great way to protect yourself from one STI, but they're not infallible. There's over one hundred strains of the HPV virus, and Gardasil 9 only protects you from nine of them. You can be infected with HPV even if you've been vaccinated.

Timing matters, too. Getting an HPV vaccine after you've already been infected won't make it go away. That's why the CDC recommends girls get the vaccines around 11 or 12 -- before they become sexually active -- for the most effective prevention. But if you're already sexually active, it's not too late; the FDA has approved all three HPV vaccines for females through the age of 26. Even if you already have one strain of HPV, the vaccines may prevent you from contracting another, different strain of the virus.

(The CDC recommends that males get vaccinated at a young age too, because they can contract the virus from one partner and pass it onto a future partner. Most HPV strains caused zero symptoms in males, so men -- even more so than women -- don't know when they're infected.)

That's why getting regular pap tests is critical if you're 21 or older.

Certain strains of HPV cause cellular changes in the cervix, which can develop into cancer if left untreated. That's why the CDC recommends that women over 21 get regular pap smears, a test that detects precancerous cell changes in the cervix before they develop into full-blown cervical cancer. Doctors can remove the abnormal cells before they progress into something far more serious.

Pap smears only test for cervical cancer, though, and HPV can also (though more rarely) lead to cancers of the vulva, vagina, penis, anus and throat. We know that sounds scary, but HPV only develops into cancer if it's been hanging around for years and years. Most of the time, your trusty immune system fights off the virus well before that. So don't freak out!

It's good to know the facts, though, to care of your sexual (and mental and physical) health.