Human papillomavirus (HPV) is an extremely common virus that infects nearly all sexually active adults at some point in their lives. Most HPV infections are fought off by the immune system and cause no symptoms. However, some high-risk strains can lead to cancer if not treated. The good news is that vaccination and regular screening can prevent most HPV-related cancers.
What is HPV?
HPV is a group of over 150 related viruses, categorized as either low-risk or high-risk based on their potential to cause cancer. Low-risk HPV types, such as HPV types 6 and 11, mostly cause benign genital warts. High-risk HPV types, like types 16 and 18, are linked to cancers of the cervix, vagina, vulva, penis, anus, and oropharynx (back of the throat, including base of the tongue and tonsils).
HPV infection statistics
Most sexually active people will get infected with HPV at some point. According to the Centers for Disease Control and Prevention (CDC):
- About 79 million Americans are currently infected with HPV.
- About 14 million people become newly infected each year.
- About half of new infections occur among people aged 15-24.
HPV is so common that nearly all men and women will get at least one type of HPV at some point in their lives.
Does the immune system fight off HPV?
In most cases, yes. The vast majority of HPV infections are cleared from the body by the immune system within 1 to 2 years:
- Around 90% of new HPV infections, including those caused by high-risk HPV types, go away by themselves within 2 years.
- HPV infections that persist for many years are more likely to cause cancer.
When the immune system fails to clear an HPV infection, it can lead to cell changes that may progress to cancer over time. But even for high-risk HPV types, cancer is considered rare.
Why does HPV sometimes persist?
Researchers aren’t exactly sure why HPV sometimes evades the immune system and persists for many years or decades. Potential reasons include:
- The strain of HPV – Some strains are better at avoiding immune detection than others.
- Weakened immunity – Immunosuppression due to HIV, organ transplant drugs, etc. can impair HPV clearance.
- Age – The immune system weakens with age, making HPV persistence more likely in older individuals.
- Genetics – Some gene mutations may make it harder for cells to recognize and destroy HPV-infected cells.
More research is still needed to fully understand the factors that allow HPV to persist.
What factors increase cancer risk from HPV?
Most HPV infections don’t lead to cancer. But persistent infection with a high-risk strain can gradually cause precancerous cell changes over many years. Factors associated with higher cancer risk include:
- High-risk HPV strain – Types 16 and 18 cause about 70% of cervical cancers.
- Older age – Long-term infections over many years raise risk.
- Smoking – Adds to the cell damage caused by HPV.
- Immunosuppression – Impairs the body’s ability to clear HPV.
- Other sexually transmitted infections – May facilitate HPV persistence.
Regular screenings are important, since HPV-related cancers often don’t have symptoms until advanced stages.
What percentage of cervical cancers are caused by HPV?
Nearly all cervical cancers are caused by HPV infection. According to the CDC:
- About 93% of cervical cancers are attributable to HPV types 16 and 18.
- The other 7% are caused by other high-risk HPV strains.
HPV is estimated to cause over 90% of anal cancers and over 70% of vaginal, vulvar, and oropharyngeal cancers. Vaccination and screening have greatly reduced HPV-related cervical cancers in recent years.
What percentage of HPV infections lead to cancer?
Only a very small fraction of HPV infections progress to cancer. Estimates vary, but according to the CDC:
- About 1% of sexually active adults in the U.S. have oral HPV infections. Only 1% of these lead to oropharyngeal cancers.
- About 1% of women have persistent HPV infections that could cause cervical cancer. But even fewer actually develop it.
So while the vast majority of adults will get HPV, only around 0.01-0.2% of infections will eventually lead to cancer. Millions of HPV infections clear on their own before causing any problems.
How many HPV strains can the vaccine protect against?
There are three HPV vaccines currently approved by the Food and Drug Administration (FDA):
- Gardasil 9 protects against 9 strains – types 6, 11, 16, 18, 31, 33, 45, 52 and 58.
- Gardasil 4 protects against 4 strains – types 6, 11, 16 and 18.
- Cervarix protects against 2 strains – types 16 and 18.
The Gardasil vaccines target the HPV types that cause about 90% of cervical and anal cancers and 90% of genital wart cases. The vaccines produce very high antibody levels that protect against persistent infection with the vaccine strains.
Table 1. Number of HPV strains covered by each vaccine
| Vaccine | Number of HPV strains protected against |
|---|---|
| Gardasil 9 | 9 |
| Gardasil 4 | 4 |
| Cervarix | 2 |
How effective is the HPV vaccine?
The HPV vaccines provide extremely effective protection against infection and precancerous lesions associated with the vaccine strains:
- Clinical trials found the vaccines to be nearly 100% effective at preventing infections and precancers caused by the strains they target.
- Population-based studies have confirmed dramatic decreases in HPV infections, genital warts, and cervical precancers in vaccinated cohorts.
- The vaccines appear to provide long-lasting protection. No signs of waning immunity have been seen over 10+ years.
However, the vaccines do not protect against all cancer-causing HPV types. They are most effective when given before exposure to HPV. Catch-up vaccination is still recommended for older groups.
Should adults get the HPV vaccine?
The CDC recommends routine HPV vaccination for girls and boys starting at ages 11-12. Catch-up vaccination is recommended for women up to age 26 and men up to age 21. Beyond these ages, vaccination decisions should be made based on discussion with a healthcare provider.
Some reasons adults may consider getting vaccinated include:
- Did not complete the full vaccine series as a preteen/teen.
- Have a new sex partner and unsure of their STI status.
- Have multiple sex partners or very new partners.
- Immunocompromised or at higher cancer risk.
The vaccine appears most beneficial when given before HPV exposure. But adults may still gain partial protection against strains they haven’t yet acquired.
Conclusion
HPV is extremely common, but most infections are cleared by the immune system within a few years. Persistent high-risk infections are responsible for nearly all cervical cancers and many other HPV-related cancers. Fortunately, we now have very effective vaccines that can prevent the vast majority of HPV cancers by protecting against the strains that cause them. Catch-up vaccination and screening remain essential for older individuals. Ongoing research is still needed to improve early detection and treatment of the small fraction of HPV infections that do progress to cancer.