A vaccine administered to adolescent girls and boys has been so successful that herd immunity against a virus that causes cervical cancer is within reach, at least in the United States. However, sustaining community resistance to the human papillomavirus (HPV) will depend on whether the US can maintain high vaccination rates, say experts. HPV is the world's most common sexually transmitted infection . Vaccines against certain strains of HPV, of which there are hundreds, have existed for almost two decades. Currently a "nine-valent" vaccine — protecting against nine strains of HPV that together account for 90% of cervical cancers — is licensed in the US. Other nations use vaccines tailored to two or four variants. Research published in the journal JAMA Pediatrics has found that the extensive HPV vaccination program in the US has driven major reductions in cervical cancer diagnoses, including among unvaccinated women. "Our study demonstrates evidence for herd immunity against cervical cancer-causing HPV in one community and in the setting of high vaccination rates and a gender-neutral vaccination strategy, that is, vaccination of both boys and girls," said the study's senior author, pediatrician Jessica Kahn from the Albert Einstein College of Medicine in New York City. Kahn's community refers to a specific group of women at higher risk of HPV infection, with almost 80% having had two or more male sexual partners and more than half having been diagnosed with an STI. The analysis of nearly 20 years of data from the group found that HPV infections covered by vaccines declined substantially. In the case of bivalent vaccines, which target two strains, there was a 98% decrease in infection. Because the sexual behaviors of the study group showed little change, it means HPV vaccines are probably responsible for curbing infection rates, including among unvaccinated women. Even though they have no immune protection, this group saw a 75% decline in infection by strains covered by the bivalent vaccine. Such a phenomenon suggests herd immunity against the disease in the US is within reach, though Kahn cautioned it would never be possible to know for sure. "There is not a specific fixed point at which we can say that HPV is eliminated," said Kahn. "Herd protection is dynamic and can wane if vaccine uptake decreases, individuals become more susceptible to HPV or transmission dynamics change." HPV: Often benign, potentially deadly Cervical cancer is the fourth most common cancer in women, with most deaths occurring in lower and middle-income countries. HPV viruses were first discovered as a cause of cervical cancer by German scientist Harald zur Hausen in 1983. His discovery led to him being a joint recipient of the 2008 Nobel Prize for Medicine. Most sexually active people are infected with HPV viruses at some point in their lives, but usually experience no symptoms. These viruses can cause wart growths on the skin of humans — often benign — and usually on the skin, feet and hands, throat and genitals. Sometimes, certain HPV viruses become incorporated into healthy cells, disrupting their normal cell processes. In cells lining the cervix, this can lead to the development of cancers. Cervical cancer rates have declined since the introduction of a vaccine called Gardasil, developed by Australia-based researchers in the 1990s and released in the 2000s. It was the first in a family of inoculations that can prevent HPV infections. Current HPV vaccines are usually administered to girls aged 9-14. Boys in this age group may also receive the vaccine, as is the case in the US, to provide more extensive community protection. Microscope image of an HPV virus